Replace RwLock<MediaList> with shared_lock::Locked<MediaList>

18 files changed, 117 insertions, 42 deletions

diff --git a/components/layout_thread/lib.rs b/components/layout_thread/lib.rs
index 566d6e437cd..ee374d18fc8 100644
--- a/components/layout_thread/lib.rs
+++ b/components/layout_thread/lib.rs
@@ -115,6 +115,7 @@ use style::logical_geometry::LogicalPoint;
 use style::media_queries::{Device, MediaType};
 use style::parser::ParserContextExtraData;
 use style::servo::restyle_damage::{REFLOW, REFLOW_OUT_OF_FLOW, REPAINT, REPOSITION, STORE_OVERFLOW};
+use style::shared_lock::SharedRwLock;
 use style::stylesheets::{Origin, Stylesheet, UserAgentStylesheets};
 use style::stylist::Stylist;
 use style::thread_state;
@@ -934,6 +935,7 @@ impl LayoutThread {
                              possibly_locked_rw_data: &mut RwData<'a, 'b>) {
         let document = unsafe { ServoLayoutNode::new(&data.document) };
         let document = document.as_document().unwrap();
+        let style_guard = document.style_shared_lock().read();
         self.quirks_mode = Some(document.quirks_mode());
 
         // FIXME(pcwalton): Combine `ReflowGoal` and `ReflowQueryType`. Then remove this assert.
@@ -1010,7 +1012,8 @@ impl LayoutThread {
 
         // Calculate the actual viewport as per DEVICE-ADAPT § 6
         let device = Device::new(MediaType::Screen, initial_viewport);
-        Arc::get_mut(&mut rw_data.stylist).unwrap().set_device(device, &data.document_stylesheets);
+        Arc::get_mut(&mut rw_data.stylist).unwrap()
+            .set_device(device, &style_guard, &data.document_stylesheets);
 
         self.viewport_size =
             rw_data.stylist.viewport_constraints().map_or(current_screen_size, |constraints| {
@@ -1528,7 +1531,8 @@ fn get_root_flow_background_color(flow: &mut Flow) -> webrender_traits::ColorF {
 }
 
 fn get_ua_stylesheets() -> Result<UserAgentStylesheets, &'static str> {
-    fn parse_ua_stylesheet(filename: &'static str) -> Result<Stylesheet, &'static str> {
+    fn parse_ua_stylesheet(shared_lock: &SharedRwLock, filename: &'static str)
+                           -> Result<Stylesheet, &'static str> {
         let res = try!(read_resource_file(filename).map_err(|_| filename));
         Ok(Stylesheet::from_bytes(
             &res,
@@ -1537,26 +1541,29 @@ fn get_ua_stylesheets() -> Result<UserAgentStylesheets, &'static str> {
             None,
             Origin::UserAgent,
             Default::default(),
+            shared_lock.clone(),
             None,
             &StdoutErrorReporter,
             ParserContextExtraData::default()))
     }
 
+    let shared_lock = SharedRwLock::new();
     let mut user_or_user_agent_stylesheets = vec!();
     // FIXME: presentational-hints.css should be at author origin with zero specificity.
     //        (Does it make a difference?)
     for &filename in &["user-agent.css", "servo.css", "presentational-hints.css"] {
-        user_or_user_agent_stylesheets.push(try!(parse_ua_stylesheet(filename)));
+        user_or_user_agent_stylesheets.push(try!(parse_ua_stylesheet(&shared_lock, filename)));
     }
     for &(ref contents, ref url) in &opts::get().user_stylesheets {
         user_or_user_agent_stylesheets.push(Stylesheet::from_bytes(
             &contents, url.clone(), None, None, Origin::User, Default::default(),
-            None, &StdoutErrorReporter, ParserContextExtraData::default()));
+            shared_lock.clone(), None, &StdoutErrorReporter, ParserContextExtraData::default()));
     }
 
-    let quirks_mode_stylesheet = try!(parse_ua_stylesheet("quirks-mode.css"));
+    let quirks_mode_stylesheet = try!(parse_ua_stylesheet(&shared_lock, "quirks-mode.css"));
 
