623362 - nanojit: avoid harmless race on nHints[]

Status: RESOLVED FIXED

(Core Graveyard :: Nanojit, defect)

Description

[Nicholas Nethercote [inactive]]

nHints[] is a static variable.  It has a harmless race that Helgrind detected.  It's written by Assembler::nInit() and read by Assembler::hint(), and there's no locking.  It's harmless because the values written by Assembler::nInit() are always the same.  But it's worth fixing just to avoid the warning.

An easy fix would be to make it non-static.  It's one int32 per opcode, so the extra space is minimal, something like 512 bytes.  It would need to be manually zeroed, as well.

Comment 1

[Nicholas Nethercote [inactive]]

Attachment: patch

Comment 2

[Julian Seward [:jseward]]

Comment on attachment 517335 [details] [diff] [review]
patch

Yes, that appears to fix the race -- is no longer detectable.
I only checked on x64-linux, although I know this race also 
exists on arm and probably x86 too.  From the look of it this
patch fixes all targets, yes?

Comment 3

[Nicholas Nethercote [inactive]]

(In reply to comment #2)
> From the look of it this patch fixes all targets, yes?

Yes.

Comment 4

[Nicholas Nethercote [inactive]]

http://hg.mozilla.org/projects/nanojit-central/rev/8ab31790e8a1

Comment 5

[Nicholas Nethercote [inactive]]

http://hg.mozilla.org/tracemonkey/rev/1c206a26f9ff

Comment 6

[Tamarin Bot]

changeset: 6225:5b8d188e84a2
user:      Nicholas Nethercote <[e-mail]>
summary:   Bug 623362 - nanojit: avoid harmless race on nHints[].  r=jseward.

http://hg.mozilla.org/tamarin-redux/rev/5b8d188e84a2