ThreadSanitizer: data race [@ mozilla::ShutdownXPCOM] vs. [@ mozilla::ThreadEventTarget::Dispatch] on gXPCOMThreadsShutDown
Categories
(Core :: XPCOM, defect)
Tracking
()
People
(Reporter: decoder, Assigned: KrisWright)
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 c0a6eb95b65c.
This is a race on the non-atomic global gXPCOMThreadsShutDown
. I don't know enough about our shutdown routines to tell how this should be fixed or if this is even expected. Depending on the code creating the timer in question here, making gXPCOMThreadsShutDown
a (non-relaxed) atomic could solve the problem. But if the timer existed pre-shutdown and is not cancelled by the shutdown, then it could still fire randomly afterwards.
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•5 years ago
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Comment 2•4 years ago
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Updated•4 years ago
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Comment 4•4 years ago
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bugherder |
Comment 5•4 years ago
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Updated•4 years ago
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Updated•4 years ago
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Comment 7•4 years ago
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bugherder |
Updated•4 years ago
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Comment 8•4 years ago
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(was supposed to be leave-open, sorry!)
Updated•4 years ago
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Updated•4 years ago
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Assignee | ||
Comment 9•4 years ago
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This commit makes gXPCOMThreadsShutdown
atomic. I've deliberated on this one for a while because I was mostly interested in how timer threads may be trying to init during shutdown, but these aren't the only places where we are making accesses into gXPCOMThreadsShutdown
so it should be made atomic regardless.
Updated•4 years ago
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Assignee | ||
Comment 10•4 years ago
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This is no longer going to be detected by tsan. I figure with full shutdown enabled we should be able to see if there were other issues around this one.
Comment 11•4 years ago
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Comment 12•4 years ago
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bugherder |
https://hg.mozilla.org/mozilla-central/rev/4c798c8a8aaf
https://hg.mozilla.org/mozilla-central/rev/40e53357bfe1
Comment 13•4 years ago
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Since the status are different for nightly and release, what's the status for beta?
For more information, please visit auto_nag documentation.
Updated•4 years ago
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Updated•4 years ago
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Description
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