From: Daniel Jordan <daniel.m.jordan@oracle.com>
To: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: "Daniel Jordan" <daniel.m.jordan@oracle.com>,
"Huang, Ying" <ying.huang@intel.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
"Hugh Dickins" <hughd@google.com>,
"Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
"Minchan Kim" <minchan@kernel.org>,
"Johannes Weiner" <hannes@cmpxchg.org>,
"Tim Chen" <tim.c.chen@linux.intel.com>,
"Mel Gorman" <mgorman@techsingularity.net>,
"Jérôme Glisse" <jglisse@redhat.com>,
"Michal Hocko" <mhocko@suse.com>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"David Rientjes" <rientjes@google.com>,
"Rik van Riel" <riel@redhat.com>, "Jan Kara" <jack@suse.cz>,
"Dave Jiang" <dave.jiang@intel.com>
Subject: Re: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations
Date: Tue, 12 Feb 2019 15:06:52 -0500 [thread overview]
Message-ID: <20190212200652.bnkiqx6t2dg7ecp5@ca-dmjordan1.us.oracle.com> (raw)
In-Reply-To: <20190212032121.GA2723@andrea>
On Tue, Feb 12, 2019 at 04:21:21AM +0100, Andrea Parri wrote:
> > > + if (!si)
> > > + goto bad_nofile;
> > > +
> > > + preempt_disable();
> > > + if (!(si->flags & SWP_VALID))
> > > + goto unlock_out;
> >
> > After Hugh alluded to barriers, it seems the read of SWP_VALID could be
> > reordered with the write in preempt_disable at runtime. Without smp_mb()
> > between the two, couldn't this happen, however unlikely a race it is?
> >
> > CPU0 CPU1
> >
> > __swap_duplicate()
> > get_swap_device()
> > // sees SWP_VALID set
> > swapoff
> > p->flags &= ~SWP_VALID;
> > spin_unlock(&p->lock); // pair w/ smp_mb
> > ...
> > stop_machine(...)
> > p->swap_map = NULL;
> > preempt_disable()
> > read NULL p->swap_map
>
>
> I don't think that that smp_mb() is necessary. I elaborate:
>
> An important piece of information, I think, that is missing in the
> diagram above is the stopper thread which executes the work queued
> by stop_machine(). We have two cases to consider, that is,
>
> 1) the stopper is "executed before" the preempt-disable section
>
> CPU0
>
> cpu_stopper_thread()
> ...
> preempt_disable()
> ...
> preempt_enable()
>
> 2) the stopper is "executed after" the preempt-disable section
>
> CPU0
>
> preempt_disable()
> ...
> preempt_enable()
> ...
> cpu_stopper_thread()
>
> Notice that the reads from p->flags and p->swap_map in CPU0 cannot
> cross cpu_stopper_thread(). The claim is that CPU0 sees SWP_VALID
> unset in (1) and that it sees a non-NULL p->swap_map in (2).
>
> I consider the two cases separately:
>
> 1) CPU1 unsets SPW_VALID, it locks the stopper's lock, and it
> queues the stopper work; CPU0 locks the stopper's lock, it
> dequeues this work, and it reads from p->flags.
>
> Diagrammatically, we have the following MP-like pattern:
>
> CPU0 CPU1
>
> lock(stopper->lock) p->flags &= ~SPW_VALID
> get @work lock(stopper->lock)
> unlock(stopper->lock) add @work
> reads p->flags unlock(stopper->lock)
>
> where CPU0 must see SPW_VALID unset (if CPU0 sees the work
> added by CPU1).
>
> 2) CPU0 reads from p->swap_map, it locks the completion lock,
> and it signals completion; CPU1 locks the completion lock,
> it checks for completion, and it writes to p->swap_map.
>
> (If CPU0 doesn't signal the completion, or CPU1 doesn't see
> the completion, then CPU1 will have to iterate the read and
> to postpone the control-dependent write to p->swap_map.)
>
> Diagrammatically, we have the following LB-like pattern:
>
> CPU0 CPU1
>
> reads p->swap_map lock(completion)
> lock(completion) read completion->done
> completion->done++ unlock(completion)
> unlock(completion) p->swap_map = NULL
>
> where CPU0 must see a non-NULL p->swap_map if CPU1 sees the
> completion from CPU0.
>
> Does this make sense?
Yes, thanks for this, Andrea! Good that smp_mb isn't needed.
next prev parent reply other threads:[~2019-02-12 20:07 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-11 8:38 [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations Huang, Ying
2019-02-11 19:06 ` Daniel Jordan
2019-02-12 3:21 ` Andrea Parri
[not found] ` <874l99ld05.fsf@yhuang-dev.intel.com>
2019-02-12 17:58 ` Tim Chen
2019-02-13 3:23 ` Huang, Ying
2019-02-12 20:06 ` Daniel Jordan [this message]
2019-02-12 6:40 ` Huang, Ying
2019-02-12 10:13 ` Andrea Parri
2019-02-14 2:38 ` Andrea Arcangeli
[not found] ` <87k1i2oks6.fsf@yhuang-dev.intel.com>
2019-02-14 21:47 ` Andrea Arcangeli
2019-02-15 7:50 ` Huang, Ying
2019-02-14 14:33 ` Michal Hocko
2019-02-14 20:30 ` Andrew Morton
2019-02-14 21:22 ` Andrea Arcangeli
[not found] ` <871s49bkaz.fsf@yhuang-dev.intel.com>
2019-02-15 13:11 ` Michal Hocko
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