[Xen-devel] [PATCH] xen: Send spinlock IPI to all waiters

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

Hopefully not mis-parsing Jan's last comments on the other thread,
this would be the fix covering things until a better implementation
is done.
This also prevents the hang on older kernels, where it could be re-
produced reliably.

-Stefan

>From 7e042a253b06da96409a0e059744c217f396a17f Mon Sep 17 00:00:00 2001
From: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Date: Fri, 15 Feb 2013 09:48:52 +0100
Subject: [PATCH] xen: Send spinlock IPI to all waiters

There is a loophole between Xen's current implementation of
pv-spinlocks and the scheduler. This was triggerable through
a testcase until v3.6 changed the TLB flushing code. The
problem potentially is still there just not observable in the
same way.

What could happen was (is):

1. CPU n tries to schedule task x away and goes into a slow
   wait for the runq lock of CPU n-# (must be one with a lower
   number).
2. CPU n-#, while processing softirqs, tries to balance domains
   and goes into a slow wait for its own runq lock (for updating
   some records). Since this is a spin_lock_irqsave in softirq
   context, interrupts will be re-enabled for the duration of
   the poll_irq hypercall used by Xen.
3. Before the runq lock of CPU n-# is unlocked, CPU n-1 receives
   an interrupt (e.g. endio) and when processing the interrupt,
   tries to wake up task x. But that is in schedule and still
   on_cpu, so try_to_wake_up goes into a tight loop.
4. The runq lock of CPU n-# gets unlocked, but the message only
   gets sent to the first waiter, which is CPU n-# and that is
   busily stuck.

To avoid this and since the unlocking code has no real sense of
which waiter is best suited to grab the lock, just send the IPI
to all of them. This causes the waiters to return from the hyper-
call (those not interrupted at least) and do active spinlocking.

BugLink: http://bugs.launchpad.net/bugs/1011792

Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/x86/xen/spinlock.c |    1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 83e866d..f7a080e 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -328,7 +328,6 @@ static noinline void xen_spin_unlock_slow(struct 
xen_spinlock *xl)
                if (per_cpu(lock_spinners, cpu) == xl) {
                        ADD_STATS(released_slow_kicked, 1);
                        xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
-                       break;
                }
        }
 }
-- 
1.7.9.5


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