1 files changed, 50 insertions, 15 deletions

diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
index 8dc4d31da67f..4884bc603aa7 100644
--- a/arch/x86/xen/spinlock.c
+++ b/arch/x86/xen/spinlock.c
@@ -47,25 +47,41 @@ static int xen_spin_trylock(struct raw_spinlock *lock)
 static DEFINE_PER_CPU(int, lock_kicker_irq) = -1;
 static DEFINE_PER_CPU(struct xen_spinlock *, lock_spinners);
 
-static inline void spinning_lock(struct xen_spinlock *xl)
+/*
+ * Mark a cpu as interested in a lock.  Returns the CPU's previous
+ * lock of interest, in case we got preempted by an interrupt.
+ */
+static inline struct xen_spinlock *spinning_lock(struct xen_spinlock *xl)
 {
+        struct xen_spinlock *prev;
+
+        prev = __get_cpu_var(lock_spinners);
         __get_cpu_var(lock_spinners) = xl;
+
         wmb();                  /* set lock of interest before count */
+
         asm(LOCK_PREFIX " incw %0"
             : "+m" (xl->spinners) : : "memory");
+
+        return prev;
 }
 
-static inline void unspinning_lock(struct xen_spinlock *xl)
+/*
+ * Mark a cpu as no longer interested in a lock.  Restores previous
+ * lock of interest (NULL for none).
+ */
+static inline void unspinning_lock(struct xen_spinlock *xl, struct xen_spinlock *prev)
 {
         asm(LOCK_PREFIX " decw %0"
             : "+m" (xl->spinners) : : "memory");
-        wmb();                  /* decrement count before clearing lock */
-        __get_cpu_var(lock_spinners) = NULL;
+        wmb();                  /* decrement count before restoring lock */
+        __get_cpu_var(lock_spinners) = prev;
 }
 
 static noinline int xen_spin_lock_slow(struct raw_spinlock *lock)
 {
         struct xen_spinlock *xl = (struct xen_spinlock *)lock;
+        struct xen_spinlock *prev;
         int irq = __get_cpu_var(lock_kicker_irq);
         int ret;
 
@@ -74,23 +90,42 @@ static noinline int xen_spin_lock_slow(struct raw_spinlock *lock)
                 return 0;
 
         /* announce we're spinning */
-        spinning_lock(xl);
+        prev = spinning_lock(xl);
 
-        /* clear pending */
-        xen_clear_irq_pending(irq);
+        do {
+                /* clear pending */
+                xen_clear_irq_pending(irq);
+
+                /* check again make sure it didn't become free while
+                   we weren't looking  */
+                ret = xen_spin_trylock(lock);
+                if (ret) {
+                        /*
+                         * If we interrupted another spinlock while it
+                         * was blocking, make sure it doesn't block
+                         * without rechecking the lock.
+                         */
+                        if (prev != NULL)
+                                xen_set_irq_pending(irq);
+                        goto out;
+                }
 
-        /* check again make sure it didn't become free while
-           we weren't looking  */
-        ret = xen_spin_trylock(lock);
-        if (ret)
-                goto out;
+                /*
+                 * Block until irq becomes pending.  If we're
+                 * interrupted at this point (after the trylock but
+                 * before entering the block), then the nested lock
+                 * handler guarantees that the irq will be left
+                 * pending if there's any chance the lock became free;
+                 * xen_poll_irq() returns immediately if the irq is
+                 * pending.
+                 */
+                xen_poll_irq(irq);
+        } while (!xen_test_irq_pending(irq)); /* check for spurious wakeups */
 
-        /* block until irq becomes pending */
-        xen_poll_irq(irq);
         kstat_this_cpu.irqs[irq]++;
 
 out:
-        unspinning_lock(xl);
+        unspinning_lock(xl, prev);
         return ret;
 }