[3.19.y-ckt stable] Patch "s390/spinlock: avoid yield to non existent cpu" has been added to the 3.19.y-ckt tree

[Kamal Mostafa]

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

    s390/spinlock: avoid yield to non existent cpu

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt20.

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

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 1d35320f671e34c5d1317b22c1667080bfc1f2f7 Mon Sep 17 00:00:00 2001
From: Heiko Carstens <heiko.carstens at de.ibm.com>
Date: Wed, 13 Apr 2016 11:05:20 +0200
Subject: s390/spinlock: avoid yield to non existent cpu

commit 8497695243f70fd19ed6cf28b63584f1b608b5f9 upstream.

arch_spin_lock_wait_flags() checks if a spinlock is not held before
trying a compare and swap instruction. If the lock is unlocked it
tries the compare and swap instruction, however if a different cpu
grabbed the lock in the meantime the instruction will fail as
expected.

Subsequently the arch_spin_lock_wait_flags() incorrectly tries to
figure out if the cpu that holds the lock is running. However it is
using the wrong cpu number for this (-1) and then will also yield the
current cpu to the wrong cpu.

Fix this by adding a missing continue statement.

Fixes: 470ada6b1a1d ("s390/spinlock: refactor arch_spin_lock_wait[_flags]")
Signed-off-by: Heiko Carstens <heiko.carstens at de.ibm.com>
Acked-by: Martin Schwidefsky <schwidefsky at de.ibm.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/s390/lib/spinlock.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/s390/lib/spinlock.c b/arch/s390/lib/spinlock.c
index 034a35a..bc73e95 100644
--- a/arch/s390/lib/spinlock.c
+++ b/arch/s390/lib/spinlock.c
@@ -75,6 +75,7 @@ void arch_spin_lock_wait_flags(arch_spinlock_t *lp, unsigned long flags)
 			if (_raw_compare_and_swap(&lp->lock, 0, cpu))
 				return;
 			local_irq_restore(flags);
+			continue;
 		}
 		/* Check if the lock owner is running. */
 		if (!smp_vcpu_scheduled(~owner)) {
--
2.7.4