1664535 - ThreadSanitizer: data race [@ JS::Zone::setNeedsIncrementalBarrier] vs. [@ needsIncrementalBarrier]

Status: RESOLVED FIXED

(Core :: JavaScript: GC, defect, P3)

Description

[Christian Holler (:decoder)]

The attached crash information was detected while running CI tests with ThreadSanitizer on try (parent revision is mozilla-central dc90a7a18c07).

For detailed crash information, see attachment.

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.

Comment 1

[Christian Holler (:decoder)]

Attachment: Detailed Crash Information

Comment 2

[Jon Coppeard (:jonco)]

The main thread is marking in the atoms zone for which barriers were disabled and are then re-enabled (here). A background thread is sweeping a another zone and deleting a HeapPtrAtom. The prebarrier is checking whether the atoms zone is marking in case it needs to mark the atom.

This is not a problem because we only need to fire the prebarrier if the second zone is still being marked (if we are removing an edge from behind the marking wavefront). This is never the case in this situation, so whether the barrier fires or not doesn't matter.

I don't know how to suppress this warning in a way that wouldn't hide legitimate races however.

Comment 3

[Christian Holler (:decoder)]

Attachment: Bug 1664535 - Temporary suppression for GC race. r?jonco

Comment 4

[Jon Coppeard (:jonco)]

I'd like to leave this open since the patch only suppresses the TSAN warning.

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/cf71cbc9e8a6
Temporary suppression for GC race. r=jonco

Comment 6

[Narcis Beleuzu [:NarcisB]]

https://hg.mozilla.org/mozilla-central/rev/cf71cbc9e8a6

Comment 7

[Jon Coppeard (:jonco)]

This race should have been fixed by bug 1681297.

Comment 8

[Jon Coppeard (:jonco)]

Attachment: Bug 1664535 - Remove TSAN suppression now write barriers are not triggered on background threads r?decoder

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/ef0570dd669b
Remove TSAN suppression now write barriers are not triggered on background threads r=decoder

Comment 10

[Razvan Maries]

Backed out for high frequency failures.

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=AStFm3IsQiyQScDZN6Xekg.0&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception&searchStr=linux%2C18.04%2Cx64%2Ctsan%2Copt%2Cmochitests%2Ctest-linux1804-64-tsan%2Fopt-mochitest-browser-chrome-e10s%2Cbc22&revision=37a6b8654987d85dc7f5d1db97cd0074143ff013

Log: https://treeherder.mozilla.org/logviewer?job_id=324965480&repo=autoland&lineNumber=3241

Backout: https://hg.mozilla.org/integration/autoland/rev/507ca4e6728999552b7ce90c7cf1513ce18f149b

Comment 11

[Jon Coppeard (:jonco)]

Well, that did not work.

Looking again, bug 1681297 fixed a race but the check added there happens after checking the zone's GC state which is what this is complaining about. Probably we need to make the GC state atomic to fix this.

Comment 13

[BugBot [:suhaib / :marco/ :calixte]]

The leave-open keyword is there and there is no activity for 6 months.
:decoder, maybe it's time to close this bug?

Comment 14

[BugBot [:suhaib / :marco/ :calixte]]

The leave-open keyword is there and there is no activity for 6 months.
:decoder, maybe it's time to close this bug?

Comment 15

[Jon Coppeard (:jonco)]

(In reply to Jon Coppeard (:jonco) from comment #11)

Probably we need to make the GC state atomic to fix this.

This state was made atomic in bug 1734801 so we should be able to remove this suppression now.

Comment 16

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/835be2209297
Remove TSAN suppression now write barriers are not triggered on background threads r=decoder

Comment 17

[Marian-Vasile Laza]

https://hg.mozilla.org/mozilla-central/rev/835be2209297