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[Xen-devel] [PATCH v6 5/8] spinlock: Introduce spin_lock_cb()

While waiting for a lock we may want to periodically run some
code. This code may, for example, allow the caller to release
resources held by it that are no longer needed in the critical
section protected by the lock.

Specifically, this feature will be needed by scrubbing code where
the scrubber, while waiting for heap lock to merge back clean
pages, may be requested by page allocator (which is currently
holding the lock) to abort merging and release the buddy page head
that the allocator wants.

We could use spin_trylock() but since it doesn't take lock ticket
it may take long time until the lock is taken. Instead we add
spin_lock_cb() that allows us to grab the ticket and execute a
callback while waiting. This callback is executed on every iteration
of the spinlock waiting loop.

Since we may be sleeping in the lock until it is released we need a
mechanism that will make sure that the callback has a chance to run.
We add spin_lock_kick() that will wake up the waiter.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
 xen/common/spinlock.c      | 9 ++++++++-
 xen/include/xen/spinlock.h | 8 ++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
index 2a06406..3c1caae 100644
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -129,7 +129,7 @@ static always_inline u16 observe_head(spinlock_tickets_t *t)
     return read_atomic(&t->head);
-void _spin_lock(spinlock_t *lock)
+void inline _spin_lock_cb(spinlock_t *lock, void (*cb)(void *), void *data)
     spinlock_tickets_t tickets = SPINLOCK_TICKET_INC;
@@ -140,6 +140,8 @@ void _spin_lock(spinlock_t *lock)
     while ( tickets.tail != observe_head(&lock->tickets) )
+        if ( unlikely(cb) )
+            cb(data);
@@ -147,6 +149,11 @@ void _spin_lock(spinlock_t *lock)
+void _spin_lock(spinlock_t *lock)
+     _spin_lock_cb(lock, NULL, NULL);
 void _spin_lock_irq(spinlock_t *lock)
diff --git a/xen/include/xen/spinlock.h b/xen/include/xen/spinlock.h
index c1883bd..91bfb95 100644
--- a/xen/include/xen/spinlock.h
+++ b/xen/include/xen/spinlock.h
@@ -153,6 +153,7 @@ typedef struct spinlock {
 #define spin_lock_init(l) (*(l) = (spinlock_t)SPIN_LOCK_UNLOCKED)
 void _spin_lock(spinlock_t *lock);
+void _spin_lock_cb(spinlock_t *lock, void (*cond)(void *), void *data);
 void _spin_lock_irq(spinlock_t *lock);
 unsigned long _spin_lock_irqsave(spinlock_t *lock);
@@ -169,6 +170,7 @@ void _spin_lock_recursive(spinlock_t *lock);
 void _spin_unlock_recursive(spinlock_t *lock);
 #define spin_lock(l)                  _spin_lock(l)
+#define spin_lock_cb(l, c, d)         _spin_lock_cb(l, c, d)
 #define spin_lock_irq(l)              _spin_lock_irq(l)
 #define spin_lock_irqsave(l, f)                                 \
     ({                                                          \
@@ -190,6 +192,12 @@ void _spin_unlock_recursive(spinlock_t *lock);
     1 : ({ local_irq_restore(flags); 0; });     \
+#define spin_lock_kick(l)                       \
+({                                              \
+    smp_mb();                                   \
+    arch_lock_signal();                         \
 /* Ensure a lock is quiescent between two critical operations. */
 #define spin_barrier(l)               _spin_barrier(l)

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