Re: [PATCH]: Fix SMP-reordering race in mark_buffer_dirty

[Mikulas Patocka]

On Wed, 2 Apr 2008, Linus Torvalds wrote:

> On Wed, 2 Apr 2008, Mikulas Patocka wrote:
> > 
> > So you're right, the gain of mfence is so little that you can remove it 
> > and use only test_set_buffer_dirty.
> 
> Well, I suspect that part of the issue is that quite often you end up 
> with *both* because the buffer wasn't already dirty from before.
> 
> Re-dirtying a dirty buffer is pretty common for things like bitmap blocks 
> etc, so it's probably a worthy optimization if it has no cost, and on 
> Core2 I suspect your version is worth it, but it's not like it's going to 
> be necessarily a 99% kind of case. I suspect quite a lot of the 
> mark_buffer_dirty() calls are actually on clean buffers.

The cost of doing the buffer lookup and update is much more than the 20 
clocks, so the 20 clocks mfence/lock difference can be forgotten.

> (Of course, a valid argument is that if it was already dirty, we'll skip 
> the other expensive parts, so only the "already dirty" case is worth 
> optimizing for. Maybe true. There might also be cases where it means one 
> less dirty cacheline in memory.)

That is another good example: when two CPUs are simultaneously updating 
the same buffer (i.e. two inodes in one block or simultaneous writes to a 
bitmap), then test_set_buffer_dirty will do cache ping-pong (that is much 
more expensive than that 20-100 cycles for an interlocked instruction), 
and smp_mb()+test_buffer_dirty will keep cache intact.

So maybe there's still a reason for smb_mb().

Mikulas

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