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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)
Date: Tue, 12 May 2020 17:09:37 +0200	[thread overview]
Message-ID: <20200512150936.GA28621@redhat.com> (raw)
In-Reply-To: <20200512000353.23653-2-dave@stgolabs.net>

On 05/11, Davidlohr Bueso wrote:
>
> Currently the tasklist_lock is shared mainly in order to observe
> the list atomically for the PRIO_PGRP and PRIO_USER cases, as
> the actual lookups are already rcu-safe,

not really...

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.

> (ii) exit (deletion), this window is small but if a task is
> deleted with the highest nice and it is not observed this would
> cause a change in return semantics. To further reduce the window
> we ignore any tasks that are PF_EXITING in the 'old' version of
> the list.

can't understand...

could you explain in details why do you think this PF_EXITING check
makes any sense?

Oleg.


  reply	other threads:[~2020-05-12 15:09 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-12  0:03 [PATCH -next v2 0/2] kernel/sys: reduce tasklist_lock usage get/set priorities Davidlohr Bueso
2020-05-12  0:03 ` [PATCH 1/2] kernel/sys: only rely on rcu for getpriority(2) Davidlohr Bueso
2020-05-12 15:09   ` Oleg Nesterov [this message]
2020-05-12 16:09     ` Davidlohr Bueso
2020-05-12 16:41       ` Oleg Nesterov
2020-05-12 16:58         ` Davidlohr Bueso
2020-05-12 18:16           ` Oleg Nesterov
2020-05-12  0:03 ` [PATCH 2/2] kernel/sys: do not grab tasklist_lock for sys_setpriority(PRIO_PROCESS) Davidlohr Bueso
2020-05-12 16:10   ` Oleg Nesterov

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