     Ok(UserAgentStylesheets {
+        shared_lock: shared_lock,
         user_or_user_agent_stylesheets: user_or_user_agent_stylesheets,
         quirks_mode_stylesheet: quirks_mode_stylesheet,
     })
diff --git a/components/script/dom/bindings/trace.rs b/components/script/dom/bindings/trace.rs
index 4d38b39d333..b0828ab6d56 100644
--- a/components/script/dom/bindings/trace.rs
+++ b/components/script/dom/bindings/trace.rs
@@ -99,6 +99,7 @@ use style::keyframes::Keyframe;
 use style::media_queries::MediaList;
 use style::properties::PropertyDeclarationBlock;
 use style::selector_parser::{PseudoElement, Snapshot};
+use style::shared_lock::SharedRwLock as StyleSharedRwLock;
 use style::stylesheets::{CssRules, KeyframesRule, MediaRule, NamespaceRule, StyleRule, ImportRule};
 use style::stylesheets::SupportsRule;
 use style::values::specified::Length;
@@ -360,6 +361,7 @@ unsafe_no_jsmanaged_fields!(HttpsState);
 unsafe_no_jsmanaged_fields!(Request);
 unsafe_no_jsmanaged_fields!(RequestInit);
 unsafe_no_jsmanaged_fields!(SharedRt);
+unsafe_no_jsmanaged_fields!(StyleSharedRwLock);
 unsafe_no_jsmanaged_fields!(TouchpadPressurePhase);
 unsafe_no_jsmanaged_fields!(USVString);
 unsafe_no_jsmanaged_fields!(ReferrerPolicy);
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index b85366a27d2..7a50fe5cf6c 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -134,6 +134,7 @@ use style::attr::AttrValue;
 use style::context::{QuirksMode, ReflowGoal};
 use style::restyle_hints::{RestyleHint, RESTYLE_STYLE_ATTRIBUTE};
 use style::selector_parser::{RestyleDamage, Snapshot};
+use style::shared_lock::SharedRwLock as StyleSharedRwLock;
 use style::str::{HTML_SPACE_CHARACTERS, split_html_space_chars, str_join};
 use style::stylesheets::Stylesheet;
 use task_source::TaskSource;
@@ -220,6 +221,9 @@ pub struct Document {
     scripts: MutNullableJS<HTMLCollection>,
     anchors: MutNullableJS<HTMLCollection>,
     applets: MutNullableJS<HTMLCollection>,
+    /// Lock use for style attributes and author-origin stylesheet objects in this document.
+    /// Can be acquired once for accessing many objects.
+    style_shared_lock: StyleSharedRwLock,
     /// List of stylesheets associated with nodes in this document. |None| if the list needs to be refreshed.
     stylesheets: DOMRefCell<Option<Vec<StylesheetInDocument>>>,
     /// Whether the list of stylesheets has changed since the last reflow was triggered.
@@ -1964,6 +1968,7 @@ pub trait LayoutDocumentHelpers {
     unsafe fn needs_paint_from_layout(&self);
     unsafe fn will_paint(&self);
     unsafe fn quirks_mode(&self) -> QuirksMode;
+    unsafe fn style_shared_lock(&self) -> &StyleSharedRwLock;
 }
 
 #[allow(unsafe_code)]
@@ -2000,6 +2005,11 @@ impl LayoutDocumentHelpers for LayoutJS<Document> {
     unsafe fn quirks_mode(&self) -> QuirksMode {
         (*self.unsafe_get()).quirks_mode()
     }
+
+    #[inline]
+    unsafe fn style_shared_lock(&self) -> &StyleSharedRwLock {
+        (*self.unsafe_get()).style_shared_lock()
+    }
 }
 
 // https://html.spec.whatwg.org/multipage/#is-a-registrable-domain-suffix-of-or-is-equal-to
@@ -2121,6 +2131,7 @@ impl Document {
             scripts: Default::default(),
             anchors: Default::default(),
             applets: Default::default(),
+            style_shared_lock: StyleSharedRwLock::new(),
             stylesheets: DOMRefCell::new(None),
             stylesheets_changed_since_reflow: Cell::new(false),
             stylesheet_list: MutNullableJS::new(None),
@@ -2250,6 +2261,11 @@ impl Document {
         };
     }
 
