1686156 - ThreadSanitizer: data race [@ operator] vs. [@ mozilla::dom::BodyConsumer::BeginConsumeBodyMainThread]

Status: RESOLVED DUPLICATE of bug 1690370

(Core :: DOM: Core & HTML, defect, P3)

Description

[Tyson Smith [:tsmith]]

Attachment: Detailed Crash Information

The attached crash information was detected by ThreadSanitizer while fuzzing with build mozilla-central 20210111-7643eaee62a3. The fuzzers have hit this issue multiple times but no reliable test case is available at this time.

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

[Andrew Sutherland [:asuth] (he/him)]

TSAN is correctly concerned about https://searchfox.org/mozilla-central/rev/014fe72eaba26dcf6082fb9bbaf208f97a38594e/dom/base/BodyConsumer.cpp#302 setting mShuttingDown in violation of our intended thread access discipline which will be idempotently also set by ShutDownMainThreadConsuming(). We can either remove this redundant setting in the callback or make the variable atomic (although in that case the method ShutDownMainThreadConsuming might want to be the one doing it?).

Medium term, this situation may be mooted by PFetch in bug 1351231.