[RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count

[Hou Tao <houtao1@huawei.com>]

Under aarch64, __this_cpu_inc() is neither IRQ-safe nor atomic, so
when percpu_up_read() is invoked under IRQ-context (e.g. aio completion),
and it interrupts the process on the same CPU which is invoking
percpu_down_read(), the decreasement on read_count may lost and
the final value of read_count on the CPU will be unexpected
as shown below:

  CPU 0          CPU 0

  io_submit_one
  __sb_start_write
  percpu_down_read
  __this_cpu_inc
  // there is already an inflight IO, so
  // reading *raw_cpu_ptr(&pcp) returns 1
  // half complete, then being interrupted
  *raw_cpu_ptr(&pcp)) += 1

  		nvme_irq
  		nvme_complete_cqes
  		blk_mq_complete_request
  		nvme_pci_complete_rq
  		nvme_complete_rq
  		blk_mq_end_request
  		blk_update_request
  		bio_endio
  		dio_bio_end_aio
  		aio_complete_rw
  		__sb_end_write
  		percpu_up_read
  		*raw_cpu_ptr(&pcp)) -= 1
  		// *raw_cpu_ptr(&pcp) is 0

  // the decreasement is overwritten by the increasement
  *raw_cpu_ptr(&pcp)) += 1
  // the final value is 1 + 1 = 2 instead of 1

Fixing it by using the IRQ-safe helper this_cpu_inc|dec() for
operations on read_count.

Another plausible fix is to state that percpu-rwsem can NOT be
used under IRQ context and convert all users which may
use it under IRQ context.

Signed-off-by: Hou Tao <houtao1@huawei.com>
---
 include/linux/percpu-rwsem.h  | 8 ++++----
 kernel/locking/percpu-rwsem.c | 4 ++--
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index 5e033fe1ff4e9..5fda40f97fe91 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -60,7 +60,7 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
 	 * anything we did within this RCU-sched read-size critical section.
 	 */
 	if (likely(rcu_sync_is_idle(&sem->rss)))
-		__this_cpu_inc(*sem->read_count);
+		this_cpu_inc(*sem->read_count);
 	else
 		__percpu_down_read(sem, false); /* Unconditional memory barrier */
 	/*
@@ -79,7 +79,7 @@ static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
 	 * Same as in percpu_down_read().
 	 */
 	if (likely(rcu_sync_is_idle(&sem->rss)))
-		__this_cpu_inc(*sem->read_count);
+		this_cpu_inc(*sem->read_count);
 	else
 		ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
 	preempt_enable();
@@ -103,7 +103,7 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
 	 * Same as in percpu_down_read().
 	 */
 	if (likely(rcu_sync_is_idle(&sem->rss))) {
-		__this_cpu_dec(*sem->read_count);
+		this_cpu_dec(*sem->read_count);
 	} else {
 		/*
 		 * slowpath; reader will only ever wake a single blocked
@@ -115,7 +115,7 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
 		 * aggregate zero, as that is the only time it matters) they
 		 * will also see our critical section.
 		 */
-		__this_cpu_dec(*sem->read_count);
+		this_cpu_dec(*sem->read_count);
 		rcuwait_wake_up(&sem->writer);
 	}
 	preempt_enable();
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 8bbafe3e5203d..70a32a576f3f2 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -45,7 +45,7 @@ EXPORT_SYMBOL_GPL(percpu_free_rwsem);
 
 static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
 {
-	__this_cpu_inc(*sem->read_count);
+	this_cpu_inc(*sem->read_count);
 
 	/*
 	 * Due to having preemption disabled the decrement happens on
@@ -71,7 +71,7 @@ static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
 	if (likely(!atomic_read_acquire(&sem->block)))
 		return true;
 
-	__this_cpu_dec(*sem->read_count);
+	this_cpu_dec(*sem->read_count);
 
 	/* Prod writer to re-evaluate readers_active_check() */
 	rcuwait_wake_up(&sem->writer);
-- 
2.25.0.4.g0ad7144999