Attachment 339088 Details for Bug 185117 – Patch

[patch] Patch

bug-185117-20180428213407.patch (text/plain), 3.14 KB, created by Geoffrey Garen on 2018-04-28 21:34:07 PDT

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Creator: Geoffrey Garen

Created: 2018-04-28 21:34:07 PDT

Size: 3.14 KB

patch

obsolete

>Index: Source/WTF/ChangeLog
>===================================================================
>--- Source/WTF/ChangeLog	(revision 231145)
>+++ Source/WTF/ChangeLog	(working copy)
>@@ -1,3 +1,36 @@
>+2018-04-28  Geoffrey Garen  <ggaren@apple.com>
>+
>+        Fixed a very unlikely race condition in WTF::WordLock
>+        https://bugs.webkit.org/show_bug.cgi?id=185117
>+
>+        Reviewed by NOBODY (OOPS!).
>+
>+        The race goes like this:
>+
>+        Thread L is in lockSlowCase() and thread U is in unlockSlowCase();
>+
>+        - U acquires queueHead->parkingLock.
>+        - U sets queueHead->shouldPark = false
>+        - U releases queueHead->parkingLock.
>+        - L spuriously wakes up from queueHead->parkingLock.wait()
>+        - L acquires queueHead->parkingLock.
>+        - L notices that queueHead->shouldPark = false, and acquires the WordLock
>+        - L finishes all its work and exits, freeing queueHead
>+        - U notifies queueHead->parkingLock (after free) and crashes or deadlocks
>+
>+        These conditions are currently so unlikely that I don't know how to test
>+        them. I noticed this race because I changed WordLock's allocation pattern
>+        to allow queueHead to be freed more often, and I crashed / deadlocked 100%.
>+
>+        Shout out to <http://en.cppreference.com/w/cpp/thread/condition_variable/notify_one>
>+        for explaining this.
>+
>+        * benchmarks/ToyLocks.h: Fixed build.
>+
>+        * wtf/WordLock.cpp:
>+        (WTF::WordLock::unlockSlow): Hold the lock a little longer to avoid
>+        this race.
>+
> 2018-04-27  David Kilzer  <ddkilzer@apple.com>
> 
>         Add logging when SpringBoard enables WebThread
>Index: Source/WTF/benchmarks/ToyLocks.h
>===================================================================
>--- Source/WTF/benchmarks/ToyLocks.h	(revision 228023)
>+++ Source/WTF/benchmarks/ToyLocks.h	(working copy)
>@@ -31,6 +31,7 @@
> #include <wtf/Atomics.h>
> #include <wtf/Lock.h>
> #include <wtf/ParkingLot.h>
>+#include <wtf/Threading.h>
> #include <wtf/WordLock.h>
> 
> #if __has_include(<os/lock.h>)
>Index: Source/WTF/wtf/WordLock.cpp
>===================================================================
>--- Source/WTF/wtf/WordLock.cpp	(revision 228023)
>+++ Source/WTF/wtf/WordLock.cpp	(working copy)
>@@ -254,12 +254,15 @@ NEVER_INLINE void WordLock::unlockSlow()
>     // We do this carefully because this may run either before or during the parkingLock critical
>     // section in lockSlow().
>     {
>+        // Be sure to hold the lock across our call to notify_one() because a spurious wakeup could
>+        // cause the thread at the head of the queue to exit and delete queueHead.
>         std::lock_guard<std::mutex> locker(queueHead->parkingLock);
>         queueHead->shouldPark = false;
>+
>+        // Doesn't matter if we notify_all() or notify_one() here since the only thread that could be
>+        // waiting is queueHead.
>+        queueHead->parkingCondition.notify_one();
>     }
>-    // Doesn't matter if we notify_all() or notify_one() here since the only thread that could be
>-    // waiting is queueHead.
>-    queueHead->parkingCondition.notify_one();
> 
>     // The old queue head can now contend for the lock again. We're done!
> }

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saam: review+

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