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SubjectRe: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations
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.

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