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From: "malteskarupke at fastmail dot fm" <sourceware-bugzilla@sourceware.org>
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Subject: [Bug nptl/25847] pthread_cond_signal failed to wake up
 pthread_cond_wait due to a bug in undoing stealing
Date: Sun, 01 Nov 2020 13:58:23 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D25847

Malte Skarupke <malteskarupke at fastmail dot fm> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |malteskarupke at fastmail =
dot fm

--- Comment #11 from Malte Skarupke <malteskarupke at fastmail dot fm> ---
I blogged about trying to understand this bug using TLA+ here:
https://probablydance.com/2020/10/31/using-tla-in-the-real-world-to-underst=
and-a-glibc-bug/

TLA+ is a tool for doing program verification. I'll make the code available=
 in
any license you want. (I presume LGPL is good?) I think it would be good to
have a TLA+ implementation of the condition variables in glibc.

My solution for this bug based on the investigation would be to broaden the
scope of g_refs, so that it gets incremented in line 411 in
pthread_cond_wait.c, and gets decremented in line 54. That makes it align
exactly with the increment and decrement of wrefs, which might make wrefs
unnecessary. (though I didn't verify that) If it does make wrefs unnecessar=
y,
this fix could both simplify the code, speed it up a little, and also get r=
id
of the "potential stealing" case.

I haven't yet done the work to actually implement and try that change. Let =
me
know if there is interest in me trying that. (or also let me know if it's
unlikely to work because I missed something)

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