On Thu, Jan 30, 2014 at 04:17:15PM +0100, Peter Zijlstra wrote:
> The below is still small and actually works.
OK, so having actually worked through the thing; I realized we can
actually do a version without MCS lock and instead use a ticket lock for
the waitqueue.
This is both smaller (back to 8 bytes for the rwlock_t), and should be
faster under moderate contention for not having to touch extra
cachelines.
Completely untested and with a rather crude generic ticket lock
implementation to illustrate the concept:
---
--- a/include/asm-generic/qrwlock_types.h
+++ b/include/asm-generic/qrwlock_types.h
@@ -3,15 +3,13 @@
#include <linux/atomic.h>
-struct mcs_spinlock;
-
/*
* The queue read/write lock data structure
*/
struct qrwlock {
atomic_t cnts;
- struct mcs_spinlock *waitq;
+ atomic_t tickets;
};
#endif /* __ASM_GENERIC_QRWLOCK_TYPES_H */
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -21,29 +21,32 @@
#include <linux/percpu.h>
#include <linux/hardirq.h>
#include <linux/mutex.h>
-#include <linux/mcs_spinlock.h>
#include <asm/qrwlock.h>
-/*
- * Compared with regular rwlock, the queue rwlock has has the following
- * advantages:
- * 1. Even though there is a slight chance of stealing the lock if come at
- * the right moment, the granting of the lock is mostly in FIFO order.
- * 2. It is usually faster in high contention situation.
- *
- * The only downside is that the lock is 4 bytes larger in 32-bit systems
- * and 12 bytes larger in 64-bit systems.
- *
- * There are two queues for writers. The writer field of the lock is a
- * one-slot wait queue. The writers that follow will have to wait in the
- * combined reader/writer queue (waitq).
- *
- * Compared with x86 ticket spinlock, the queue rwlock is faster in high
- * contention situation. The writer lock is also faster in single thread
- * operations. Therefore, queue rwlock can be considered as a replacement
- * for those spinlocks that are highly contended as long as an increase
- * in lock size is not an issue.
- */
+#define TICKET_MSB 0x8000
+#define TICKET_MASK 0x7FFF
+
+#define atomic_xadd(v, a) (atomic_add_return((v), (a)) - (v))
+
+static inline void ticket_spin_lock(atomic_t *tickets)
+{
+ u32 val = atomic_xadd(1 << 16, tickets);
+ u32 ticket = (val >> 16) & TICKET_MASK;
+
+ if (unlikely(val & TICKET_MSB))
+ atomic_clear_mask(TICKET_MSB, tickets);
+
+ if (unlikely((val & TICKET_MASK) != ticket)) {
+ while ((smp_load_acquire((u32 *)tickets) & TICKET_MASK) != ticket)
+ cpu_relax();
+ }
+}
+
+static inline void ticket_spin_unlock(atomic_t *tickets)
+{
+ smp_mb__before_atomic_inc();
+ atomic_inc(tickets);
+}
/**
* rspin_until_writer_unlock - inc reader count & spin until writer is gone
@@ -68,7 +71,6 @@ rspin_until_writer_unlock(struct qrwlock
*/
void queue_read_lock_slowpath(struct qrwlock *lock)
{
- struct mcs_spinlock node;
u32 cnts;
/*
@@ -88,7 +90,7 @@ void queue_read_lock_slowpath(struct qrw
/*
* Put the reader into the wait queue
*/
- mcs_spin_lock(&lock->waitq, &node);
+ ticket_spin_lock(&lock->tickets);
/*
* At the head of the wait queue now, wait until the writer state
@@ -100,13 +102,13 @@ void queue_read_lock_slowpath(struct qrw
while (atomic_read(&lock->cnts) & _QW_WMASK)
arch_mutex_cpu_relax();
- cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
+ cnts = atomic_xadd(_QR_BIAS, &lock->cnts);
rspin_until_writer_unlock(lock, cnts);
/*
* Signal the next one in queue to become queue head
*/
- mcs_spin_unlock(&lock->waitq, &node);
+ ticket_spin_unlock(&lock->tickets);
}
EXPORT_SYMBOL(queue_read_lock_slowpath);
@@ -116,11 +118,10 @@ EXPORT_SYMBOL(queue_read_lock_slowpath);
*/
void queue_write_lock_slowpath(struct qrwlock *lock)
{
- struct mcs_spinlock node;
u32 cnts;
/* Put the writer into the wait queue */
- mcs_spin_lock(&lock->waitq, &node);
+ ticket_spin_lock(&lock->tickets);
/* Try to acquire the lock directly if no reader is present */
if (!atomic_read(&lock->cnts) &&
@@ -152,6 +153,6 @@ void queue_write_lock_slowpath(struct qr
arch_mutex_cpu_relax();
}
unlock:
- mcs_spin_unlock(&lock->waitq, &node);
+ ticket_spin_unlock(&lock->tickets);
}
EXPORT_SYMBOL(queue_write_lock_slowpath);
--
To unsubscribe from this list: send the line "unsubscribe linux-arch" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
