1607224 - ThreadSanitizer: data race [@ FileSizeInK] vs. [@ mozilla::net::CacheFileIOManager::CreateFile] in telemetry code

Status: RESOLVED FIXED

(Core :: Networking: Cache, defect, P2)

Description

[Christian Holler (:decoder)]

The attached crash information was detected while running CI tests with ThreadSanitizer on mozilla-central revision c0a6eb95b65c.

We are racing on mFileSize which sounds bad, but it looks like it might be less of a problem because the reading code goes through nsHttpChannel::ReportNetVSCacheTelemetry. Worst case is probably that this code reports a wrong file size in telemetry.

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

[Andy Grover [:grover]]

Michal wdyt?

Comment 3

[Michal Novotny [:michal]]

Attachment: Bug 1607224 - Data race on CacheFileHandle::mFileSize

This patch fixes data race on CacheFileHandle::mFileSize when accessed on main thread by nsHttpChannel::ReportNetVSCacheTelemetry(). All other usages are on cache I/O thread.

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/c5b764728ebc
Data race on CacheFileHandle::mFileSize r=valentin

Comment 5

[Stefan Hindli [:stefan_hindli]]

https://hg.mozilla.org/mozilla-central/rev/c5b764728ebc