[PATCH 1/4] irqchip: armada-370-xp: fix IPI race condition

[Thomas Petazzoni]

From: Lior Amsalem <alior at marvell.com>

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>
Cc: stable at vger.kernel.org
Cc: Thomas Gleixner <tglx at linutronix.de>
---
The problem has been present since 344e873e5657e8dc0 ('arm: mvebu: Add
IPI support via doorbells'), that is since v3.8. However, notice that
the IRQ driver was moved from arch/arm/mach-mvebu/ to drivers/irqchip
in the process, and also that the very line being changed was slightly
modified in 5ec69017cc944f3ed8 ('irqchip: armada-370-xp: slightly
cleanup irq controller driver').
---
 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 433cc85..f5e49a2 100644
--- a/drivers/irqchip/irq-armada-370-xp.c
+++ b/drivers/irqchip/irq-armada-370-xp.c
@@ -407,7 +407,7 @@ 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 */
-- 
1.8.1.2