linux-kernel - [PATCH 0/1] ptrace: task_clear_jobctl_trapping()->wake_up

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Message-ID: <20140516135116.GA19210@redhat.com>
Date:	Fri, 16 May 2014 15:51:16 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>
Cc:	Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 0/1] ptrace: task_clear_jobctl_trapping()->wake_up_bit()
	needs mb()

On 05/14, Peter Zijlstra wrote:
>
> On Wed, May 14, 2014 at 06:11:52PM +0200, Oleg Nesterov wrote:
> >
> > I mean, we do not need mb() before __wake_up(). We need it only because
> > __wake_up_bit() checks waitqueue_active().
> >
> >
> > And at least
> >
> > 	fs/cachefiles/namei.c:cachefiles_delete_object()
> > 	fs/block_dev.c:blkdev_get()
> > 	kernel/signal.c:task_clear_jobctl_trapping()
> > 	security/keys/gc.c:key_garbage_collector()
> >
> > look obviously wrong.
> >
> > I would be happy to send the fix, but do I need to split it per-file?
> > Given that it is trivial, perhaps I can send a single patch?
>
> Since its all the same issue a single patch would be fine I think.

Actually blkdev_get() is fine, it relies on bdev_lock. But bd_prepare_to_claim()
is the good example of abusing bit_waitqueue(). Not only it is itself suboptimal,
this doesn't allow to optimize wake_up_bit-like paths. And there are more, say,
inode_sleep_on_writeback(). Plus we have wait_on_atomic_t() which I think should
be generalized or even unified with the regular wait_on_bit(). Perhaps I'll try
to do this later, fortunately the recent patch from Neil greatly reduced the
number of "action" functions.

As for cachefiles_walk_to_object() and key_garbage_collector(), it still seems
to me they need smp_mb__after_clear_bit() but I'll leave this to David, I am
not comfortable to change the code I absolutely do not understand. In particular,
I fail to understand why key_garbage_collector() does smp_mb() before clear_bit().
At least it could be smp_mb__before_clear_bit().

So let me send a trivial patch which only changes task_clear_jobctl_trapping().

Oleg.

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