LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Suresh Siddha <suresh.b.siddha@intel.com>
To: jens.axboe@oracle.com, paulmck@linux.vnet.ibm.com, mingo@elte.hu,
	jeremy.fitzhardinge@citrix.com, nickpiggin@yahoo.com.au,
	torvalds@linux-foundation.org
Cc: linux-kernel@vger.kernel.org, asit.k.mallick@intel.com
Subject: [patch] generic-ipi: fix the smp_mb() placement
Date: Wed, 29 Oct 2008 15:42:29 -0700	[thread overview]
Message-ID: <20081029224229.GK30573@linux-os.sc.intel.com> (raw)

While looking at some other issue recently, we encountered this smp_mb()
placement issue.  x86 specific code also needs some similar fixes. Patch for
that will follow soon.

Please review the appended generic-ipi fix.

thanks,
suresh
---

From: Suresh Siddha <suresh.b.siddha@intel.com>
Subject: generic-ipi: fix the smp_mb() placement

smp_mb() is needed (to make the memory operations visible globally) before
sending the ipi on the sender and the receiver (on Alpha atleast) needs
smp_read_barrier_depends() in the handler before reading the call_single_queue
list in a lock-free fashion.

On x86, x2apic mode register accesses for sending IPI's don't have serializing
semantics. So the need for smp_mb() before sending the IPI becomes more
critical in x2apic mode.

Remove the unnecessary smp_mb() in csd_flag_wait(), as the presence of that
smp_mb() doesn't mean anything on the sender, when the ipi receiver is not
doing any thing special (like memory fence) after clearing the CSD_FLAG_WAIT.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
---

diff --git a/kernel/smp.c b/kernel/smp.c
index f362a85..75c8dde 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -51,10 +51,6 @@ static void csd_flag_wait(struct call_single_data *data)
 {
 	/* Wait for response */
 	do {
-		/*
-		 * We need to see the flags store in the IPI handler
-		 */
-		smp_mb();
 		if (!(data->flags & CSD_FLAG_WAIT))
 			break;
 		cpu_relax();
@@ -76,6 +72,11 @@ static void generic_exec_single(int cpu, struct call_single_data *data)
 	list_add_tail(&data->list, &dst->list);
 	spin_unlock_irqrestore(&dst->lock, flags);
 
+	/*
+	 * Make the list addition visible before sending the ipi.
+	 */
+	smp_mb();
+
 	if (ipi)
 		arch_send_call_function_single_ipi(cpu);
 
@@ -157,7 +158,7 @@ void generic_smp_call_function_single_interrupt(void)
 	 * Need to see other stores to list head for checking whether
 	 * list is empty without holding q->lock
 	 */
-	smp_mb();
+	smp_read_barrier_depends();
 	while (!list_empty(&q->list)) {
 		unsigned int data_flags;
 
@@ -191,7 +192,7 @@ void generic_smp_call_function_single_interrupt(void)
 		/*
 		 * See comment on outer loop
 		 */
-		smp_mb();
+		smp_read_barrier_depends();
 	}
 }
 
@@ -370,6 +371,11 @@ int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,
 	list_add_tail_rcu(&data->csd.list, &call_function_queue);
 	spin_unlock_irqrestore(&call_function_lock, flags);
 
+	/*
+	 * Make the list addition visible before sending the ipi.
+	 */
+	smp_mb();
+
 	/* Send a message to all CPUs in the map */
 	arch_send_call_function_ipi(mask);
 

             reply	other threads:[~2008-10-29 22:43 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-10-29 22:42 Suresh Siddha [this message]
2008-10-30  7:20 ` Jens Axboe
2008-10-30 16:30   ` Suresh Siddha
2008-10-30 17:25     ` Jens Axboe
2008-10-30 18:53 ` Ingo Molnar
2008-10-30 20:23   ` Suresh Siddha
2008-10-31  5:10     ` Jeremy Fitzhardinge
2008-10-31  9:39       ` Ingo Molnar
2008-10-31 11:12         ` Jeremy Fitzhardinge
2008-10-31 16:53           ` Suresh Siddha
2008-10-31 20:30             ` Jeremy Fitzhardinge
2008-11-03 10:17               ` Ingo Molnar
2008-11-03 23:48                 ` Jeremy Fitzhardinge
2008-11-04  9:19                   ` Ingo Molnar
2008-11-04 22:25                     ` Suresh Siddha
2008-11-05  9:20                       ` Jens Axboe

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20081029224229.GK30573@linux-os.sc.intel.com \
    --to=suresh.b.siddha@intel.com \
    --cc=asit.k.mallick@intel.com \
    --cc=jens.axboe@oracle.com \
    --cc=jeremy.fitzhardinge@citrix.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@elte.hu \
    --cc=nickpiggin@yahoo.com.au \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=torvalds@linux-foundation.org \
    --subject='Re: [patch] generic-ipi: fix the smp_mb() placement' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).