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authorOleg Nesterov <oleg@redhat.com>2012-12-17 16:01:32 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-12-17 17:15:18 -0800
commita1fd3e24d8a484b3265a6d485202afe093c058f3 (patch)
tree472f6480a81abbc04b27eccdb798d80b1685bee0
parent53809751ac230a3611b5cdd375f3389f3207d471 (diff)
downloadvexpress-lsk-a1fd3e24d8a484b3265a6d485202afe093c058f3.tar.gz
percpu_rw_semaphore: reimplement to not block the readers unnecessarily
Currently the writer does msleep() plus synchronize_sched() 3 times to acquire/release the semaphore, and during this time the readers are blocked completely. Even if the "write" section was not actually started or if it was already finished. With this patch down_write/up_write does synchronize_sched() twice and down_read/up_read are still possible during this time, just they use the slow path. percpu_down_write() first forces the readers to use rw_semaphore and increment the "slow" counter to take the lock for reading, then it takes that rw_semaphore for writing and blocks the readers. Also. With this patch the code relies on the documented behaviour of synchronize_sched(), it doesn't try to pair synchronize_sched() with barrier. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mikulas Patocka <mpatocka@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@elte.hu> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com> Cc: Anton Arapov <anton@redhat.com> Cc: Jens Axboe <axboe@kernel.dk> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/percpu-rwsem.h83
-rw-r--r--lib/Makefile2
-rw-r--r--lib/percpu-rwsem.c154
3 files changed, 168 insertions, 71 deletions
diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index bd1e86071e5..592f0d610d8 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -2,82 +2,25 @@
#define _LINUX_PERCPU_RWSEM_H
#include <linux/mutex.h>
+#include <linux/rwsem.h>
#include <linux/percpu.h>
-#include <linux/rcupdate.h>
-#include <linux/delay.h>
+#include <linux/wait.h>
struct percpu_rw_semaphore {
- unsigned __percpu *counters;
- bool locked;
- struct mutex mtx;
+ unsigned int __percpu *fast_read_ctr;
+ struct mutex writer_mutex;
+ struct rw_semaphore rw_sem;
+ atomic_t slow_read_ctr;
+ wait_queue_head_t write_waitq;
};
-#define light_mb() barrier()
-#define heavy_mb() synchronize_sched_expedited()
+extern void percpu_down_read(struct percpu_rw_semaphore *);
+extern void percpu_up_read(struct percpu_rw_semaphore *);
-static inline void percpu_down_read(struct percpu_rw_semaphore *p)
-{
- rcu_read_lock_sched();
- if (unlikely(p->locked)) {
- rcu_read_unlock_sched();
- mutex_lock(&p->mtx);
- this_cpu_inc(*p->counters);
- mutex_unlock(&p->mtx);
- return;
- }
- this_cpu_inc(*p->counters);
- rcu_read_unlock_sched();
- light_mb(); /* A, between read of p->locked and read of data, paired with D */
-}
+extern void percpu_down_write(struct percpu_rw_semaphore *);
+extern void percpu_up_write(struct percpu_rw_semaphore *);
-static inline void percpu_up_read(struct percpu_rw_semaphore *p)
-{
- light_mb(); /* B, between read of the data and write to p->counter, paired with C */
- 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_sched_expedited(); /* make sure that all readers exit the rcu_read_lock_sched region */
- while (__percpu_count(p->counters))
- msleep(1);
- heavy_mb(); /* C, between read of p->counter and write to data, paired with B */
-}
-
-static inline void percpu_up_write(struct percpu_rw_semaphore *p)
-{
- heavy_mb(); /* D, between write to data and write to p->locked, paired with A */
- 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 */
-}
+extern int percpu_init_rwsem(struct percpu_rw_semaphore *);
+extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
#endif
diff --git a/lib/Makefile b/lib/Makefile
index e2152fa7ff4..e959c20efb2 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -9,7 +9,7 @@ endif
lib-y := ctype.o string.o vsprintf.o cmdline.o \
rbtree.o radix-tree.o dump_stack.o timerqueue.o\
- idr.o int_sqrt.o extable.o \
+ idr.o int_sqrt.o extable.o percpu-rwsem.o \
sha1.o md5.o irq_regs.o reciprocal_div.o argv_split.o \
proportions.o flex_proportions.o prio_heap.o ratelimit.o show_mem.o \
is_single_threaded.o plist.o decompress.o kobject_uevent.o \
diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c
new file mode 100644
index 00000000000..2e03bcfe48f
--- /dev/null
+++ b/lib/percpu-rwsem.c
@@ -0,0 +1,154 @@
+#include <linux/mutex.h>
+#include <linux/rwsem.h>
+#include <linux/percpu.h>
+#include <linux/wait.h>
+#include <linux/percpu-rwsem.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/errno.h>
+
+int percpu_init_rwsem(struct percpu_rw_semaphore *brw)
+{
+ brw->fast_read_ctr = alloc_percpu(int);
+ if (unlikely(!brw->fast_read_ctr))
+ return -ENOMEM;
+
+ mutex_init(&brw->writer_mutex);
+ init_rwsem(&brw->rw_sem);
+ atomic_set(&brw->slow_read_ctr, 0);
+ init_waitqueue_head(&brw->write_waitq);
+ return 0;
+}
+
+void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
+{
+ free_percpu(brw->fast_read_ctr);
+ brw->fast_read_ctr = NULL; /* catch use after free bugs */
+}
+
+/*
+ * This is the fast-path for down_read/up_read, it only needs to ensure
+ * there is no pending writer (!mutex_is_locked() check) and inc/dec the
+ * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
+ * serialize with the preempt-disabled section below.
