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

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Message-ID: <11571.1269419842@redhat.com>
Date:	Wed, 24 Mar 2010 08:37:22 +0000
From:	David Howells <dhowells@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	dhowells@...hat.com, 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

Oleg Nesterov <oleg@...hat.com> wrote:

> > 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.

Ah, yes.  I read that as you meant how procfs accessed the actual data
structures, not how the user accessed procfs.  It might be worth clarifying
that.

> 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 ?

I don't know of anything this will affect adversely.  In fact, I'm not sure
there was a guarantee that it would be atomic anyway.

So as far as I'm concerned, you can add:

Acked-by: David Howells <dhowells@...hat.com>

> > Probably we can change do_task_stat() to avod ->siglock too, except
> > we can't get tty_nr lockless.

Btw, avoid has an 'i' in it... :-)
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