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

From: Michal Hocko
Date: Wed Jul 25 2018 - 04:21:05 EST

[Please do not top post - thank you]
[CC Hugh - the original patch was]

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