Re: [PATCH] zsmalloc: use workqueue to destroy pool in zpool callback

[Yu Zhao]

On Thu, Mar 31, 2016 at 05:46:39PM +0900, Sergey Senozhatsky wrote:
> On (03/30/16 08:59), Minchan Kim wrote:
> > On Tue, Mar 29, 2016 at 03:02:57PM -0700, Yu Zhao wrote:
> > > zs_destroy_pool() might sleep so it shouldn't be used in zpool
> > > destroy callback which can be invoked in softirq context when
> > > zsmalloc is configured to work with zswap.
> > 
> > I think it's a limitation of zswap design, not zsmalloc.
> > Could you handle it in zswap?
> 
> agree. hm, looking at this backtrace
> 
> >   [<ffffffffaea0224b>] mutex_lock+0x1b/0x2f
> >   [<ffffffffaebca4f0>] kmem_cache_destroy+0x50/0x130
> >   [<ffffffffaec10405>] zs_destroy_pool+0x85/0xe0
> >   [<ffffffffaec1046e>] zs_zpool_destroy+0xe/0x10
> >   [<ffffffffaec101a4>] zpool_destroy_pool+0x54/0x70
> >   [<ffffffffaebedac2>] __zswap_pool_release+0x62/0x90
> >   [<ffffffffaeb1037e>] rcu_process_callbacks+0x22e/0x640
> >   [<ffffffffaeb15a3e>] ? run_timer_softirq+0x3e/0x280
> >   [<ffffffffaeabe13b>] __do_softirq+0xcb/0x250
> >   [<ffffffffaeabe4dc>] irq_exit+0x9c/0xb0
> >   [<ffffffffaea03e7a>] smp_apic_timer_interrupt+0x6a/0x80
> >   [<ffffffffaf0a394f>] apic_timer_interrupt+0x7f/0x90
> 
> it also can hit the following path
> 
> 	rcu_process_callbacks()
> 		__zswap_pool_release()
> 			zswap_pool_destroy()
> 				zswap_cpu_comp_destroy()
> 					cpu_notifier_register_begin()
> 						mutex_lock(&cpu_add_remove_lock);  <<<
> 
> can't it?
> 
> 	-ss

Thanks, Sergey. Now I'm convinced the problem should be fixed in
zswap. Since the rcu callback is already executed asynchronously,
using workqueue to defer the callback further more doesn't seem
to cause additional race condition at least.