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SubjectRe: [PATCH] locking/rwsem: Prevent non-first waiter from spinning in down_write() slowpath
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Hi,

On 9/29/2022 11:34 PM, Waiman Long wrote:
> A non-first waiter can potentially spin in the for loop of
> rwsem_down_write_slowpath() without sleeping but fail to acquire the
> lock even if the rwsem is free if the following sequence happens:
>
> Non-first waiter First waiter Lock holder
> ---------------- ------------ -----------
> Acquire wait_lock
> rwsem_try_write_lock():
> Set handoff bit if RT or
> wait too long
> Set waiter->handoff_set
> Release wait_lock
> Acquire wait_lock
> Inherit waiter->handoff_set
> Release wait_lock
> Clear owner
> Release lock
> if (waiter.handoff_set) {
> rwsem_spin_on_owner(();
> if (OWNER_NULL)
> goto trylock_again;
> }
> trylock_again:
> Acquire wait_lock
> rwsem_try_write_lock():
> if (first->handoff_set && (waiter != first))
> return false;
> Release wait_lock
>
> It is especially problematic if the non-first waiter is an RT task and
> it is running on the same CPU as the first waiter as this can lead to
> live lock.
>
> Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more consistent")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> kernel/locking/rwsem.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 65f0262f635e..ad676e99e0b3 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -628,6 +628,11 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
> new = count;
>
> if (count & RWSEM_LOCK_MASK) {
> + /*
> + * A waiter (first or not) can set the handoff bit
> + * if it is an RT task or wait in the wait queue
> + * for too long.
> + */
> if (has_handoff || (!rt_task(waiter->task) &&
> !time_after(jiffies, waiter->timeout)))

Not related to this issue, however wanted to understand the idea about this.

If RT task comes in any order either come first or later it is setting
the RWSEM_FLAG_HANDOFF bit.
So, here we are giving some priority right a way to RT task however it
can not get waiter->handoff_set=true since it is not the first
waiter.(after this patch), is it not conflicting ?


Why can't we just keep like as below and not set
new |= RWSEM_FLAG_HANDOFF; and return false from here.

--------------0<------------------------------------
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 65f0262..dbe3e16 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -628,8 +628,8 @@ static inline bool rwsem_try_write_lock(struct
rw_semaphore *sem,
new = count;

if (count & RWSEM_LOCK_MASK) {
- if (has_handoff || (!rt_task(waiter->task) &&
- !time_after(jiffies,
waiter->timeout)))
+ if (has_handoff || (rt_task(waiter->task) &&
waiter != first) ||
+ (!rt_task(waiter->task) &&
!time_after(jiffies, waiter->timeout)))
return false;


-Mukesh

> return false;
> @@ -643,11 +648,13 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
> } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
>
> /*
> - * We have either acquired the lock with handoff bit cleared or
> - * set the handoff bit.
> + * We have either acquired the lock with handoff bit cleared or set
> + * the handoff bit. Only the first waiter can have its handoff_set
> + * set here to enable optimistic spinning in slowpath loop.
> */
> if (new & RWSEM_FLAG_HANDOFF) {
> - waiter->handoff_set = true;
> + if (waiter == first)
> + waiter->handoff_set = true;
> lockevent_inc(rwsem_wlock_handoff);
> return false;
> }

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