+    /// Return a reference to the per-document shared lock used in stylesheets.
+    pub fn style_shared_lock(&self) -> &StyleSharedRwLock {
+        &self.style_shared_lock
+    }
+
     /// Returns the list of stylesheets associated with nodes in the document.
     pub fn stylesheets(&self) -> Vec<Arc<Stylesheet>> {
         self.ensure_stylesheets();
diff --git a/components/script/dom/htmlmetaelement.rs b/components/script/dom/htmlmetaelement.rs
index a751486de7d..2f78e74d3b4 100644
--- a/components/script/dom/htmlmetaelement.rs
+++ b/components/script/dom/htmlmetaelement.rs
@@ -99,12 +99,15 @@ impl HTMLMetaElement {
             let content = content.value();
             if !content.is_empty() {
                 if let Some(translated_rule) = ViewportRule::from_meta(&**content) {
+                    let document = self.upcast::<Node>().owner_doc();
+                    let shared_lock = document.style_shared_lock();
                     *self.stylesheet.borrow_mut() = Some(Arc::new(Stylesheet {
                         rules: CssRules::new(vec![CssRule::Viewport(Arc::new(RwLock::new(translated_rule)))]),
                         origin: Origin::Author,
+                        shared_lock: shared_lock.clone(),
                         base_url: window_from_node(self).get_url(),
                         namespaces: Default::default(),
-                        media: Default::default(),
+                        media: Arc::new(shared_lock.wrap(Default::default())),
                         // Viewport constraints are always recomputed on resize; they don't need to
                         // force all styles to be recomputed.
                         dirty_on_viewport_size_change: AtomicBool::new(false),
diff --git a/components/script/dom/htmlstyleelement.rs b/components/script/dom/htmlstyleelement.rs
index f9c6412f854..13721feba58 100644
--- a/components/script/dom/htmlstyleelement.rs
+++ b/components/script/dom/htmlstyleelement.rs
@@ -84,9 +84,10 @@ impl HTMLStyleElement {
 
         let data = node.GetTextContent().expect("Element.textContent must be a string");
         let mq = parse_media_query_list(&mut CssParser::new(&mq_str));
+        let shared_lock = node.owner_doc().style_shared_lock().clone();
         let loader = StylesheetLoader::for_element(self.upcast());
         let sheet = Stylesheet::from_str(&data, url, Origin::Author, mq,
-                                         Some(&loader),
+                                         shared_lock, Some(&loader),
                                          win.css_error_reporter(),
                                          ParserContextExtraData::default());
 
diff --git a/components/script/layout_wrapper.rs b/components/script/layout_wrapper.rs
index 98ce86745f2..5a9a834667b 100644
--- a/components/script/layout_wrapper.rs
+++ b/components/script/layout_wrapper.rs
@@ -69,6 +69,7 @@ use style::dom::UnsafeNode;
 use style::element_state::*;
 use style::properties::{ComputedValues, PropertyDeclarationBlock};
 use style::selector_parser::{NonTSPseudoClass, PseudoElement, SelectorImpl};
+use style::shared_lock::SharedRwLock as StyleSharedRwLock;
 use style::sink::Push;
 use style::str::is_whitespace;
 use style::stylist::ApplicableDeclarationBlock;
@@ -330,6 +331,10 @@ impl<'ld> ServoLayoutDocument<'ld> {
         unsafe { self.document.quirks_mode() }
     }
 
+    pub fn style_shared_lock(&self) -> &StyleSharedRwLock {
+        unsafe { self.document.style_shared_lock() }
+    }
+
     pub fn from_layout_js(doc: LayoutJS<Document>) -> ServoLayoutDocument<'ld> {
         ServoLayoutDocument {
             document: doc,
diff --git a/components/script/stylesheet_loader.rs b/components/script/stylesheet_loader.rs
index cdc93c9808b..26dffa5fe56 100644
--- a/components/script/stylesheet_loader.rs
+++ b/components/script/stylesheet_loader.rs
@@ -152,12 +152,14 @@ impl FetchResponseListener for StylesheetContext {
                     let is_stylesheet_load_applicable =
                         self.request_generation_id.map_or(true, |gen| gen == link.get_request_generation_id());
                     if is_stylesheet_load_applicable {
+                        let shared_lock = document.style_shared_lock().clone();
                         let sheet =
                             Arc::new(Stylesheet::from_bytes(&data, final_url,
                                                             protocol_encoding_label,
                                                             Some(environment_encoding),
                                                             Origin::Author,
                                                             media.take().unwrap(),
+                                                            shared_lock,
                                                             Some(&loader),
                                                             win.css_error_reporter(),
                                                             ParserContextExtraData::default()));
diff --git a/components/style/encoding_support.rs b/components/style/encoding_support.rs
index 62323ec2314..141291c896e 100644
--- a/components/style/encoding_support.rs
+++ b/components/style/encoding_support.rs
@@ -12,6 +12,7 @@ use media_queries::MediaList;
 use parser::ParserContextExtraData;
 use self::encoding::{EncodingRef, DecoderTrap};
 use servo_url::ServoUrl;
+use shared_lock::SharedRwLock;
 use std::str;
 use stylesheets::{Stylesheet, StylesheetLoader, Origin};
 
