linux-kernel - futex_cmpxchg_enabled not set in futex

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Date:	Sun, 29 Nov 2009 11:46:43 -0600 (CST)
From:	Joseph Parmelee <jparmele@...dbear.com>
To:	linux-kernel@...r.kernel.org
Subject: futex_cmpxchg_enabled not set in futex_init on pentium3

Greetings all:

Sometime between 2.6.28.6 and 2.6.31.5 a regression (feature?) in the futex
system now causes futex test failures on glibc-2.9 which where not present
before.  That is, recompiling the binaries of glibc-2.9 and rerunning its
test suite now produces futex errors that passed previously.  The problem
appears now with glibc-2.9 compiled with either gcc-4.1.2 or gcc-4.4.2, and
with glibc-2.11 compiled with gcc-4.4.2, which is what I am currently
running on this machine, failures and all.

The system under discussion is a uniprocessor pentium3 with an AMI BIOS. 
Full details available on request should that prove necessary.

I have tracked the test failures down to the fact that futex_cmpxchg_enabled
is not set because the test in futex_init now "fails" (actually succeeds). 
This appears to be happening because the expected page fault intentionally
provoked by a null dereference appears to be working now in kernel mode. 
This *may* (rank speculation) be associated with the AMI BIOS low-memory
corruption protection added sometime during this gap, and which is activated
on this machine.

Before I muck any further with this, especially involving the quite tricky
futex mess, I would appreciate some insight into the idea behind the test in
futex_init.  I don't understand why you would bother to invoke a fault in
what is apparently a test to determine if the cmpxchg instruction works. 
The fault is supposed to occur as a result of a null dereference that takes
place *before* the cmpxchg instruction is even executed.  If you want to
test that cmpxchg works, why not just make a little test in futex_init that
uses it and fails (not succeeds) if it doesn't behave as expected, or if
there is a fault of some kind (like illegal instruction)?  Or is the fact
that we don't get a fault the whole point here?

Regards,

Joseph

Please CC me directly as I am no longer subscribed to the list.
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