[3.11.y.z extended stable] Patch "irqchip: armada-370-xp: fix IPI race condition" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    irqchip: armada-370-xp: fix IPI race condition

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 28cfabd3014dcd63da4c4977e5d6fd12b1794008 Mon Sep 17 00:00:00 2001
From: Lior Amsalem <alior at marvell.com>
Date: Mon, 25 Nov 2013 17:26:44 +0100
Subject: irqchip: armada-370-xp: fix IPI race condition

commit a6f089e95b1e08cdea9633d50ad20aa5d44ba64d upstream.

In the Armada 370/XP driver, when we receive an IRQ 0, we read the
list of doorbells that caused the interrupt from register
ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS. This gives the list of IPIs that
were generated. However, instead of acknowledging only the IPIs that
were generated, we acknowledge *all* the IPIs, by writing
~IPI_DOORBELL_MASK in the ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS register.

This creates a race condition: if a new IPI that isn't part of the
ones read into the temporary "ipimask" variable is fired before we
acknowledge all IPIs, then we will simply loose it. This is causing
scheduling hangs on SMP intensive workloads.

It is important to mention that this ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS
register has the following behavior: "A CPU write of 0 clears the bits
in this field. A CPU write of 1 has no effect". This is what allows us
to simply write ~ipimask to acknoledge the handled IPIs.

Notice that the same problem is present in the MSI implementation, but
it will be fixed as a separate patch, so that this IPI fix can be
pushed to older stable versions as appropriate (all the way to 3.8),
while the MSI code only appeared in 3.13.

Signed-off-by: Lior Amsalem <alior at marvell.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
Fixes: 344e873e5657e8dc0 'arm: mvebu: Add IPI support via doorbells'
Cc: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Jason Cooper <jason at lakedaemon.net>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/irqchip/irq-armada-370-xp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/irqchip/irq-armada-370-xp.c b/drivers/irqchip/irq-armada-370-xp.c
index bb328a3..a51ee00 100644
--- a/drivers/irqchip/irq-armada-370-xp.c
+++ b/drivers/irqchip/irq-armada-370-xp.c
@@ -229,7 +229,7 @@ armada_370_xp_handle_irq(struct pt_regs *regs)
 						ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS)
 				& IPI_DOORBELL_MASK;

-			writel(~IPI_DOORBELL_MASK, per_cpu_int_base +
+			writel(~ipimask, per_cpu_int_base +
 				ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS);

 			/* Handle all pending doorbells */
--
1.8.3.2