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

Hou Tao
2020-09-16T00:24:19Z

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