#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
--- /dev/null
+#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);
+}
projects / GitHub / LineageOS / android_kernel_samsung_universal7580.git / commitdiff