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Date:   Tue, 12 May 2020 18:41:31 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     akpm@linux-foundation.org, peterz@infradead.org,
        paulmck@kernel.org, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH 1/2] kernel/sys: only rely on rcu for getpriority(2)
Message-ID: <20200512164130.GC28621@redhat.com>
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 <20200512000353.23653-2-dave@stgolabs.net>
 <20200512150936.GA28621@redhat.com>
 <20200512160915.n3plwrwwrlpfqyrs@linux-p48b>
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On 05/12, Davidlohr Bueso wrote:
>
> On Tue, 12 May 2020, Oleg Nesterov wrote:
>
> >do_each_pid_task(PIDTYPE_PGID) can race with change_pid(PIDTYPE_PGID)
> >which moves the task from one hlist to another. Yes, it is safe in
> >that task_struct can't go away. But still this is not right because
> >do_each_pid_task() can scan the wrong (2nd) hlist.
>
> Hmm I didn't think about this case, I guess this is also busted in
> ioprio_get(2) then.

agreed...

> >
> >could you explain in details why do you think this PF_EXITING check
> >makes any sense?
>
> My logic was that if the task with the highest prio exited while we
> were iterating the list, it would not be necessarily seen with rcu
> and the syscall would return the highest prio of a task that exited;
> and checking against PF_EXITING was a way to ignore such scenarios
> as we were going to race with it anyway.

Sorry, still can't understand. The PF_EXITING flag is not protected by
tasklist_lock or rcu_lock.


OK, if nothing else. Suppose that a prgp has a single process P, this
proces has already exited but its parent didn't do wait().

Currently getpriority() returns task_nice(P). With the PF_EXITING check
it will return -ESRCH. Hmm?

Oleg.