linux-kernel - Re: [RFC 1/2] page-flags: Make page lock operation atomic

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Date:   Tue, 12 Feb 2019 08:45:35 +0100
From:   Jan Kara <jack@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jan Kara <jack@...e.cz>,
        Linux Upstream <linux.upstream@...plus.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Chintan Pandya <chintan.pandya@...plus.com>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "mawilcox@...rosoft.com" <mawilcox@...rosoft.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC 1/2] page-flags: Make page lock operation atomic

On Mon 11-02-19 09:56:53, Matthew Wilcox wrote:
> On Mon, Feb 11, 2019 at 06:48:46PM +0100, Jan Kara wrote:
> > On Mon 11-02-19 13:59:24, Linux Upstream wrote:
> > > > 
> > > >> Signed-off-by: Chintan Pandya <chintan.pandya@...plus.com>
> > > > 
> > > > NAK.
> > > > 
> > > > This is bound to regress some stuff. Now agreed that using non-atomic
> > > > ops is tricky, but many are in places where we 'know' there can't be
> > > > concurrency.
> > > > 
> > > > If you can show any single one is wrong, we can fix that one, but we're
> > > > not going to blanket remove all this just because.
> > > 
> > > Not quite familiar with below stack but from crash dump, found that this
> > > was another stack running on some other CPU at the same time which also
> > > updates page cache lru and manipulate locks.
> > > 
> > > [84415.344577] [20190123_21:27:50.786264]@1 preempt_count_add+0xdc/0x184
> > > [84415.344588] [20190123_21:27:50.786276]@1 workingset_refault+0xdc/0x268
> > > [84415.344600] [20190123_21:27:50.786288]@1 add_to_page_cache_lru+0x84/0x11c
> > > [84415.344612] [20190123_21:27:50.786301]@1 ext4_mpage_readpages+0x178/0x714
> > > [84415.344625] [20190123_21:27:50.786313]@1 ext4_readpages+0x50/0x60
> > > [84415.344636] [20190123_21:27:50.786324]@1 
> > > __do_page_cache_readahead+0x16c/0x280
> > > [84415.344646] [20190123_21:27:50.786334]@1 filemap_fault+0x41c/0x588
> > > [84415.344655] [20190123_21:27:50.786343]@1 ext4_filemap_fault+0x34/0x50
> > > [84415.344664] [20190123_21:27:50.786353]@1 __do_fault+0x28/0x88
> > > 
> > > Not entirely sure if it's racing with the crashing stack or it's simply
> > > overrides the the bit set by case 2 (mentioned in 0/2).
> > 
> > So this is interesting. Looking at __add_to_page_cache_locked() nothing
> > seems to prevent __SetPageLocked(page) in add_to_page_cache_lru() to get
> > reordered into __add_to_page_cache_locked() after page is actually added to
> > the xarray. So that one particular instance might benefit from atomic
> > SetPageLocked or a barrier somewhere between __SetPageLocked() and the
> > actual addition of entry into the xarray.
> 
> There's a write barrier when you add something to the XArray, by virtue
> of the call to rcu_assign_pointer().

OK, I've missed rcu_assign_pointer(). Thanks for correction... but...
rcu_assign_pointer() is __smp_store_release(&p, v) and that on x86 seems to
be:

        barrier();                                                      \
        WRITE_ONCE(*p, v);                                              \

which seems to provide a compiler barrier but not an SMP barrier? So is x86
store ordering strong enough to make writes appear in the right order? So far
I didn't think so... What am I missing?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR