linux-kernel - Re: [PATCH] It may not be assumed that wake_up(), finish

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: <20090424172845.GB13026@redhat.com>
Date:	Fri, 24 Apr 2009 19:28:45 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, torvalds@...l.org,
	Andrew Morton <akpm@...ux-foundation.org>, serue@...ibm.com,
	viro@...iv.linux.org.uk,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] It may not be assumed that wake_up(), finish_wait()
	and co. imply a memory barrier

On 04/24, David Howells wrote:
>
>  (2) wake_up() interpolates a write memory barrier before clearing the task
>      state - _if_ it wakes anything up - then there's no problem in the waker
>      either.
>
[...snip...]
>
> +A write memory barrier is implied by wake_up() and co. if and only if they wake
> +something up.  The barrier occurs before the task state is cleared, and so sits
> +between the STORE to indicate the event and the STORE to set TASK_RUNNING:

Very minor nit. Perhaps it makes sense to mention that we also need the
barrier before _reading_ the task->state as well. Or not, I am not sure ;)
Just in case...

	event_indicated = 1;
	wake_up_process(event_daemon);

Suppose that "event_indicated = 1" leaks into try_to_wake_up() after we
read p->state. In this case we have

	try_to_wake_up:

		if (!(p->state & state))
			goto out;		// do nothing

		// WINDOW

		event_indicated = 1;		// leaked

In that case the whole

	set_current_state(TASK_UNINTERRUPTIBLE);
	if (event_indicated)
		break;
	schedule();

can happen in the WINDOW above.

But again, this is the real nitpick, and probably just the "implementation
details" which should not be documented.

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/