Re: Re: [PATCH] [PATCH] mm: disable preemption before swapcache_free

[Michal Hocko]

On Wed 25-07-18 10:21:00, Michal Hocko wrote:
> [Please do not top post - thank you]
> [CC Hugh - the original patch was http://lkml.kernel.org/r/2018072514375722198958@xxxxxxxxxxxx]

now for real

> On Wed 25-07-18 15:57:55, zhaowuyun@xxxxxxxxxxxx wrote:
> > That is a BUG we found in mm/vmscan.c at KERNEL VERSION 4.9.82
> 
> The code is quite similar in the current tree as well.
> 
> > Sumary is TASK A (normal priority) doing __remove_mapping page preempted by TASK B (RT priority) doing __read_swap_cache_async,
> > the TASK A preempted before swapcache_free, left SWAP_HAS_CACHE flag in the swap cache,
> > the TASK B which doing __read_swap_cache_async, will not success at swapcache_prepare(entry) because the swap cache was exist, then it will loop forever because it is a RT thread...
> > the spin lock unlocked before swapcache_free, so disable preemption until swapcache_free executed ...
> 
> OK, I see your point now. I have missed the lock is dropped before
> swapcache_free. How can preemption disabling prevent this race to happen
> while the code is preempted by an IRQ?

-- 
Michal Hocko
SUSE Labs