ThreadSanitizer: data race [@ mozilla::ScriptPreloader::MaybeFinishOffThreadDecode] vs. [@ mozilla::ScriptPreloader::OffThreadDecodeCallback] on mToken
Categories
(Core :: XPConnect, defect, P3)
Tracking
()
People
(Reporter: decoder, Assigned: tcampbell)
References
(Blocks 1 open bug)
Details
Attachments
(4 files, 1 obsolete file)
The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision 6989fcd6bab3.
Filing this because it is amongst the frequent races in Mochitests. This race is apparently on updating the mToken
pointer. Should that be an atomic to ensure that the update is properly visible?
General information about TSan reports
Why fix races?
Data races are undefined behavior and can cause crashes as well as correctness issues. Compiler optimizations can cause racy code to have unpredictable and hard-to-reproduce behavior.
Rating
If you think this race can cause crashes or correctness issues, it would be great to rate the bug appropriately as P1/P2 and/or indicating this in the bug. This makes it a lot easier for us to assess the actual impact that these reports make and if they are helpful to you.
False Positives / Benign Races
Typically, races reported by TSan are not false positives [1], but it is possible that the race is benign. Even in this case it would be nice to come up with a fix if it is easily doable and does not regress performance. Every race that we cannot fix will have to remain on the suppression list and slows down the overall TSan performance. Also note that seemingly benign races can possibly be harmful (also depending on the compiler, optimizations and the architecture) [2][3].
[1] One major exception is the involvement of uninstrumented code from third-party libraries.
[2] http://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong
[3] How to miscompile programs with "benign" data races: https://www.usenix.org/legacy/events/hotpar11/tech/final_files/Boehm.pdf
Suppressing unfixable races
If the bug cannot be fixed, then a runtime suppression needs to be added in mozglue/build/TsanOptions.cpp
. The suppressions match on the full stack, so it should be picked such that it is unique to this particular race. The bug number of this bug should also be included so we have some documentation on why this suppression was added.
Reporter | ||
Comment 1•4 years ago
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Updated•4 years ago
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Comment 2•4 years ago
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Hmm. Looks real; ScriptPreloader::WaitForCachedScript
calls MaybeFinishOffThreadDecode()
three times, sometimes with mMonitor
held, sometimes not, which seems fishy.
The first line of MaybeFinishOffThreadDecode
reads mToken
which I think has to be done with the lock held, since it is set here with the mutex held.
Kris, can you take this?
Updated•4 years ago
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Comment hidden (Intermittent Failures Robot) |
Comment 4•4 years ago
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Depends on D95128
Updated•4 years ago
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Pushed by abeingessner@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/2f14d44bacf2 Remove supression for seemingly fixed issue. r=decoder
Comment 6•4 years ago
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Backed out 2 changesets (bug 1601632, bug 1608068) for ScriptPreloader related tsan failures.
Backout link: https://hg.mozilla.org/integration/autoland/rev/d01cd574728d7c07dbfab85f48987f2d0a4b1fd6
Failure log: https://treeherder.mozilla.org/logviewer?job_id=320532020&repo=autoland&lineNumber=10577
Updated•4 years ago
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Updated•4 years ago
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Assignee | ||
Comment 8•3 years ago
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The mToken
is used across threads without full locking so it must be marked
as atomic. At the same time, we can use exchange
operations to read or
write outside of the mMonitor lock.
Depends on D99402
Updated•3 years ago
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Assignee | ||
Comment 9•3 years ago
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Add mWaitingForDecode
to indicate that main-thread is blocked on the
Monitor and should be woken. This avoids generating some of the unused
Runnables. Also ensure mFinishDecodeRunnablePending
is only accessed while
holding the lock.
Depends on D99403
Comment 10•3 years ago
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Pushed by tcampbell@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/2497e2dd2ae8 Make ScriptPreloader::mToken updates Atomic. r=kmag,Gankro,decoder https://hg.mozilla.org/integration/autoland/rev/9e1d4db4e5d0 Cleanup ScriptPreloader Runnable behaviour. r=kmag,Gankro,decoder
Comment 11•3 years ago
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bugherder |
https://hg.mozilla.org/mozilla-central/rev/2497e2dd2ae8
https://hg.mozilla.org/mozilla-central/rev/9e1d4db4e5d0
Updated•3 years ago
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Description
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