From: Waiman Long longman@redhat.com
commit 2f06f702925b512a95b95dca3855549c047eef58 upstream
The lock handoff bit is added in commit 4f23dbc1e657 ("locking/rwsem: Implement lock handoff to prevent lock starvation") to avoid lock starvation. However, allowing readers to do optimistic spinning does introduce an unlikely scenario where lock starvation can happen.
The lock handoff bit may only be set when a waiter is being woken up. In the case of reader unlock, wakeup happens only when the reader count reaches 0. If there is a continuous stream of incoming readers acquiring read lock via optimistic spinning, it is possible that the reader count may never reach 0 and so the handoff bit will never be asserted.
One way to prevent this scenario from happening is to disallow optimistic spinning if the rwsem is currently owned by readers. If the previous or current owner is a writer, optimistic spinning will be allowed.
If the previous owner is a reader but the reader count has reached 0 before, a wakeup should have been issued. So the handoff mechanism will be kicked in to prevent lock starvation. As a result, it should be OK to do optimistic spinning in this case.
This patch may have some impact on reader performance as it reduces reader optimistic spinning especially if the lock critical sections are short the number of contending readers are small.
Signed-off-by: Waiman Long longman@redhat.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Davidlohr Bueso dbueso@suse.de Link: https://lkml.kernel.org/r/20201121041416.12285-3-longman@redhat.com Cc: stable@vger.kernel.org # 5.10 Signed-off-by: Shaoying Xu shaoyi@amazon.com --- kernel/locking/rwsem.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index 94d0fa715214..029a832f725e 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -1010,16 +1010,27 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable) static struct rw_semaphore __sched * rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, int state) { - long adjustment = -RWSEM_READER_BIAS; + long owner, adjustment = -RWSEM_READER_BIAS; + long rcnt = (count >> RWSEM_READER_SHIFT); struct rwsem_waiter waiter; DEFINE_WAKE_Q(wake_q); bool wake = false;
+ /* + * To prevent a constant stream of readers from starving a sleeping + * waiter, don't attempt optimistic spinning if the lock is currently + * owned by readers. + */ + owner = atomic_long_read(&sem->owner); + if ((owner & RWSEM_READER_OWNED) && (rcnt > 1) && + !(count & RWSEM_WRITER_LOCKED)) + goto queue; + /* * Save the current read-owner of rwsem, if available, and the * reader nonspinnable bit. */ - waiter.last_rowner = atomic_long_read(&sem->owner); + waiter.last_rowner = owner; if (!(waiter.last_rowner & RWSEM_READER_OWNED)) waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;