linux-kernel - Re: [PATCH -mm 3/3] proc: make task

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Date:	Tue, 23 Mar 2010 11:57:07 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 3/3] proc: make task_sig() lockless

On 03/23, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > task_sig() doesn't need ->siglock.
>
> Except that the data returned might then be inconsistent because you don't
> hold a lock as you read the various bits of it.

Yes. From the changelog:

	Of course, this means we read pending/blocked/etc nonatomically,
	but I hope this is OK for fs/proc.

But I don't think the returned data could be "really" inconsistent
from the /bin/ps pov. Yes, it is possible that, say, some signal is
seen as both pending and ignored without ->siglock. Or we can report
user->sigpending != 0 while pending/shpending are empty.

But this looks harmless to me. We never guaranteed /proc/pid/status
can't report the "intermediate" state, and I don't think we can
confuse the user-space.

Do you agree? Or do you think this can make problems ?

Oleg.

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