linux-kernel - Re: [PATCH v3] wait: prevent waiter starvation 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: <20090121143602.GA16584@redhat.com>
Date:	Wed, 21 Jan 2009 15:36:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Matthew Wilcox <matthew@....cx>,
	Chuck Lever <cel@...i.umich.edu>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3] wait: prevent waiter starvation in
	__wait_on_bit_lock

On 01/20, Johannes Weiner wrote:
>
> > But, more importantly, I'm afraid we can also have the false negative,
> > this "if (!test_bit())" test lacks the barriers. This can't happen with
> > sync_page_killable() because it always calls schedule(). But let's
> > suppose we modify it to check signal_pending() first:
> >
> > 	static int sync_page_killable(void *word)
> > 	{
> > 		if (fatal_signal_pending(current))
> > 			return -EINTR;
> > 		return sync_page(word);
> > 	}
> >
> > It is still correct, but unless I missed something now __wait_on_bit_lock()
> > has problems again.
>
> Hm, this would require the lock bit to be set without someone else
> doing the wakeup.  How could this happen?
>
> I could think of wake_up_page() happening BEFORE clear_bit_unlock()
> and we have to be on the front of the waitqueue.  Then we are already
> running, the wake up is a nop, the !test_bit() is false and noone
> wakes up the next real contender.
>
> But the wake up side uses a smp barrier after clearing the bit, so if
> the bit is not cleared we can expect a wake up, no?

Yes we have the barriers on the "wakeup", but this doesn't mean the
woken task must see the result of clear_bit() (unless it was really
unscheduled of course).

> Or do we still need a read-side barrier before the test bit?

Even this can't help afaics.

Because the the whole clear_bit + wakeup sequence can happen after
the "if (!test_bit()) check and before finish_wait(). Please note
that from the waker's pov we are sleeping in TASK_KILLABLE state,
it will wake up us if we are at the front of the waitqueue.

(to clarify, I am talking about the imaginary sync_page_killable()
 above).

Oleg.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/