@@ -54,6 +55,7 @@ impl Stylesheet {
                       environment_encoding: Option<EncodingRef>,
                       origin: Origin,
                       media: MediaList,
+                      shared_lock: SharedRwLock,
                       stylesheet_loader: Option<&StylesheetLoader>,
                       error_reporter: &ParseErrorReporter,
                       extra_data: ParserContextExtraData)
@@ -64,6 +66,7 @@ impl Stylesheet {
                              base_url,
                              origin,
                              media,
+                             shared_lock,
                              stylesheet_loader,
                              error_reporter,
                              extra_data)
diff --git a/components/style/stylesheets.rs b/components/style/stylesheets.rs
index 5b5c47e152f..3d487ec8db1 100644
--- a/components/style/stylesheets.rs
+++ b/components/style/stylesheets.rs
@@ -21,6 +21,7 @@ use selector_parser::{SelectorImpl, SelectorParser};
 use selectors::parser::SelectorList;
 use servo_config::prefs::PREFS;
 use servo_url::ServoUrl;
+use shared_lock::{SharedRwLock, Locked, SharedRwLockReadGuard};
 use std::cell::Cell;
 use std::fmt;
 use std::sync::Arc;
@@ -175,11 +176,13 @@ pub struct Stylesheet {
     /// cascading order)
     pub rules: Arc<RwLock<CssRules>>,
     /// List of media associated with the Stylesheet.
-    pub media: Arc<RwLock<MediaList>>,
+    pub media: Arc<Locked<MediaList>>,
     /// The origin of this stylesheet.
     pub origin: Origin,
     /// The base url this stylesheet should use.
     pub base_url: ServoUrl,
+    /// The lock used for objects inside this stylesheet
+    pub shared_lock: SharedRwLock,
     /// The namespaces that apply to this stylesheet.
     pub namespaces: RwLock<Namespaces>,
     /// Whether this stylesheet would be dirty when the viewport size changes.
@@ -191,6 +194,8 @@ pub struct Stylesheet {
 
 /// This structure holds the user-agent and user stylesheets.
 pub struct UserAgentStylesheets {
+    /// The lock used for user-agent stylesheets.
+    pub shared_lock: SharedRwLock,
     /// The user or user agent stylesheets.
     pub user_or_user_agent_stylesheets: Vec<Stylesheet>,
     /// The quirks mode stylesheet.
@@ -291,12 +296,12 @@ impl CssRule {
     /// used for others.
     ///
     /// This will not recurse down unsupported @supports rules
-    pub fn with_nested_rules_and_mq<F, R>(&self, mut f: F) -> R
+    pub fn with_nested_rules_and_mq<F, R>(&self, guard: &SharedRwLockReadGuard, mut f: F) -> R
     where F: FnMut(&[CssRule], Option<&MediaList>) -> R {
         match *self {
             CssRule::Import(ref lock) => {
                 let rule = lock.read();
-                let media = rule.stylesheet.media.read();
+                let media = rule.stylesheet.media.read_with(guard);
                 let rules = rule.stylesheet.rules.read();
                 // FIXME(emilio): Include the nested rules if the stylesheet is
                 // loaded.
@@ -349,6 +354,7 @@ impl CssRule {
         let mut rule_parser = TopLevelRuleParser {
             stylesheet_origin: parent_stylesheet.origin,
             context: context,
+            shared_lock: &parent_stylesheet.shared_lock,
             loader: None,
             state: Cell::new(state),
             namespaces: &mut namespaces,
@@ -424,7 +430,8 @@ impl ToCss for ImportRule {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         try!(dest.write_str("@import "));
         try!(self.url.to_css(dest));
-        let media = self.stylesheet.media.read();
+        let guard = self.stylesheet.shared_lock.read();  // FIXME: have the caller pass this?
+        let media = self.stylesheet.media.read_with(&guard);
         if !media.is_empty() {
             try!(dest.write_str(" "));
             try!(media.to_css(dest));
@@ -554,6 +561,7 @@ impl Stylesheet {
         let rule_parser = TopLevelRuleParser {
             stylesheet_origin: existing.origin,
             namespaces: &mut namespaces,
+            shared_lock: &existing.shared_lock,
             loader: stylesheet_loader,
             context: ParserContext::new_with_extra_data(existing.origin,
                                                         &existing.base_url,
@@ -591,6 +599,7 @@ impl Stylesheet {
                     base_url: ServoUrl,
                     origin: Origin,
                     media: MediaList,
+                    shared_lock: SharedRwLock,
                     stylesheet_loader: Option<&StylesheetLoader>,
                     error_reporter: &ParseErrorReporter,
                     extra_data: ParserContextExtraData) -> Stylesheet {
@@ -599,7 +608,8 @@ impl Stylesheet {
             base_url: base_url,
             namespaces: RwLock::new(Namespaces::default()),
             rules: CssRules::new(vec![]),
-            media: Arc::new(RwLock::new(media)),
+            media: Arc::new(shared_lock.wrap(media)),
+            shared_lock: shared_lock.clone(),
             dirty_on_viewport_size_change: AtomicBool::new(false),
             disabled: AtomicBool::new(false),
         };
@@ -638,7 +648,8 @@ impl Stylesheet {
     ///
     /// Always true if no associated MediaList exists.
     pub fn is_effective_for_device(&self, device: &Device) -> bool {
-        self.media.read().evaluate(device)
+        let guard = self.shared_lock.read();  // FIXME: have the caller pass this?
+        self.media.read_with(&guard).evaluate(device)
     }
 
