svn commit: r244732 - head/sys/sys

[Gleb Smirnoff]

  Attilio,

On Thu, Dec 27, 2012 at 12:36:58PM +0000, Attilio Rao wrote:
A> Author: attilio
A> Date: Thu Dec 27 12:36:58 2012
A> New Revision: 244732
A> URL: http://svnweb.freebsd.org/changeset/base/244732
A> 
A> Log:
A>   br_prod_tail and br_cons_tail members are used as barrier to
A>   signal bug_ring ownership. However, instructions can be reordered
A>   around members write leading to stale values for ie. br_prod_bufs.
A>   
A>   Use correct memory barriers to ensure proper ordering of the
A>   ownership tokens updates.
A>   
A>   Sponsored by:	EMC / Isilon storage division
A>   MFC after:	2 weeks

Have you profiled this?

After this change the buf_ring actually gains its own hand-rolled
mutex:

	while (atomic_load_acq_32(&br->br_prod_tail) != prod_head)
		cpu_spinwait();

The only difference with mutex(9) is that this one isn't monitored
by WITNESS.

The idea behind buf_ring was lockless storing and lockless fetching
from a ring and now this vanished.

What does your change actually fixes except precise accounting of
br_prod_bufs that are actually unused and should be better garbage
collected rather than fixed?

A> Modified:
A>   head/sys/sys/buf_ring.h
A> 
A> Modified: head/sys/sys/buf_ring.h
A> ==============================================================================
A> --- head/sys/sys/buf_ring.h	Thu Dec 27 09:15:21 2012	(r244731)
A> +++ head/sys/sys/buf_ring.h	Thu Dec 27 12:36:58 2012	(r244732)
A> @@ -114,10 +114,10 @@ buf_ring_enqueue(struct buf_ring *br, vo
A>  	 * that preceeded us, we need to wait for them
A>  	 * to complete 
A>  	 */   
A> -	while (br->br_prod_tail != prod_head)
A> +	while (atomic_load_acq_32(&br->br_prod_tail) != prod_head)
A>  		cpu_spinwait();
A>  	br->br_prod_bufs++;
A> -	br->br_prod_tail = prod_next;
A> +	atomic_store_rel_32(&br->br_prod_tail, prod_next);
A>  	critical_exit();
A>  	return (0);
A>  }
A> @@ -161,10 +161,10 @@ buf_ring_dequeue_mc(struct buf_ring *br)
A>  	 * that preceeded us, we need to wait for them
A>  	 * to complete 
A>  	 */   
A> -	while (br->br_cons_tail != cons_head)
A> +	while (atomic_load_acq_32(&br->br_cons_tail) != cons_head)
A>  		cpu_spinwait();
A>  
A> -	br->br_cons_tail = cons_next;
A> +	atomic_store_rel_32(&br->br_cons_tail, cons_next);
A>  	critical_exit();
A>  
A>  	return (buf);
A> @@ -209,7 +209,7 @@ buf_ring_dequeue_sc(struct buf_ring *br)
A>  		panic("inconsistent list cons_tail=%d cons_head=%d",
A>  		    br->br_cons_tail, cons_head);
A>  #endif
A> -	br->br_cons_tail = cons_next;
A> +	atomic_store_rel_32(&br->br_cons_tail, cons_next);
A>  	return (buf);
A>  }
A>  

-- 
Totus tuus, Glebius.