Open Bug 1671574 Opened 7 months ago Updated 5 months ago

ThreadSanitizer: thread leak [@ pthread_create] for StartupCache Thread

Categories

(Core :: XPCOM, defect)

x86_64
Linux
defect
Not set
normal

Tracking

()

ASSIGNED
Tracking Status
firefox83 --- affected

People

(Reporter: decoder, Assigned: decoder)

References

(Blocks 1 open bug)

Details

(Keywords: leave-open)

Attachments

(2 files)

The attached crash information was detected while running CI tests with ThreadSanitizer on try revision 1fdd4f52d08fd1fb01b6c77c4a86b03af275855d.

I saw this thread leak intermittently in mochitest-bc but nowhere else. It means we didn't join the StartupCache thread properly, is this expected? If so, that's fine and we can just suppress this. However, I was skeptical since it only happens intermittently and only in some tests.

Doug, maybe you can answer this?

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.

Assignee: nobody → choller
Status: NEW → ASSIGNED
Keywords: leave-open
Pushed by choller@mozilla.com:
https://hg.mozilla.org/integration/autoland/rev/7a39a3000939
Add temporary suppression for StartupCache thread leak. r=Gankro

The "StartupCache" thread races with main-thread and tries to touch pages of an mmap'd buffer in order to trigger OS-level paging. It has no effect on correctness and so is never joined with. Perhaps there is a better way to indicate this? Otherwise it may just be a justifiable suppression.

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