     /// Return an iterator over the effective rules within the style-sheet, as
@@ -649,7 +660,8 @@ impl Stylesheet {
     /// examined.
     #[inline]
     pub fn effective_rules<F>(&self, device: &Device, mut f: F) where F: FnMut(&CssRule) {
-        effective_rules(&self.rules.read().0, device, &mut f);
+        let guard = self.shared_lock.read();  // FIXME: have the caller pass this?
+        effective_rules(&self.rules.read().0, device, &guard, &mut f);
     }
 
     /// Returns whether the stylesheet has been explicitly disabled through the
@@ -670,16 +682,17 @@ impl Stylesheet {
     }
 }
 
-fn effective_rules<F>(rules: &[CssRule], device: &Device, f: &mut F) where F: FnMut(&CssRule) {
+fn effective_rules<F>(rules: &[CssRule], device: &Device, guard: &SharedRwLockReadGuard, f: &mut F)
+where F: FnMut(&CssRule) {
     for rule in rules {
         f(rule);
-        rule.with_nested_rules_and_mq(|rules, mq| {
+        rule.with_nested_rules_and_mq(guard, |rules, mq| {
             if let Some(media_queries) = mq {
                 if !media_queries.evaluate(device) {
                     return
                 }
             }
-            effective_rules(rules, device, f)
+            effective_rules(rules, device, guard, f)
         })
     }
 }
@@ -724,6 +737,7 @@ pub trait StylesheetLoader {
 struct TopLevelRuleParser<'a> {
     stylesheet_origin: Origin,
     namespaces: &'a mut Namespaces,
+    shared_lock: &'a SharedRwLock,
     loader: Option<&'a StylesheetLoader>,
     context: ParserContext<'a>,
     state: Cell<State>,
@@ -780,7 +794,7 @@ impl<'a> AtRuleParser for TopLevelRuleParser<'a> {
                                                              &self.context));
 
                     let media =
-                        Arc::new(RwLock::new(parse_media_query_list(input)));
+                        Arc::new(self.shared_lock.wrap(parse_media_query_list(input)));
 
                     let is_valid_url = url.url().is_some();
 
@@ -790,6 +804,7 @@ impl<'a> AtRuleParser for TopLevelRuleParser<'a> {
                             stylesheet: Arc::new(Stylesheet {
                                 rules: Arc::new(RwLock::new(CssRules(vec![]))),
                                 media: media,
+                                shared_lock: self.shared_lock.clone(),
                                 origin: self.context.stylesheet_origin,
                                 base_url: self.context.base_url.clone(),
                                 namespaces: RwLock::new(Namespaces::default()),
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index f7dd149e7d3..e28bb641cd0 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -28,6 +28,7 @@ use selectors::matching::{AFFECTED_BY_STYLE_ATTRIBUTE, AFFECTED_BY_PRESENTATIONA
 use selectors::matching::{ElementSelectorFlags, StyleRelations, matches_complex_selector};
 use selectors::parser::{Selector, SimpleSelector, LocalName as LocalNameSelector, ComplexSelector};
 use selectors::parser::SelectorMethods;
+#[cfg(feature = "servo")] use shared_lock::SharedRwLockReadGuard;
 use sink::Push;
 use smallvec::VecLike;
 use std::borrow::Borrow;
@@ -461,7 +462,8 @@ impl Stylist {
     /// FIXME(emilio): The semantics of the device for Servo and Gecko are
     /// different enough we may want to unify them.
     #[cfg(feature = "servo")]
-    pub fn set_device(&mut self, mut device: Device, stylesheets: &[Arc<Stylesheet>]) {
+    pub fn set_device(&mut self, mut device: Device, guard: &SharedRwLockReadGuard,
+                      stylesheets: &[Arc<Stylesheet>]) {
         let cascaded_rule = ViewportRule {
             declarations: viewport::Cascade::from_stylesheets(stylesheets, &device).finish(),
         };
@@ -473,15 +475,16 @@ impl Stylist {
             device.account_for_viewport_rule(constraints);
         }
 
