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authorMikulas Patocka <mpatocka@redhat.com>2012-09-26 07:46:43 +0200
committerJens Axboe <axboe@kernel.dk>2012-09-26 07:46:43 +0200
commit62ac665ff9fc07497ca524bd20d6a96893d11071 (patch)
treedfd697e488fde4b46f1cb2ebfb380bb881115827 /include/linux/percpu-rwsem.h
parentb87570f5d349661814b262dd5fc40787700f80d6 (diff)
downloadop-kernel-dev-62ac665ff9fc07497ca524bd20d6a96893d11071.zip
op-kernel-dev-62ac665ff9fc07497ca524bd20d6a96893d11071.tar.gz
blockdev: turn a rw semaphore into a percpu rw semaphore
This avoids cache line bouncing when many processes lock the semaphore for read. New percpu lock implementation The lock consists of an array of percpu unsigned integers, a boolean variable and a mutex. When we take the lock for read, we enter rcu read section, check for a "locked" variable. If it is false, we increase a percpu counter on the current cpu and exit the rcu section. If "locked" is true, we exit the rcu section, take the mutex and drop it (this waits until a writer finished) and retry. Unlocking for read just decreases percpu variable. Note that we can unlock on a difference cpu than where we locked, in this case the counter underflows. The sum of all percpu counters represents the number of processes that hold the lock for read. When we need to lock for write, we take the mutex, set "locked" variable to true and synchronize rcu. Since RCU has been synchronized, no processes can create new read locks. We wait until the sum of percpu counters is zero - when it is, there are no readers in the critical section. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'include/linux/percpu-rwsem.h')
-rw-r--r--include/linux/percpu-rwsem.h89
1 files changed, 89 insertions, 0 deletions
diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
new file mode 100644
index 0000000..cf80f7e
--- /dev/null
+++ b/include/linux/percpu-rwsem.h
@@ -0,0 +1,89 @@
+#ifndef _LINUX_PERCPU_RWSEM_H
+#define _LINUX_PERCPU_RWSEM_H
+
+#include <linux/mutex.h>
+#include <linux/percpu.h>
+#include <linux/rcupdate.h>
+#include <linux/delay.h>
+
+struct percpu_rw_semaphore {
+ unsigned __percpu *counters;
+ bool locked;
+ struct mutex mtx;
+};
+
+static inline void percpu_down_read(struct percpu_rw_semaphore *p)
+{
+ rcu_read_lock();
+ if (unlikely(p->locked)) {
+ rcu_read_unlock();
+ mutex_lock(&p->mtx);
+ this_cpu_inc(*p->counters);
+ mutex_unlock(&p->mtx);
+ return;
+ }
+ this_cpu_inc(*p->counters);
+ rcu_read_unlock();
+}
+
+static inline void percpu_up_read(struct percpu_rw_semaphore *p)
+{
+ /*
+ * On X86, write operation in this_cpu_dec serves as a memory unlock
+ * barrier (i.e. memory accesses may be moved before the write, but
+ * no memory accesses are moved past the write).
+ * On other architectures this may not be the case, so we need smp_mb()
+ * there.
+ */
+#if defined(CONFIG_X86) && (!defined(CONFIG_X86_PPRO_FENCE) && !defined(CONFIG_X86_OOSTORE))
+ barrier();
+#else
+ smp_mb();
+#endif
+ this_cpu_dec(*p->counters);
+}
+
+static inline unsigned __percpu_count(unsigned __percpu *counters)
+{
+ unsigned total = 0;
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ total += ACCESS_ONCE(*per_cpu_ptr(counters, cpu));
+
+ return total;
+}
+
+static inline void percpu_down_write(struct percpu_rw_semaphore *p)
+{
+ mutex_lock(&p->mtx);
+ p->locked = true;
+ synchronize_rcu();
+ while (__percpu_count(p->counters))
+ msleep(1);
+ smp_rmb(); /* paired with smp_mb() in percpu_sem_up_read() */
+}
+
+static inline void percpu_up_write(struct percpu_rw_semaphore *p)
+{
+ p->locked = false;
+ mutex_unlock(&p->mtx);
+}
+
+static inline int percpu_init_rwsem(struct percpu_rw_semaphore *p)
+{
+ p->counters = alloc_percpu(unsigned);
+ if (unlikely(!p->counters))
+ return -ENOMEM;
+ p->locked = false;
+ mutex_init(&p->mtx);
+ return 0;
+}
+
+static inline void percpu_free_rwsem(struct percpu_rw_semaphore *p)
+{
+ free_percpu(p->counters);
+ p->counters = NULL; /* catch use after free bugs */
+}
+
+#endif
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