linux-kernel - Re: [PATCH v2 20/22] selftests: futex: Test sys_futex

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Message-ID: <e73b9309-b575-37b4-519c-d326b764cf38@linaro.org>
Date:   Tue, 9 Nov 2021 10:00:59 -0300
From:   Adhemerval Zanella <adhemerval.zanella@...aro.org>
To:     André Almeida <andrealmeid@...labora.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     Davidlohr Bueso <dave@...olabs.net>, libc-alpha@...rceware.org,
        Peter Zijlstra <peterz@...radead.org>,
        linux-api@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, mtk.manpages@...il.com,
        Darren Hart <dvhart@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, kernel@...labora.com,
        krisman@...labora.com
Subject: Re: [PATCH v2 20/22] selftests: futex: Test sys_futex_waitv() timeout



On 09/11/2021 09:52, André Almeida via Libc-alpha wrote:
> Hi Vasily,
> 
> Às 08:18 de 09/11/21, Vasily Gorbik escreveu:
>> On Thu, Sep 23, 2021 at 02:11:09PM -0300, André Almeida wrote:
>>> Test if the futex_waitv timeout is working as expected, using the
>>> supported clockid options.
>>
>>> +	/* futex_waitv with CLOCK_MONOTONIC */
>>> +	if (futex_get_abs_timeout(CLOCK_MONOTONIC, &to, timeout_ns))
>>> +		return RET_FAIL;
>>> +	res = futex_waitv(&waitv, 1, 0, &to, CLOCK_MONOTONIC);
>>> +	test_timeout(res, &ret, "futex_waitv monotonic", ETIMEDOUT);
>>> +
>>> +	/* futex_waitv with CLOCK_REALTIME */
>>> +	if (futex_get_abs_timeout(CLOCK_REALTIME, &to, timeout_ns))
>>> +		return RET_FAIL;
>>> +	res = futex_waitv(&waitv, 1, 0, &to, CLOCK_REALTIME);
>>> +	test_timeout(res, &ret, "futex_waitv realtime", ETIMEDOUT);
>>
>> Hi André,
>>
>> when built with -m32 and run as compat this two futex_waitv calls hang
>> on x86 and s390 (noticed while wiring up futex_waitv). The rest of the
>> futex selftests pass. This suggests some common compat issue? Any ideas?
> 
> The issue is that futex_waitv() only accepts struct timespec that uses
> 64bit members. When using -m32, glibc will give you a 32bit timespec,
> thus the timeout won't wort. Someday glibc will provide 64bit timespec
> to 32bit userspace, given that this is affected by y2038 bug.

We do since glibc 2.34, but you need to opt-in by defining -D_TIME_SIZE=64.
The default might change in a future release, so hopefully we will have
both LFS and 64-bit as the default ABI.

> 
> In previous submissions I added a workaround for that in the
> selftest[0]. Search for "Y2038 section for 32-bit applications" in that
> link. I'll submit something like that for futex_waitv() timeout test.
> 
> [0]
> https://lore.kernel.org/lkml/20210709001328.329716-6-andrealmeid@collabora.com/
>