linux-kernel - Re: [PATCH v3 7/7] locking/rtmutex: Acquire the hb lock via trylock after wait-proxylock.

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Message-ID: <9f75eb59-9b7a-4b49-9081-e6a3cbb00187@kernel.org>
Date: Mon, 15 Jan 2024 12:52:49 +0100
From: Jiri Slaby <jirislaby@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-kernel@...r.kernel.org, boqun.feng@...il.com, bristot@...hat.com,
 bsegall@...gle.com, dietmar.eggemann@....com, jstultz@...gle.com,
 juri.lelli@...hat.com, longman@...hat.com, mgorman@...e.de,
 mingo@...hat.com, rostedt@...dmis.org, swood@...hat.com,
 vincent.guittot@...aro.org, vschneid@...hat.com, will@...nel.org
Subject: Re: [PATCH v3 7/7] locking/rtmutex: Acquire the hb lock via trylock
 after wait-proxylock.

On 15. 01. 24, 12:40, Jiri Slaby wrote:
> On 15. 09. 23, 17:19, Peter Zijlstra wrote:
>> On Fri, Sep 15, 2023 at 02:58:35PM +0200, Thomas Gleixner wrote:
>>
>>> I spent quite some time to convince myself that this is correct. I was
>>> not able to poke a hole into it. So that really should be safe to
>>> do. Famous last words ...
>>
>> IKR :-/
>>
>> Something like so then...
>>
>> ---
>> Subject: futex/pi: Fix recursive rt_mutex waiter state
> 
> So this breaks some random test in APR:
> 
>  From 
> https://build.opensuse.org/package/live_build_log/openSUSE:Factory:Staging:G/apr/standard/x86_64:
> testprocmutex       :  Line 122: child did not terminate with success
> 
> The child in fact terminates on 
> https://github.com/apache/apr/blob/trunk/test/testprocmutex.c#L93:
>                  while ((rv = apr_proc_mutex_timedlock(proc_lock, 1))) {
>                      if (!APR_STATUS_IS_TIMEUP(rv))
>                          exit(1); <----- here
> 
> The test creates 6 children and does some 
> pthread_mutex_timedlock/unlock() repeatedly (200 times) in parallel 
> while sleeping 1 us inside the lock. The timeout is 1 us above. And the 
> test expects all them to fail (to time out). But the time out does not 
> always happen in 6.7 (it's racy, so the failure is semi-random: like 1 
> of 1000 attempts is bad).

This is not precise as I misinterpreted. The test is: either it succeeds 
or times out.

But since the commit, futex() yields 22/EINVAL, i.e. fails.

> If I revert this patch (commit fbeb558b0dd0d), the test works.
> 
> I know, the test could be broken too, but I have no idea, really. The 
> testsuite is sort of hairy and I could not come up with a simple repro.
> 
> Note APR sets up PTHREAD_PROCESS_SHARED, _ROBUST, and _PRIO_INHERIT 
> attrs for the mutex.
> 
> Anyway:
> downstream report: https://bugzilla.suse.com/show_bug.cgi?id=1218801
> APR report: https://bz.apache.org/bugzilla/show_bug.cgi?id=68481
> 
> Any idea if this patch should cause the above (or even is a desired 
> behavior)?
> 
> Thanks.

-- 
js