Re: [PATCH] percpu-rwsem: use barrier in unlock path

[Steven Rostedt]

On Wed, Oct 17, 2012 at 11:07:21AM -0400, Mikulas Patocka wrote:
> > 
> > Even the previous patch is applied, percpu_down_read() still
> > needs mb() to pair with it.
> 
> percpu_down_read uses rcu_read_lock which should guarantee that memory 
> accesses don't escape in front of a rcu-protected section.

You do realize that rcu_read_lock() does nothing more that a barrier(),
right?

Paul worked really hard to get rcu_read_locks() to not call HW barriers.

> 
> If rcu_read_unlock has only an unlock barrier and not a full barrier, 
> memory accesses could be moved in front of rcu_read_unlock and reordered 
> with this_cpu_inc(*p->counters), but it doesn't matter because 
> percpu_down_write does synchronize_rcu(), so it never sees these accesses 
> halfway through.

Looking at the patch, you are correct. The read side doesn't need the
memory barrier as the worse thing that will happen is that it sees the
locked = false, and will just grab the mutex unnecessarily.

> > 
> > I suggest any new synchronization should stay in -tip for 2 or more cycles
> > before merged to mainline.
> 
> But the bug that this synchronization is fixing is quite serious (it 
> causes random crashes when block size is being changed, the crash happens 
> regularly at multiple important business sites) so it must be fixed soon 
> and not wait half a year.

I don't think Lai was suggesting to wait on this fix, but instead to
totally rip out the percpu_rwsems and work on them some more, and then
re-introduce them in a half a year.

-- Steve

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