-        fn mq_eval_changed(rules: &[CssRule], before: &Device, after: &Device) -> bool {
+        fn mq_eval_changed(guard: &SharedRwLockReadGuard, rules: &[CssRule],
+                           before: &Device, after: &Device) -> bool {
             for rule in rules {
-                let changed = rule.with_nested_rules_and_mq(|rules, mq| {
+                let changed = rule.with_nested_rules_and_mq(guard, |rules, mq| {
                     if let Some(mq) = mq {
                         if mq.evaluate(before) != mq.evaluate(after) {
                             return true
                         }
                     }
-                    mq_eval_changed(rules, before, after)
+                    mq_eval_changed(guard, rules, before, after)
                 });
                 if changed {
                     return true
@@ -490,12 +493,12 @@ impl Stylist {
             false
         }
         self.is_device_dirty |= stylesheets.iter().any(|stylesheet| {
-            let mq = stylesheet.media.read();
+            let mq = stylesheet.media.read_with(guard);
             if mq.evaluate(&self.device) != mq.evaluate(&device) {
                 return true
             }
 
-            mq_eval_changed(&stylesheet.rules.read().0, &self.device, &device)
+            mq_eval_changed(guard, &stylesheet.rules.read().0, &self.device, &device)
         });
 
         self.device = Arc::new(device);
diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index 16d8b0c9f36..7330d8f63de 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -76,6 +76,7 @@ use style::properties::parse_one_declaration;
 use style::restyle_hints::{self, RestyleHint};
 use style::selector_parser::PseudoElementCascadeType;
 use style::sequential;
+use style::shared_lock::SharedRwLock;
 use style::string_cache::Atom;
 use style::stylesheets::{CssRule, CssRules, ImportRule, MediaRule, NamespaceRule};
 use style::stylesheets::{Origin, Stylesheet, StyleRule};
@@ -85,7 +86,7 @@ use style::thread_state;
 use style::timer::Timer;
 use style::traversal::{resolve_style, DomTraversal, TraversalDriver};
 use style_traits::ToCss;
-use stylesheet_loader::StylesheetLoader;
+use super::stylesheet_loader::StylesheetLoader;
 
 /*
  * For Gecko->Servo function calls, we need to redeclare the same signature that was declared in
@@ -95,12 +96,15 @@ use stylesheet_loader::StylesheetLoader;
  * depend on but good enough for our purposes.
  */
 
-struct GlobalStyleData {
+pub struct GlobalStyleData {
     // How many threads parallel styling can use.
     pub num_threads: usize,
 
     // The parallel styling thread pool.
     pub style_thread_pool: Option<rayon::ThreadPool>,
+
+    // Shared RWLock for CSSOM objects
+    pub shared_lock: SharedRwLock,
 }
 
 impl GlobalStyleData {
@@ -124,12 +128,13 @@ impl GlobalStyleData {
         GlobalStyleData {
             num_threads: num_threads,
             style_thread_pool: pool,
+            shared_lock: SharedRwLock::new(),
         }
     }
 }
 
 lazy_static! {
-    static ref GLOBAL_STYLE_DATA: GlobalStyleData = {
+    pub static ref GLOBAL_STYLE_DATA: GlobalStyleData = {
         GlobalStyleData::new()
     };
 }
@@ -335,8 +340,9 @@ pub extern "C" fn Servo_StyleSheet_Empty(mode: SheetParsingMode) -> RawServoStyl
         SheetParsingMode::eUserSheetFeatures => Origin::User,
         SheetParsingMode::eAgentSheetFeatures => Origin::UserAgent,
     };
+    let shared_lock = GLOBAL_STYLE_DATA.shared_lock.clone();
     Arc::new(Stylesheet::from_str(
-        "", url, origin, Default::default(), None,
+        "", url, origin, Default::default(), shared_lock, None,
         &StdoutErrorReporter, extra_data)
     ).into_strong()
 }
@@ -378,8 +384,9 @@ pub extern "C" fn Servo_StyleSheet_FromUTF8Bytes(loader: *mut Loader,
         Some(ref s) => Some(s),
     };
 
+    let shared_lock = GLOBAL_STYLE_DATA.shared_lock.clone();
     Arc::new(Stylesheet::from_str(
-        input, url, origin, Default::default(), loader,
+        input, url, origin, Default::default(), shared_lock, loader,
         &StdoutErrorReporter, extra_data)
     ).into_strong()
 }
diff --git a/ports/geckolib/stylesheet_loader.rs b/ports/geckolib/stylesheet_loader.rs
index 3e6fb421c86..bcd55cce2fd 100644
--- a/ports/geckolib/stylesheet_loader.rs
+++ b/ports/geckolib/stylesheet_loader.rs
@@ -9,6 +9,7 @@ use style::gecko_bindings::structs::{Loader, ServoStyleSheet};
 use style::gecko_bindings::sugar::ownership::HasArcFFI;
 use style::stylesheets::{ImportRule, StylesheetLoader as StyleStylesheetLoader};
 use style_traits::ToCss;
+use super::glue::GLOBAL_STYLE_DATA;
 
 pub struct StylesheetLoader(*mut Loader, *mut ServoStyleSheet);
 
