linux-kernel - Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in __wait_on_bit

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20160826124756.GC28904@redhat.com>
Date:   Fri, 26 Aug 2016 14:47:56 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Al Viro <viro@...IV.linux.org.uk>,
        Bart Van Assche <bvanassche@....org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Neil Brown <neilb@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in
 __wait_on_bit_lock()

On 08/26, Oleg Nesterov wrote:
>
> We do not need anything tricky to avoid the race, we can just call
> finish_wait() if action() fails. test_and_set_bit() implies mb() so
> the lockless list_empty_careful() case is fine, we can not miss the
> condition if we race with unlock_page().

To simplify the review see the code with the patch applied:


int __sched
__wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q,
			wait_bit_action_f *action, unsigned mode)
{
	int ret = 0;

	for (;;) {
		prepare_to_wait_exclusive(wq, &q->wait, mode);
		if (test_bit(q->key.bit_nr, q->key.flags)) {
			ret = action(&q->key, mode);
			/*
			 * Ensure that clear_bit() + wake_up() right after
			 * test_and_set_bit() below can't see us; it should
			 * wake up another exclusive waiter if we fail.
			 */
			if (ret)
				finish_wait(wq, &q->wait);
		}
		if (!test_and_set_bit(q->key.bit_nr, q->key.flags)) {
			if (!ret)
				finish_wait(wq, &q->wait);
			return 0;
		} else if (ret) {
			return ret;
		}
	}
}