xen: only enable interrupts while actually blocking for spinlock
Where possible we enable interrupts while waiting for a spinlock to
become free, in order to reduce big latency spikes in interrupt handling.
However, at present if we manage to pick up the spinlock just before
blocking, we'll end up holding the lock with interrupts enabled for a
while. This will cause a deadlock if we recieve an interrupt in that
window, and the interrupt handler tries to take the lock too.
Solve this by shrinking the interrupt-enabled region to just around the
blocking call.
[ Impact: avoid race/deadlock when using Xen PV spinlocks ]
Reported-by: "Yang, Xiaowei" <xiaowei.yang@intel.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 5601506..2f91e56 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -187,7 +187,6 @@
struct xen_spinlock *prev;
int irq = __get_cpu_var(lock_kicker_irq);
int ret;
- unsigned long flags;
u64 start;
/* If kicker interrupts not initialized yet, just spin */
@@ -199,16 +198,12 @@
/* announce we're spinning */
prev = spinning_lock(xl);
- flags = __raw_local_save_flags();
- if (irq_enable) {
- ADD_STATS(taken_slow_irqenable, 1);
- raw_local_irq_enable();
- }
-
ADD_STATS(taken_slow, 1);
ADD_STATS(taken_slow_nested, prev != NULL);
do {
+ unsigned long flags;
+
/* clear pending */
xen_clear_irq_pending(irq);
@@ -228,6 +223,12 @@
goto out;
}
+ flags = __raw_local_save_flags();
+ if (irq_enable) {
+ ADD_STATS(taken_slow_irqenable, 1);
+ raw_local_irq_enable();
+ }
+
/*
* Block until irq becomes pending. If we're
* interrupted at this point (after the trylock but
@@ -238,13 +239,15 @@
* pending.
*/
xen_poll_irq(irq);
+
+ raw_local_irq_restore(flags);
+
ADD_STATS(taken_slow_spurious, !xen_test_irq_pending(irq));
} while (!xen_test_irq_pending(irq)); /* check for spurious wakeups */
kstat_incr_irqs_this_cpu(irq, irq_to_desc(irq));
out:
- raw_local_irq_restore(flags);
unspinning_lock(xl, prev);
spin_time_accum_blocked(start);