linux-kernel - Re: [PATCH v3 2/6] posix-cpu-timers: Use PIDTYPE_TGID to simplify the logic in lookup

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Message-ID: <20200428180316.GA29960@redhat.com>
Date:   Tue, 28 Apr 2020 20:03:17 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Alexey Gladkov <legion@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v3 2/6] posix-cpu-timers: Use PIDTYPE_TGID to simplify
 the logic in lookup_task

On 04/27, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > Eric,
> >
> > I am sick today and can't read the code, but I feel this patch is not
> > right ... please correct me.
>
>
> > So, iiuc when posix_cpu_timer_create() is called and CPUCLOCK_PERTHREAD
> > is false we roughly have
> >
> > 	task = pid_task(pid, PIDTYPE_TGID);			// lookup_task()
> >
> > 	/* WINDOW */
> >
> > 	timer->it.cpu.pid = = get_task_pid(task, PIDTYPE_TGID)	// posix_cpu_timer_create()
> >
> > Now suppose that we race with mt-exec and this "task" is the old leader;
> > it can be release_task()'ed in the WINDOW above and then get_task_pid()
> > will return NULL.
>
> Except it is asking for PIDTYPE_TGID.
>
> task->signal

Ah yes, I knew I missed something...

Oleg.