Re: [PATCH v2 2/2] locking/rwsem: Limit # of null owner retries for handoff writer

[Mukesh Ojha <quic_mojha@quicinc.com>]

Hi,

On 10/12/2022 7:03 PM, Waiman Long wrote:
> Commit 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically
> spin on owner") assumes that when the owner field is changed to NULL,
> the lock will become free soon.  That assumption may not be correct
> especially if the handoff writer doing the spinning is a RT task which
> may preempt another task from completing its action of either freeing
> the rwsem or properly setting up owner.
> 
> To prevent this live lock scenario, we have to limit the number of
> trylock attempts without sleeping. The current limit is now set to 8
> to allow enough time for the other task to hopefully complete its action.
> 
> By adding new lock events to track the number of NULL owner retries with
> handoff flag set before a successful trylock when running a 96 threads
> locking microbenchmark with equal number of readers and writers running
> on a 2-core 96-thread system for 15 seconds, the following stats are
> obtained. Note that none of locking threads are RT tasks.
> 
>    Retries of successful trylock    Count
>    -----------------------------    -----
>               1                     1738
>               2                       19
>               3                       11
>               4                        2
>               5                        1
>               6                        1
>               7                        1
>               8                        0
>               X                        1
> 
> The last row is the one failed attempt that needs more than 8 retries.
> So a retry count maximum of 8 should capture most of them if no RT task
> is in the mix.
> 
> Fixes: 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically spin on owner")
> Reported-by: Mukesh Ojha <quic_mojha@quicinc.com>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/locking/rwsem.c | 18 ++++++++++++++++--
>   1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 3839b38608da..12eb093328f2 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -1123,6 +1123,7 @@ static struct rw_semaphore __sched *
>   rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>   {
>   	struct rwsem_waiter waiter;
> +	int null_owner_retries;
>   	DEFINE_WAKE_Q(wake_q);
>   
>   	/* do optimistic spinning and steal lock if possible */
> @@ -1164,7 +1165,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>   	set_current_state(state);
>   	trace_contention_begin(sem, LCB_F_WRITE);
>   
> -	for (;;) {
> +	for (null_owner_retries = 0;;) {
>   		if (rwsem_try_write_lock(sem, &waiter)) {
>   			/* rwsem_try_write_lock() implies ACQUIRE on success */
>   			break;
> @@ -1190,8 +1191,21 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>   			owner_state = rwsem_spin_on_owner(sem);
>   			preempt_enable();
>   
> -			if (owner_state == OWNER_NULL)
> +			/*
> +			 * owner is NULL doesn't guarantee the lock is free.
> +			 * An incoming reader will temporarily increment the
> +			 * reader count without changing owner and the
> +			 * rwsem_try_write_lock() will fails if the reader
> +			 * is not able to decrement it in time. Allow 8
> +			 * trylock attempts when hitting a NULL owner before
> +			 * going to sleep.
> +			 */
> +			if ((owner_state == OWNER_NULL) &&
> +			    (null_owner_retries < 8)) {

define MAX_NULL_OWNER_RETRY		8

??

> +				null_owner_retries++;
>   				goto trylock_again;
> +			}
> +			null_owner_retries = 0;
>   		}
>   

Thanks for considering this patch.

LGTM.
Reviewed-and-Tested-by: Mukesh Ojha <quic_mojha@quicinc.com>

-Mukesh

>   		schedule();