+ *
+ * The nontrivial part is that we should guarantee acquire/release semantics
+ * in case when
+ *
+ * R_W: down_write() comes after up_read(), the writer should see all
+ * changes done by the reader
+ * or
+ * W_R: down_read() comes after up_write(), the reader should see all
+ * changes done by the writer
+ *
+ * If this helper fails the callers rely on the normal rw_semaphore and
+ * atomic_dec_and_test(), so in this case we have the necessary barriers.
+ *
+ * But if it succeeds we do not have any barriers, mutex_is_locked() or
+ * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
+ * reader inside the critical section. See the comments in down_write and
+ * up_write below.
+ */
+static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
+{
+ bool success = false;
+
+ preempt_disable();
+ if (likely(!mutex_is_locked(&brw->writer_mutex))) {
+ __this_cpu_add(*brw->fast_read_ctr, val);
+ success = true;
+ }
+ preempt_enable();
+
+ return success;
+}
+
+/*
+ * Like the normal down_read() this is not recursive, the writer can
+ * come after the first percpu_down_read() and create the deadlock.
+ */
+void percpu_down_read(struct percpu_rw_semaphore *brw)
+{
+ if (likely(update_fast_ctr(brw, +1)))
+ return;
+
+ down_read(&brw->rw_sem);
+ atomic_inc(&brw->slow_read_ctr);
+ up_read(&brw->rw_sem);
+}
+
+void percpu_up_read(struct percpu_rw_semaphore *brw)
+{
+ if (likely(update_fast_ctr(brw, -1)))
+ return;
+
+ /* false-positive is possible but harmless */
+ if (atomic_dec_and_test(&brw->slow_read_ctr))
+ wake_up_all(&brw->write_waitq);
+}
+
+static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
+{
+ unsigned int sum = 0;
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ sum += per_cpu(*brw->fast_read_ctr, cpu);
+ per_cpu(*brw->fast_read_ctr, cpu) = 0;
+ }
+
+ return sum;
+}
+
+/*
+ * A writer takes ->writer_mutex to exclude other writers and to force the
+ * readers to switch to the slow mode, note the mutex_is_locked() check in
+ * update_fast_ctr().
+ *
+ * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
+ * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
+ * counter it represents the number of active readers.
+ *
+ * Finally the writer takes ->rw_sem for writing and blocks the new readers,
+ * then waits until the slow counter becomes zero.
+ */
+void percpu_down_write(struct percpu_rw_semaphore *brw)
+{
+ /* also blocks update_fast_ctr() which checks mutex_is_locked() */
+ mutex_lock(&brw->writer_mutex);
+
+ /*
+ * 1. Ensures mutex_is_locked() is visible to any down_read/up_read
+ * so that update_fast_ctr() can't succeed.
+ *
+ * 2. Ensures we see the result of every previous this_cpu_add() in
+ * update_fast_ctr().
+ *
+ * 3. Ensures that if any reader has exited its critical section via
+ * fast-path, it executes a full memory barrier before we return.
+ * See R_W case in the comment above update_fast_ctr().
+ */
+ synchronize_sched_expedited();
+
+ /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
+ atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
+
+ /* block the new readers completely */
+ down_write(&brw->rw_sem);
+
+ /* wait for all readers to complete their percpu_up_read() */
+ wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
+}
+
+void percpu_up_write(struct percpu_rw_semaphore *brw)
+{
+ /* allow the new readers, but only the slow-path */
+ up_write(&brw->rw_sem);
+
+ /*
+ * Insert the barrier before the next fast-path in down_read,
+ * see W_R case in the comment above update_fast_ctr().
+ */
+ synchronize_sched_expedited();
+ mutex_unlock(&brw->writer_mutex);
+}