Re: kernel bug: futex_wait hang

[Andrew Morton]

Jamie Lokier <jamie@xxxxxxxxxxxxx> wrote:
>
> Andrew Morton wrote:
> > iirc we ended up deciding that the futex problems around that time were due
> > to userspace problems (a version of libc).  But then, there's no discussion
> > around Seto's patch and it didn't get applied.  So I don't know what
> > happened to that work - it's all a bit mysterious.
> 
> It can be fixed _either_ in Glibc, or by changing the kernel.
> 
> That problem is caused by differing assumptions between Glibc and the
> kernel about subtle futex semantics.  Which goes to show they are
> really clever, or something.
> 
> I provided pseudo-code for the Glibc fix, but not an actual patch
> because NPTL is quite complicated and I wanted to know the Glibc
> people were interested, but apparently they were too busy at the time
> - benchmarks would have made sense for such a patch.
> 
> Scott Snyder started fixing part of Glibc, and that did fix his
> instance of this problem so we know the approach works.  But a full
> patch for Glibc was not prepared.
> 
> The most recent messages under "Futex queue_me/get_user ordering",
> with a patch from Jakub Jelinek will fix this problem by changing the
> kernel.  Yes, you should apply Jakub's most recent patch, message-ID
> "<20050318165326.GB32746@xxxxxxxxxxxxxxxxxxxxxxxx>".
> 
> I have not tested the patch, but it looks convincing.

OK, thanks.  Lee && Paul, that's at
ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.12-rc1/2.6.12-rc1-mm1/broken-out/futex-queue_me-get_user-ordering-fix.patch


> I argued for fixing Glibc on the grounds that the changed kernel
> behaviour, or more exactly having Glibc depend on it, loses a certain
> semantic property useful for unusual operations on multiple futexes at
> the same time.  But I appear to have lost the argument, and Jakub's
> latest patch does clean up some cruft quite nicely, with no expected
> performance hit.

Futexes were initially so simple.
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