1 files changed, 192 insertions, 0 deletions
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
new file mode 100644
index 000000000..883cf1b92
--- /dev/null
+++ b/kernel/locking/percpu-rwsem.c
@@ -0,0 +1,192 @@
+#include <linux/atomic.h>
+#include <linux/rwsem.h>
+#include <linux/percpu.h>
+#include <linux/lockdep.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 *sem,
+			const char *name, struct lock_class_key *rwsem_key)
+{
+	sem->read_count = alloc_percpu(int);
+	if (unlikely(!sem->read_count))
+		return -ENOMEM;
+
+	/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
+	rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
+	__init_rwsem(&sem->rw_sem, name, rwsem_key);
+	rcuwait_init(&sem->writer);
+	sem->readers_block = 0;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
+
+void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
+{
+	/*
+	 * XXX: temporary kludge. The error path in alloc_super()
+	 * assumes that percpu_free_rwsem() is safe after kzalloc().
+	 */
+	if (!sem->read_count)
+		return;
+
+	rcu_sync_dtor(&sem->rss);
+	free_percpu(sem->read_count);
+	sem->read_count = NULL; /* catch use after free bugs */
+}
+EXPORT_SYMBOL_GPL(percpu_free_rwsem);
+
+int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
+{
+	/*
+	 * Due to having preemption disabled the decrement happens on
+	 * the same CPU as the increment, avoiding the
+	 * increment-on-one-CPU-and-decrement-on-another problem.
+	 *
+	 * If the reader misses the writer's assignment of readers_block, then
+	 * the writer is guaranteed to see the reader's increment.
+	 *
+	 * Conversely, any readers that increment their sem->read_count after
+	 * the writer looks are guaranteed to see the readers_block value,
+	 * which in turn means that they are guaranteed to immediately
+	 * decrement their sem->read_count, so that it doesn't matter that the
+	 * writer missed them.
+	 */
+
+	smp_mb(); /* A matches D */
+
+	/*
+	 * If !readers_block the critical section starts here, matched by the
+	 * release in percpu_up_write().
+	 */
+	if (likely(!smp_load_acquire(&sem->readers_block)))
+		return 1;
+
+	/*
+	 * Per the above comment; we still have preemption disabled and
+	 * will thus decrement on the same CPU as we incremented.
+	 */
+	__percpu_up_read(sem);
+
+	if (try)
+		return 0;
+
+	/*
+	 * We either call schedule() in the wait, or we'll fall through
+	 * and reschedule on the preempt_enable() in percpu_down_read().
+	 */
+	preempt_enable_no_resched();
+
+	/*
+	 * Avoid lockdep for the down/up_read() we already have them.
+	 */
+	__down_read(&sem->rw_sem);
+	this_cpu_inc(*sem->read_count);
+	__up_read(&sem->rw_sem);
+
+	preempt_disable();
+	return 1;
+}
+EXPORT_SYMBOL_GPL(__percpu_down_read);
+
+void __percpu_up_read(struct percpu_rw_semaphore *sem)
+{
+	smp_mb(); /* B matches C */
+	/*
+	 * In other words, if they see our decrement (presumably to aggregate
+	 * zero, as that is the only time it matters) they will also see our
+	 * critical section.
+	 */
+	__this_cpu_dec(*sem->read_count);
+
+	/* Prod writer to recheck readers_active */
+	rcuwait_wake_up(&sem->writer);
+}
+EXPORT_SYMBOL_GPL(__percpu_up_read);
+
+#define per_cpu_sum(var)						\
+({									\
+	typeof(var) __sum = 0;						\
+	int cpu;							\
+	compiletime_assert_atomic_type(__sum);				\
+	for_each_possible_cpu(cpu)					\
+		__sum += per_cpu(var, cpu);				\
+	__sum;								\
+})
+
+/*
+ * Return true if the modular sum of the sem->read_count per-CPU variable is
+ * zero.  If this sum is zero, then it is stable due to the fact that if any
+ * newly arriving readers increment a given counter, they will immediately
+ * decrement that same counter.
+ */
+static bool readers_active_check(struct percpu_rw_semaphore *sem)
+{
+	if (per_cpu_sum(*sem->read_count) != 0)
+		return false;
+
+	/*
+	 * If we observed the decrement; ensure we see the entire critical
+	 * section.
+	 */
+
+	smp_mb(); /* C matches B */
+
+	return true;
+}
+
+void percpu_down_write(struct percpu_rw_semaphore *sem)
+{
+	/* Notify readers to take the slow path. */
+	rcu_sync_enter(&sem->rss);
+
+	down_write(&sem->rw_sem);
+
+	/*
+	 * Notify new readers to block; up until now, and thus throughout the
+	 * longish rcu_sync_enter() above, new readers could still come in.
+	 */
+	WRITE_ONCE(sem->readers_block, 1);
+
+	smp_mb(); /* D matches A */
+
+	/*
+	 * If they don't see our writer of readers_block, then we are
+	 * guaranteed to see their sem->read_count increment, and therefore
+	 * will wait for them.
+	 */
+
+	/* Wait for all now active readers to complete. */
+	rcuwait_wait_event(&sem->writer, readers_active_check(sem));
+}
+EXPORT_SYMBOL_GPL(percpu_down_write);
+
+void percpu_up_write(struct percpu_rw_semaphore *sem)
+{
+	/*
+	 * Signal the writer is done, no fast path yet.
+	 *
+	 * One reason that we cannot just immediately flip to readers_fast is
+	 * that new readers might fail to see the results of this writer's
+	 * critical section.
+	 *
+	 * Therefore we force it through the slow path which guarantees an
+	 * acquire and thereby guarantees the critical section's consistency.
+	 */
+	smp_store_release(&sem->readers_block, 0);
+
+	/*
+	 * Release the write lock, this will allow readers back in the game.
+	 */
+	up_write(&sem->rw_sem);
+
+	/*
+	 * Once this completes (at least one RCU-sched grace period hence) the
+	 * reader fast path will be available again. Safe to use outside the
+	 * exclusive write lock because its counting.
+	 */
+	rcu_sync_exit(&sem->rss);
+}
+EXPORT_SYMBOL_GPL(percpu_up_write);