Messages in this thread | | | Date | Wed, 14 May 2014 18:11:52 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath |
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The subsequent discussion was "off-topic", and it seems that the patch itself needs a bit more discussion,
On 05/13, Peter Zijlstra wrote: > > On Tue, May 13, 2014 at 01:53:13PM +0100, Mel Gorman wrote: > > On Tue, May 13, 2014 at 10:45:50AM +0100, Mel Gorman wrote: > > > void unlock_page(struct page *page) > > > { > > > + wait_queue_head_t *wqh = clear_page_waiters(page); > > > + > > > VM_BUG_ON_PAGE(!PageLocked(page), page); > > > + > > > + /* > > > + * No additional barrier needed due to clear_bit_unlock barriering all updates > > > + * before waking waiters > > > + */ > > > clear_bit_unlock(PG_locked, &page->flags); > > > - smp_mb__after_clear_bit(); > > > - wake_up_page(page, PG_locked); > > > > This is wrong.
Yes,
> > The smp_mb__after_clear_bit() is still required to ensure > > that the cleared bit is visible before the wakeup on all architectures.
But note that "the cleared bit is visible before the wakeup" is confusing. 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?
Oleg.
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