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

[Andrew Morton]

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?