linux-kernel - Re: [PATCH v4] posix-timers: Prefer delivery of signals to the current thread

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Message-ID: <20230130164929.GB20353@redhat.com>
Date:   Mon, 30 Jan 2023 17:49:30 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Marco Elver <elver@...gle.com>, linux-kernel@...r.kernel.org,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v4] posix-timers: Prefer delivery of signals to the
 current thread

On 01/30, Dmitry Vyukov wrote:
>
> On Sat, 28 Jan 2023 at 20:56, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > Dmitry,
> >
> > I agree with what you said, just one note...
> >
> > On 01/27, Dmitry Vyukov wrote:
> > >
> > > After this change the test passes quickly (within a second for me).
> >
> > yet perhaps it makes sense to slightly change it? It does
> >
> >         +static void *distribution_thr(void *arg) {
> >         +       while (__atomic_load_n(&remain, __ATOMIC_RELAXED));
> >         +       return NULL;
> >         +}
> >
> > so distribution_thr() eats CPU even after this thread gets a signal and thus
> > (in theory) it can "steal" cpu_timer_fire() from other threads unpredictably
> > long ? How about
> >
> >         -       while (__atomic_load_n(&remain, __ATOMIC_RELAXED));
> >         +       while (__atomic_load_n(&got_signal, __ATOMIC_RELAXED));
> > ?
>
> But why?
> IIUC this makes the test even "weaker". As Thomas notes it's already
> somewhat "weak". And this would make it even "weaker".

Not sure I understand why can this change make the test more weak...

IIUC, _in theory_ the test-case can "hang" forever, since all threads
are running nothing guarentees that every thread will have a chance to
call cpu_timer_fire() and get a signal.

With this change this is not possible, and the test-case will still
verify that all threads must get a signal.

Nevermind,

> So if it passes
> in the current version, I would keep it as is.

OK, I won't insist, please forget.

Oleg.