linux-kernel - Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs

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Date:	Mon, 18 Jan 2016 08:54:07 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Minchan Kim <minchan@...nel.org>
Cc:	Junil Lee <junil0814.lee@....com>, ngupta@...are.org,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition

On 18.1.2016 8:39, Sergey Senozhatsky wrote:
> On (01/18/16 16:11), Minchan Kim wrote:
> [..]
>>> so, even if clear_bit_unlock/test_and_set_bit_lock do smp_mb or
>>> barrier(), there is no corresponding barrier from record_obj()->WRITE_ONCE().
>>> so I don't think WRITE_ONCE() will help the compiler, or am I missing
>>> something?
>>
>> We need two things
>> 2. memory barrier.
>>
>> As compiler barrier, WRITE_ONCE works to prevent store tearing here
>> by compiler.
>> However, if we omit unpin_tag here, we lose memory barrier(e,g, smp_mb)
>> so another CPU could see stale data caused CPU memory reordering.
> 
> oh... good find! lost release semantic of unpin_tag()...

Ah, release semantic, good point indeed. OK then we need the v2 approach again,
with WRITE_ONCE() in record_obj(). Or some kind of record_obj_release() with
release semantic, which would be a bit more effective, but I guess migration is
not that critical path to be worth introducing it.

Thanks,
Vlastimil

> 
> 	-ss
>