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

[Minchan Kim <minchan@kernel.org>]

On Mon, Jan 18, 2016 at 08:54:07AM +0100, Vlastimil Babka wrote:
> 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.

WRITE_ONCE in record_obj would add more memory operations in obj_malloc
but I don't feel it's too heavy in this phase so,

How about this? Junil, Could you resend patch if others agree this?
Thanks.

+/*
+ * record_obj updates handle's value to free_obj and it shouldn't
+ * invalidate lock bit(ie, HANDLE_PIN_BIT) of handle, otherwise
+ * it breaks synchronization using pin_tag(e,g, zs_free) so let's
+ * keep the lock bit.
+ */
 static void record_obj(unsigned long handle, unsigned long obj)
 {
-	*(unsigned long *)handle = obj;
+	int locked = (*(unsigned long *)handle) & (1<<HANDLE_PIN_BIT);
+	unsigned long val = obj | locked;
+
+	/*
+	 * WRITE_ONCE could prevent store tearing like below
+	 * *(unsigned long *)handle = free_obj
+	 * *(unsigned long *)handle |= locked;
+	 */
+	WRITE_ONCE(*(unsigned long *)handle, val);
 }



> 
> Thanks,
> Vlastimil
> 
> > 
> > 	-ss
> > 
> 

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