@@ -20,6 +21,8 @@ impl StylesheetLoader {
 
 impl StyleStylesheetLoader for StylesheetLoader {
     fn request_stylesheet(&self, import_rule: &Arc<RwLock<ImportRule>>) {
+        let global_style_data = &*GLOBAL_STYLE_DATA;
+        let guard = global_style_data.shared_lock.read();
         let import = import_rule.read();
         let (spec_bytes, spec_len) = import.url.as_slice_components()
             .expect("Import only loads valid URLs");
@@ -32,7 +35,7 @@ impl StyleStylesheetLoader for StylesheetLoader {
         // evaluate them on the main thread.
         //
         // Meanwhile, this works.
-        let media = import.stylesheet.media.read().to_css_string();
+        let media = import.stylesheet.media.read_with(&guard).to_css_string();
 
         unsafe {
             Gecko_LoadStyleSheet(self.0,
diff --git a/tests/unit/style/media_queries.rs b/tests/unit/style/media_queries.rs
index 95182676e3c..485ccd1d486 100644
--- a/tests/unit/style/media_queries.rs
+++ b/tests/unit/style/media_queries.rs
@@ -11,6 +11,7 @@ use style::error_reporting::ParseErrorReporter;
 use style::media_queries::*;
 use style::parser::ParserContextExtraData;
 use style::servo::media_queries::*;
+use style::shared_lock::{SharedRwLock, SharedRwLockReadGuard};
 use style::stylesheets::{Stylesheet, Origin, CssRule};
 use style::values::specified;
 use style_traits::ToCss;
@@ -29,26 +30,27 @@ fn test_media_rule<F>(css: &str, callback: F)
     let url = ServoUrl::parse("http://localhost").unwrap();
     let css_str = css.to_owned();
     let stylesheet = Stylesheet::from_str(
-        css, url, Origin::Author, Default::default(),
+        css, url, Origin::Author, Default::default(), SharedRwLock::new(),
         None, &CSSErrorReporterTest,
         ParserContextExtraData::default());
     let mut rule_count = 0;
-    media_queries(&stylesheet.rules.read().0, &mut |mq| {
+    let guard = stylesheet.shared_lock.read();
+    media_queries(&guard, &stylesheet.rules.read().0, &mut |mq| {
         rule_count += 1;
         callback(mq, css);
     });
     assert!(rule_count > 0, css_str);
 }
 
-fn media_queries<F>(rules: &[CssRule], f: &mut F)
+fn media_queries<F>(guard: &SharedRwLockReadGuard, rules: &[CssRule], f: &mut F)
     where F: FnMut(&MediaList),
 {
     for rule in rules {
-        rule.with_nested_rules_and_mq(|rules, mq| {
+        rule.with_nested_rules_and_mq(guard, |rules, mq| {
             if let Some(mq) = mq {
                 f(mq)
             }
-            media_queries(rules, f)
+            media_queries(guard, rules, f)
         })
     }
 }
@@ -56,7 +58,7 @@ fn media_queries<F>(rules: &[CssRule], f: &mut F)
 fn media_query_test(device: &Device, css: &str, expected_rule_count: usize) {
     let url = ServoUrl::parse("http://localhost").unwrap();
     let ss = Stylesheet::from_str(
-        css, url, Origin::Author, Default::default(),
+        css, url, Origin::Author, Default::default(), SharedRwLock::new(),
         None, &CSSErrorReporterTest,
         ParserContextExtraData::default());
     let mut rule_count = 0;
diff --git a/tests/unit/style/rule_tree/bench.rs b/tests/unit/style/rule_tree/bench.rs
index 37bef590a64..623a534159a 100644
--- a/tests/unit/style/rule_tree/bench.rs
+++ b/tests/unit/style/rule_tree/bench.rs
@@ -12,6 +12,7 @@ use style::media_queries::MediaList;
 use style::parser::ParserContextExtraData;
 use style::properties::{longhands, Importance, PropertyDeclaration, PropertyDeclarationBlock};
 use style::rule_tree::{CascadeLevel, RuleTree, StrongRuleNode, StyleSource};
+use style::shared_lock::SharedRwLock;
 use style::stylesheets::{Origin, Stylesheet, CssRule};
 use test::{self, Bencher};
 
