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

[Huang\, Ying]

Hi, Pual,

"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> writes:

> 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.

Thanks a lot for explanation!  Now I understand this.

Another question, for this specific problem, I think both stop_machine()
based solution and rcu_read_lock/unlock() + synchronize_rcu() based
solution work.  If so, what is the difference between them?  I guess rcu
based solution will be a little better for real-time applications?  So
what is the advantage of stop_machine() based solution?

Best Regards,
Huang, Ying