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

[Mukesh Ojha]

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@xxxxxxxxxx>
> ---
>   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;
>   	}