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From: Thomas Gleixner <tglx@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Hyunwoo Kim <imv4bel@gmail.com>, Frederic Weisbecker
 <frederic@kernel.org>, Anna-Maria Behnsen <anna-maria@linutronix.de>, Kees
 Cook <kees@kernel.org>, Christian Brauner <brauner@kernel.org>, Peter
 Zijlstra <peterz@infradead.org>, John Stultz <jstultz@google.com>, Ingo
 Molnar <mingo@kernel.org>, Alexander Viro <viro@zeniv.linux.org.uk>, Jan
 Kara <jack@suse.cz>, "Eric W. Biederman" <ebiederm@xmission.com>,
 linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
 linux-mm@kvack.org
Subject: Re: [PATCH v2] posix-cpu-timers: Dequeue per-thread timers before
 exchange_tids()
In-Reply-To: <87qzjdauv4.ffs@fw13>
References: <ao7Q8miiuLAPVnWv@v4bel> <ao7iQ_wOLghOvTsq@redhat.com>
 <875x0qclcx.ffs@fw13> <apXGzcA9BmrbrQZv@redhat.com> <87qzjdauv4.ffs@fw13>
Date: Tue, 01 Sep 2026 17:11:45 +0200
Message-ID: <87o6ehatvy.ffs@fw13>
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On Tue, Sep 01 2026 at 16:50, Thomas Gleixner wrote:
> On Mon, Aug 31 2026 at 20:24, Oleg Nesterov wrote:
>> On 08/31, Thomas Gleixner wrote:
>>>
>>> On Wed, Aug 26 2026 at 14:55, Oleg Nesterov wrote:
>>> > Can we move posix_cpu_timers_exit() or the whole CONFIG_POSIX_TIMERS
>>> > sequence in begin_new_exec() up, right after de_thread() before the
>>> > "if (retval)" check?
>>>
>>> Yes, but that does not buy us anything.
>>
>> I didn't mean this makes sense, I just tried to understand the Hyunwoo's
>> fix, in particular the placement of additional posix_cpu_timers_exit().
>
> Fair enough. But as I was staring at it some more, I thought more about
> it. Yes, we can do:
>
>      ret = de_thread()
>      if (ret)
>      	goto out;
>
>      mop_up_posix_timers();
>
> de_thread() only returns failure _before_ the TID swap, so that's safe
> and spares sprinkling this extra invocation into de_thread().
>
> What a mess.

I'm still thinking that an indicator for a task doing exec(), which is
set _before_ de_thread() and removed afterwards would be more sensible
than relying on subtle ordering constraints.

Along with the nonsense to keep timers queued on a task until
release_task(). There is no real benefit from keeping them around at
after PF_EXITING is set. Sure someone might argue that the precious CPU
cycles consumed in do_exit() afterwards won't be accounted, but
seriously?

Thanks,

        tglx