Replace the initial set-pending cmpxchg() loop with an unconditional
test-and-set bit (x86: bts) instruction.
It looses the direct trylock state transition; however since that should
be very unlikely (we've just done a trylock) that shouldn't be a
problem.
Signed-off-by: Peter Zijlstra <pet...@infradead.org>
---
include/asm-generic/qspinlock_types.h | 2 +
kernel/locking/qspinlock.c | 60 +++++++++++++++++++---------------
2 files changed, 36 insertions(+), 26 deletions(-)
--- a/include/asm-generic/qspinlock_types.h
+++ b/include/asm-generic/qspinlock_types.h
@@ -59,6 +59,8 @@ typedef struct qspinlock {
#define _Q_TAIL_CPU_BITS (32 - _Q_TAIL_CPU_OFFSET)
#define _Q_TAIL_CPU_MASK (((1U << _Q_TAIL_CPU_BITS) - 1) <<
_Q_TAIL_CPU_OFFSET)
+#define _Q_TAIL_MASK (_Q_TAIL_IDX_MASK | _Q_TAIL_CPU_MASK)
+
#define _Q_LOCKED_VAL (1U << _Q_LOCKED_OFFSET)
#define _Q_PENDING_VAL (1U << _Q_PENDING_OFFSET)
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -83,6 +83,37 @@ static inline struct mcs_spinlock *decod
return per_cpu_ptr(&mcs_nodes[idx], cpu);
}
+/*
+ * 0,0,1 -> 0,1,* ; pending
+ *
+ * Ignore the locked bit; if we set pending and locked happens to be clear
+ * we'll fall through on the subsequent wait.
+ */
+static int __always_inline
+try_set_pending(struct qspinlock *lock, u32 val)
+{
+ if (val & ~_Q_LOCKED_MASK)
+ return 0; /* fail; queue */
+
+ /*
+ * If we find the pending bit was already set; fail and queue.
+ */
+ if (atomic_test_and_set_bit(_Q_PENDING_OFFSET, &lock->val))
+ return 0;
+
+ /*
+ * If we raced and someone concurrently set the tail; no problem. He
+ * need not have observed our pending bit and can have claimed the
+ * lock.
+ *
+ * The next node in line however will wait for the pending to go away
+ * again though, so in effect we've just flipped order between two
+ * contenders which already had undetermined order as per the race.
+ */
+
+ return 1;
+}
+
#define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK)
/**
@@ -115,34 +146,10 @@ void queue_spin_lock_slowpath(struct qsp
BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
/*
- * trylock || pending
- *
- * 0,0,0 -> 0,0,1 ; trylock
* 0,0,1 -> 0,1,1 ; pending
*/
- for (;;) {
- /*
- * If we observe any contention; queue.
- */
- if (val & ~_Q_LOCKED_MASK)
- goto queue;
-
- new = _Q_LOCKED_VAL;
- if (val == new)
- new |= _Q_PENDING_VAL;
-
- old = atomic_cmpxchg(&lock->val, val, new);
- if (old == val)
- break;
-
- val = old;
- }
-
- /*
- * we won the trylock
- */
- if (new == _Q_LOCKED_VAL)
- return;
+ if (!try_set_pending(lock, val))
+ goto queue;
/*
* we're pending, wait for the owner to go away.
@@ -186,6 +193,7 @@ void queue_spin_lock_slowpath(struct qsp
* 0,0,0 -> 0,0,1 ; trylock
* p,y,x -> n,y,x ; prev = xchg(lock, node)
*/
+ val = atomic_read(&lock->val);
for (;;) {
new = _Q_LOCKED_VAL;
if (val)
--
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