linux-kernel - Re: [Openipmi-developer] [PATCH] IPMI: Fix some RCU problems

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Date:	Wed, 3 Jan 2007 21:34:40 -0600
From:	Corey Minyard <minyard@....org>
To:	Andrew Morton <akpm@...l.org>
Cc:	Carol Hebert <cah@...ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Developers <openipmi-developer@...ts.sourceforge.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	OpenIPMI@...-sf-spam2.sourceforge.net
Subject: Re: [Openipmi-developer] [PATCH] IPMI: Fix some RCU problems

On Wed, Jan 03, 2007 at 01:22:32PM -0800, Andrew Morton wrote:
> It's nice to always have a comment explaining the use of open-coded
> barriers.  Because often the reader is left wondered what on earth it's
> barriering against what on earth else.
> 

Ok, here it is...


Andrew asked that the open-coded barriers be commented, so here it
is.  I also realized that one of the read barriers was in an area
where the protecting mutex was held, so no read barrier was needed.

Signed-off-by: Corey Minyard <minyard@....org>

Index: linux-2.6.19/drivers/char/ipmi/ipmi_msghandler.c
===================================================================
--- linux-2.6.19.orig/drivers/char/ipmi/ipmi_msghandler.c
+++ linux-2.6.19/drivers/char/ipmi/ipmi_msghandler.c
@@ -839,7 +839,6 @@ int ipmi_create_user(unsigned int       
 	goto out_kfree;
 
  found:
-	smp_rmb();
 	/* Note that each existing user holds a refcount to the interface. */
 	kref_get(&intf->refcount);
 
@@ -2762,10 +2761,15 @@ int ipmi_register_smi(struct ipmi_smi_ha
 		synchronize_rcu();
 		kref_put(&intf->refcount, intf_free);
 	} else {
-		/* After this point the interface is legal to use. */
-		smp_wmb(); /* Keep memory order straight for RCU readers. */
+		/*
+		 * Keep memory order straight for RCU readers.  Make
+		 * sure everything else is committed to memory before
+		 * setting intf_num to mark the interface valid.
+		 */
+		smp_wmb();
 		intf->intf_num = i;
 		mutex_unlock(&ipmi_interfaces_mutex);
+		/* After this point the interface is legal to use. */
 		call_smi_watchers(i, intf->si_dev);
 		mutex_unlock(&smi_watchers_mutex);
 	}
@@ -3927,6 +3931,12 @@ static void send_panic_events(char *str)
 			/* Interface was not ready yet. */
 			continue;
 
+		/*
+		 * intf_num is used as an marker to tell if the
+		 * interface is valid.  Thus we need a read barrier to
+		 * make sure data fetched before checking intf_num
+		 * won't be used.
+		 */
 		smp_rmb();
 
 		/* First job here is to figure out where to send the
-
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