Re: [PATCH] rcu_process_callbacks: don't cli() while testing ->nxtlist

Re: [PATCH] rcu_process_callbacks: don't cli() while testing ->nxtlist

[Dipankar Sarma]

On Sat, Jan 28, 2006 at 10:02:01PM +0300, Oleg Nesterov wrote:
> __rcu_process_callbacks() disables interrupts to protect itself
> from call_rcu() which adds new entries to ->nxtlist.
> 
> However we can check "->nxtlist != NULL" with interrupts enabled,
> we can't get "false positives" because call_rcu() can only change
> this condition from 0 to 1.
> 
> Tested with rcutorture.ko.
> 
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

Looks good to me.

Thanks
Dipankar

[PATCH] rcu_process_callbacks: don't cli() while testing ->nxtlist

[Oleg Nesterov]

__rcu_process_callbacks() disables interrupts to protect itself
from call_rcu() which adds new entries to ->nxtlist.

However we can check "->nxtlist != NULL" with interrupts enabled,
we can't get "false positives" because call_rcu() can only change
this condition from 0 to 1.

Tested with rcutorture.ko.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- RC-1/kernel/rcupdate.c~	2006-01-19 18:13:07.000000000 +0300
+++ RC-1/kernel/rcupdate.c	2006-01-29 00:13:24.000000000 +0300
@@ -381,8 +381,8 @@ static void __rcu_process_callbacks(stru
 		rdp->curtail = &rdp->curlist;
 	}
 
-	local_irq_disable();
 	if (rdp->nxtlist && !rdp->curlist) {
+		local_irq_disable();
 		rdp->curlist = rdp->nxtlist;
 		rdp->curtail = rdp->nxttail;
 		rdp->nxtlist = NULL;
@@ -407,9 +407,8 @@ static void __rcu_process_callbacks(stru
 			rcu_start_batch(rcp);
 			spin_unlock(&rcp->lock);
 		}
-	} else {
-		local_irq_enable();
 	}
+
 	rcu_check_quiescent_state(rcp, rdp);
 	if (rdp->donelist)
 		rcu_do_batch(rdp);