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

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Date:   Sun, 26 Apr 2020 19:22:07 +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

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.

That is why I suggested to change lookup_task() to return "struct pid*"
to eliminate the pid -> task -> pid transition.

Apart from the same_thread_group() check for the "thread" case we do not
need task_struct at all, lookup_task() can do

	if (thread) {
		p = pid_task(pid, PIDTYPE_PID);
		if (p && !same_thread_group(p, current))
			pid = NULL;
	} else {
		... gettime check ...

		if (!pid_has_task(pid, PIDTYPE_TGID))
			pid = NULL;
	}

	return pid;

No?

Oleg.