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

[Paul E. McKenney]

On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote:
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
> 
> > On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" <ying.huang@xxxxxxxxx> wrote:
> >
> >> > Why do we need srcu here? Is it enough with rcu like below?
> >> >
> >> > It might have a bug/room to be optimized about performance/naming.
> >> > I just wanted to show my intention.
> >> 
> >> Yes.  rcu should work too.  But if we use rcu, it may need to be called
> >> several times to make sure the swap device under us doesn't go away, for
> >> example, when checking si->max in __swp_swapcount() and
> >> add_swap_count_continuation().  And I found we need rcu to protect swap
> >> cache radix tree array too.  So I think it may be better to use one
> >> calling to srcu_read_lock/unlock() instead of multiple callings to
> >> rcu_read_lock/unlock().
> >
> > Or use stop_machine() ;)  It's very crude but it sure is simple.  Does
> > anyone have a swapoff-intensive workload?
> 
> Sorry, I don't know how to solve the problem with stop_machine().
> 
> The problem we try to resolved is that, we have a swap entry, but that
> swap entry can become invalid because of swappoff between we check it
> and we use it.  So we need to prevent swapoff to be run between checking
> and using.
> 
> I don't know how to use stop_machine() in swapoff to wait for all users
> of swap entry to finish.  Anyone can help me on this?

You can think of stop_machine() as being sort of like a reader-writer
lock.  The readers can be any section of code with preemption disabled,
and the writer is the function passed to stop_machine().

Users running real-time applications on Linux don't tend to like
stop_machine() much, but perhaps it is nevertheless the right tool
for this particular job.

							Thanx, Paul