/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
__init_rwsem(&brw->rw_sem, name, rwsem_key);
- atomic_set(&brw->write_ctr, 0);
+ rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
atomic_set(&brw->slow_read_ctr, 0);
init_waitqueue_head(&brw->write_waitq);
return 0;
}
+EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
{
+ /*
+ * XXX: temporary kludge. The error path in alloc_super()
+ * assumes that percpu_free_rwsem() is safe after kzalloc().
+ */
+ if (!brw->fast_read_ctr)
+ return;
+
+ rcu_sync_dtor(&brw->rss);
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 (atomic_read(write_ctr) == 0) 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
+ * This is the fast-path for down_read/up_read. If it succeeds we rely
+ * on the barriers provided by rcu_sync_enter/exit; see the comments in
+ * percpu_down_write() and percpu_up_write().
*
* 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, atomic_read(write_ctr) 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;
+ bool success;
preempt_disable();
- if (likely(!atomic_read(&brw->write_ctr))) {
+ success = rcu_sync_is_idle(&brw->rss);
+ if (likely(success))
__this_cpu_add(*brw->fast_read_ctr, val);
- success = true;
- }
preempt_enable();
return success;
void percpu_down_read(struct percpu_rw_semaphore *brw)
{
might_sleep();
- if (likely(update_fast_ctr(brw, +1))) {
- rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
+ rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
+
+ if (likely(update_fast_ctr(brw, +1)))
return;
- }
- down_read(&brw->rw_sem);
+ /* Avoid rwsem_acquire_read() and rwsem_release() */
+ __down_read(&brw->rw_sem);
atomic_inc(&brw->slow_read_ctr);
- /* avoid up_read()->rwsem_release() */
__up_read(&brw->rw_sem);
}
+EXPORT_SYMBOL_GPL(percpu_down_read);
int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
{
if (atomic_dec_and_test(&brw->slow_read_ctr))
wake_up_all(&brw->write_waitq);
}
+EXPORT_SYMBOL_GPL(percpu_up_read);
static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
{
return sum;
}
-/*
- * A writer increments ->write_ctr to force the readers to switch to the
- * slow mode, note the atomic_read() 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)
{
- /* tell update_fast_ctr() there is a pending writer */
- atomic_inc(&brw->write_ctr);
/*
- * 1. Ensures that write_ctr != 0 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().
+ * Make rcu_sync_is_idle() == F and thus disable the fast-path in
+ * percpu_down_read() and percpu_up_read(), and wait for gp pass.
*
- * 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().
+ * The latter synchronises us with the preceding readers which used
+ * the fast-past, so we can not miss the result of __this_cpu_add()
+ * or anything else inside their criticial sections.
*/
- synchronize_sched_expedited();
+ rcu_sync_enter(&brw->rss);
/* exclude other writers, and 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));
}
+EXPORT_SYMBOL_GPL(percpu_down_write);
void percpu_up_write(struct percpu_rw_semaphore *brw)
{
/* release the lock, but the readers can't use the fast-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().
+ * Enable the fast-path in percpu_down_read() and percpu_up_read()
+ * but only after another gp pass; this adds the necessary barrier
+ * to ensure the reader can't miss the changes done by us.
*/
- synchronize_sched_expedited();
- /* the last writer unblocks update_fast_ctr() */
- atomic_dec(&brw->write_ctr);
+ rcu_sync_exit(&brw->rss);
}
+EXPORT_SYMBOL_GPL(percpu_up_write);
projects / karo-tx-linux.git / blobdiff