@@ -44,6 +45,7 @@ fn parse_rules(css: &str) -> Vec<(StyleSource, CascadeLevel)> {
                                  MediaList {
                                      media_queries: vec![],
                                  },
+                                 SharedRwLock::new(),
                                  None,
                                  &ErrorringErrorReporter,
                                  ParserContextExtraData {});
diff --git a/tests/unit/style/stylesheets.rs b/tests/unit/style/stylesheets.rs
index a1d649c099c..590e93185f4 100644
--- a/tests/unit/style/stylesheets.rs
+++ b/tests/unit/style/stylesheets.rs
@@ -20,6 +20,7 @@ use style::properties::Importance;
 use style::properties::{CSSWideKeyword, DeclaredValueOwned, PropertyDeclaration, PropertyDeclarationBlock};
 use style::properties::longhands;
 use style::properties::longhands::animation_play_state;
+use style::shared_lock::SharedRwLock;
 use style::stylesheets::{Origin, Namespaces};
 use style::stylesheets::{Stylesheet, NamespaceRule, CssRule, CssRules, StyleRule, KeyframesRule};
 use style::values::specified::{LengthOrPercentageOrAuto, Percentage};
@@ -62,14 +63,15 @@ fn test_parse_stylesheet() {
         }";
     let url = ServoUrl::parse("about::test").unwrap();
     let stylesheet = Stylesheet::from_str(css, url.clone(), Origin::UserAgent, Default::default(),
-                                          None,
+                                          SharedRwLock::new(), None,
                                           &CSSErrorReporterTest,
                                           ParserContextExtraData::default());
     let mut namespaces = Namespaces::default();
     namespaces.default = Some(ns!(html));
     let expected = Stylesheet {
         origin: Origin::UserAgent,
-        media: Default::default(),
+        media: Arc::new(stylesheet.shared_lock.wrap(Default::default())),
+        shared_lock: stylesheet.shared_lock.clone(),
         namespaces: RwLock::new(namespaces),
         base_url: url,
         dirty_on_viewport_size_change: AtomicBool::new(false),
@@ -324,7 +326,7 @@ fn test_report_error_stylesheet() {
     let errors = error_reporter.errors.clone();
 
     Stylesheet::from_str(css, url.clone(), Origin::UserAgent, Default::default(),
-                         None,
+                         SharedRwLock::new(), None,
                          &error_reporter,
                          ParserContextExtraData::default());
 
diff --git a/tests/unit/style/viewport.rs b/tests/unit/style/viewport.rs
index 166094ebdc1..9fe53ebb40c 100644
--- a/tests/unit/style/viewport.rs
+++ b/tests/unit/style/viewport.rs
@@ -9,6 +9,7 @@ use servo_config::prefs::{PREFS, PrefValue};
 use servo_url::ServoUrl;
 use style::media_queries::{Device, MediaType};
 use style::parser::{ParserContext, ParserContextExtraData};
+use style::shared_lock::SharedRwLock;
 use style::stylesheets::{Stylesheet, Origin};
 use style::values::specified::LengthOrPercentageOrAuto::{self, Auto};
 use style::values::specified::NoCalcLength::{self, ViewportPercentage};
@@ -24,6 +25,7 @@ macro_rules! stylesheet {
             ServoUrl::parse("http://localhost").unwrap(),
             Origin::$origin,
             Default::default(),
+            SharedRwLock::new(),
             None,
             &$error_reporter,
             ParserContextExtraData::default()
diff --git a/tests/unit/stylo/lib.rs b/tests/unit/stylo/lib.rs
index c89687918ef..c0d04ffcbec 100644
--- a/tests/unit/stylo/lib.rs
+++ b/tests/unit/stylo/lib.rs
@@ -21,8 +21,5 @@ extern crate style_traits;
 mod sanity_checks;
 mod size_of;
 
-#[path = "../../../ports/geckolib/stylesheet_loader.rs"]
-mod stylesheet_loader;
-
 mod servo_function_signatures;
 
diff --git a/tests/unit/stylo/servo_function_signatures.rs b/tests/unit/stylo/servo_function_signatures.rs
index 6bdeb6d5af7..30d6d02439b 100644
--- a/tests/unit/stylo/servo_function_signatures.rs
+++ b/tests/unit/stylo/servo_function_signatures.rs
@@ -10,6 +10,9 @@ use style::gecko_properties::*;
 
 include!(concat!(env!("OUT_DIR"), "/check_bindings.rs"));
 
+#[path = "../../../ports/geckolib/stylesheet_loader.rs"]
+mod stylesheet_loader;
+
 #[allow(non_snake_case, unused_unsafe, private_no_mangle_fns)]
 mod glue {
     // this module pretends to be glue.rs, with the safe functions swapped for unsafe ones. This is