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Wed, 4 Apr 2018 12:18:21 -0400 Date: Wed, 4 Apr 2018 18:18:18 +0200 From: Peter Zijlstra To: Kirill Tkhai Cc: mingo@redhat.com, stable@vger.kernel.org, linux-kernel@vger.kernel.org, Waiman Long , Boqun Feng Subject: Re: [PATCH] locking/qrwlock: Give priority to readers with irqs disabled to prevent deadlock Message-ID: <20180404161818.GJ4043@hirez.programming.kicks-ass.net> References: <152285543936.4080.5025412328012618076.stgit@localhost.localdomain> <20180404153534.GI4043@hirez.programming.kicks-ass.net> <3c9b507c-c6bb-3e13-2a82-f3504626d016@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3c9b507c-c6bb-3e13-2a82-f3504626d016@virtuozzo.com> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Wed, Apr 04, 2018 at 06:51:08PM +0300, Kirill Tkhai wrote: > On 04.04.2018 18:35, Peter Zijlstra wrote: > > On Wed, Apr 04, 2018 at 06:24:39PM +0300, Kirill Tkhai wrote: > >> The following situation leads to deadlock: > >> > >> [task 1] [task 2] [task 3] > >> kill_fasync() mm_update_next_owner() copy_process() > >> spin_lock_irqsave(&fa->fa_lock) read_lock(&tasklist_lock) write_lock_irq(&tasklist_lock) > >> send_sigio() ... > >> read_lock(&fown->lock) kill_fasync() ... > >> read_lock(&tasklist_lock) spin_lock_irqsave(&fa->fa_lock) ... > >> > >> Task 1 can't acquire read locked tasklist_lock, since there is > >> already task 3 expressed its wish to take the lock exclusive. > >> Task 2 holds the read locked lock, but it can't take the spin lock. > >> > >> The patch makes queued_read_lock_slowpath() to give task 1 the same > >> priority as it was an interrupt handler, and to take the lock > > > > That re-introduces starvation scenarios. And the above looks like a > > proper deadlock that should be sorted by fixing the locking order. > > We can move tasklist_lock out of send_sigio(), but I'm not sure > it's possible for read_lock(&fown->lock). So the scenario is: CPU0 CPU1 CPU2 spin_lock_irqsave(&fa->fa_lock); read_lock(&tasklist_lock) write_lock_irq(&tasklist_lock) read_lock(&tasklist_lock) spin_lock_irqsave(&fa->fa_lock); Right? (where the row now signifies time) That doesn't seem to include fown->lock, you're saying it has an identical issue? > Is there another solution? Is there reliable way to iterate do_each_pid_task() > with rcu_read_lock()? Depends on what you call reliable :-), Yes you can use do_each_pid_task() with RCU, but as always you're prone to see tasks that are dead and miss tasks that just came in. If that is sufficient for the signal muck, dunno :/ Typically signals use sighand lock, not tasklist_lock.