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

[Andrea Parri <andrea.parri@amarulasolutions.com>]

> > +	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?

  Andrea