Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition

[Sergey Senozhatsky]

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

thanks.

> 1. compiler barrier

um... probably gcc can reorder that sequence to something like this

	*handle = obj_malloc()   /* unpin the object */
	zs_object_copy(*handle, used_obj, class) /* now use it*/

ok.


> 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()...

	-ss