From: Andrew Morton <akpm@linux-foundation.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
David Howells <dhowells@redhat.com>, Mel Gorman <mgorman@suse.de>,
Johannes Weiner <hannes@cmpxchg.org>,
Vlastimil Babka <vbabka@suse.cz>, Jan Kara <jack@suse.cz>,
Michal Hocko <mhocko@suse.cz>, Hugh Dickins <hughd@google.com>,
Dave Hansen <dave.hansen@intel.com>,
Linux Kernel <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
Linux-FSDevel <linux-fsdevel@vger.kernel.org>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 0/1] ptrace: task_clear_jobctl_trapping()->wake_up_bit() needs mb()
Date: Wed, 21 May 2014 12:18:37 -0700 [thread overview]
Message-ID: <20140521121837.7efc35b2bb455bd45bbc8970@linux-foundation.org> (raw)
In-Reply-To: <20140516135116.GA19210@redhat.com>
On Fri, 16 May 2014 15:51:16 +0200 Oleg Nesterov <oleg@redhat.com> wrote:
> 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().
This is all quite convincing evidence that these interfaces are too
tricky for regular kernel developers to use.
Can we fix them?
One way would be to make the interfaces safe to use and provide
lower-level no-barrier interfaces for use by hot-path code where the
author knows what he/she is doing. And there are probably other ways.
next prev parent reply other threads:[~2014-05-21 19:18 UTC|newest]
Thread overview: 103+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-13 9:45 [PATCH 00/19] Misc page alloc, shmem, mark_page_accessed and page_waitqueue optimisations v3r33 Mel Gorman
2014-05-13 9:45 ` [PATCH 01/19] mm: page_alloc: Do not update zlc unless the zlc is active Mel Gorman
2014-05-13 9:45 ` [PATCH 02/19] mm: page_alloc: Do not treat a zone that cannot be used for dirty pages as "full" Mel Gorman
2014-05-13 9:45 ` [PATCH 03/19] jump_label: Expose the reference count Mel Gorman
2014-05-13 9:45 ` [PATCH 04/19] mm: page_alloc: Use jump labels to avoid checking number_of_cpusets Mel Gorman
2014-05-13 10:58 ` Peter Zijlstra
2014-05-13 12:28 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 05/19] mm: page_alloc: Calculate classzone_idx once from the zonelist ref Mel Gorman
2014-05-13 22:25 ` Andrew Morton
2014-05-14 6:32 ` Mel Gorman
2014-05-14 20:29 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 06/19] mm: page_alloc: Only check the zone id check if pages are buddies Mel Gorman
2014-05-13 9:45 ` [PATCH 07/19] mm: page_alloc: Only check the alloc flags and gfp_mask for dirty once Mel Gorman
2014-05-13 9:45 ` [PATCH 08/19] mm: page_alloc: Take the ALLOC_NO_WATERMARK check out of the fast path Mel Gorman
2014-05-13 9:45 ` [PATCH 09/19] mm: page_alloc: Use word-based accesses for get/set pageblock bitmaps Mel Gorman
2014-05-22 9:24 ` Vlastimil Babka
2014-05-22 18:23 ` Andrew Morton
2014-05-22 18:45 ` Vlastimil Babka
2014-05-13 9:45 ` [PATCH 10/19] mm: page_alloc: Reduce number of times page_to_pfn is called Mel Gorman
2014-05-13 13:27 ` Vlastimil Babka
2014-05-13 14:09 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 11/19] mm: page_alloc: Lookup pageblock migratetype with IRQs enabled during free Mel Gorman
2014-05-13 13:36 ` Vlastimil Babka
2014-05-13 14:23 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 12/19] mm: page_alloc: Use unsigned int for order in more places Mel Gorman
2014-05-13 9:45 ` [PATCH 13/19] mm: page_alloc: Convert hot/cold parameter and immediate callers to bool Mel Gorman
2014-05-13 9:45 ` [PATCH 14/19] mm: shmem: Avoid atomic operation during shmem_getpage_gfp Mel Gorman
2014-05-13 9:45 ` [PATCH 15/19] mm: Do not use atomic operations when releasing pages Mel Gorman
2014-05-13 9:45 ` [PATCH 16/19] mm: Do not use unnecessary atomic operations when adding pages to the LRU Mel Gorman
2014-05-13 9:45 ` [PATCH 17/19] fs: buffer: Do not use unnecessary atomic operations when discarding buffers Mel Gorman
2014-05-13 11:09 ` Peter Zijlstra
2014-05-13 12:50 ` Mel Gorman
2014-05-13 13:49 ` Jan Kara
2014-05-13 14:30 ` Mel Gorman
2014-05-13 14:01 ` Peter Zijlstra
2014-05-13 14:46 ` Mel Gorman
2014-05-13 13:50 ` Jan Kara
2014-05-13 22:29 ` Andrew Morton
2014-05-14 6:12 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 18/19] mm: Non-atomically mark page accessed during page cache allocation where possible Mel Gorman
2014-05-13 14:29 ` Theodore Ts'o
2014-05-20 15:49 ` [PATCH] mm: non-atomically mark page accessed during page cache allocation where possible -fix Mel Gorman
2014-05-20 19:34 ` Andrew Morton
2014-05-21 12:09 ` Mel Gorman
2014-05-21 22:11 ` Andrew Morton
2014-05-22 0:07 ` Mel Gorman
2014-05-22 5:35 ` Prabhakar Lad
2014-05-13 9:45 ` [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath Mel Gorman
2014-05-13 12:53 ` Mel Gorman
2014-05-13 14:17 ` Peter Zijlstra
2014-05-13 15:27 ` Paul E. McKenney
2014-05-13 15:44 ` Peter Zijlstra
2014-05-13 16:14 ` Paul E. McKenney
2014-05-13 18:57 ` Oleg Nesterov
2014-05-13 20:24 ` Paul E. McKenney
2014-05-14 14:25 ` Oleg Nesterov
2014-05-13 18:22 ` Oleg Nesterov
2014-05-13 18:18 ` Oleg Nesterov
2014-05-13 18:24 ` Peter Zijlstra
2014-05-13 18:52 ` Paul E. McKenney
2014-05-13 19:31 ` Oleg Nesterov
2014-05-13 20:32 ` Paul E. McKenney
2014-05-14 16:11 ` Oleg Nesterov
2014-05-14 16:17 ` Peter Zijlstra
2014-05-16 13:51 ` [PATCH 0/1] ptrace: task_clear_jobctl_trapping()->wake_up_bit() needs mb() Oleg Nesterov
2014-05-16 13:51 ` [PATCH 1/1] " Oleg Nesterov
2014-05-21 9:29 ` Peter Zijlstra
2014-05-21 19:19 ` Andrew Morton
2014-05-21 19:18 ` Andrew Morton [this message]
2014-05-14 19:29 ` [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath Oleg Nesterov
2014-05-14 20:53 ` Mel Gorman
2014-05-15 10:48 ` [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v4 Mel Gorman
2014-05-15 13:20 ` Peter Zijlstra
2014-05-15 13:29 ` Peter Zijlstra
2014-05-15 15:34 ` Oleg Nesterov
2014-05-15 15:45 ` Peter Zijlstra
2014-05-15 16:18 ` Mel Gorman
2014-05-15 15:03 ` Oleg Nesterov
2014-05-15 21:24 ` Andrew Morton
2014-05-21 12:15 ` [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v5 Mel Gorman
2014-05-21 13:02 ` Peter Zijlstra
2014-05-21 15:33 ` Mel Gorman
2014-05-21 16:08 ` Peter Zijlstra
2014-05-21 21:26 ` Andrew Morton
2014-05-21 21:33 ` Peter Zijlstra
2014-05-21 21:50 ` Andrew Morton
2014-05-22 0:07 ` Mel Gorman
2014-05-22 7:20 ` Peter Zijlstra
2014-05-22 10:40 ` [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue lookups in unlock_page fastpath v7 Mel Gorman
2014-05-22 10:56 ` Peter Zijlstra
2014-05-22 13:00 ` Mel Gorman
2014-05-22 14:40 ` Mel Gorman
2014-05-22 15:04 ` Peter Zijlstra
2014-05-22 15:36 ` Mel Gorman
2014-05-22 16:58 ` [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue lookups in unlock_page fastpath v8 Mel Gorman
2014-05-22 6:45 ` [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v5 Peter Zijlstra
2014-05-22 8:46 ` Mel Gorman
2014-05-22 17:47 ` Andrew Morton
2014-05-22 19:53 ` Mel Gorman
2014-05-21 23:35 ` Mel Gorman
2014-05-13 16:52 ` [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath Peter Zijlstra
2014-05-14 7:31 ` Mel Gorman
2014-05-19 8:57 ` [PATCH] mm: Avoid unnecessary atomic operations during end_page_writeback Mel Gorman
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