linux-kernel - Re: [PATCH v2] locking/rwsem: Disable preemption while trying for rwsem lock

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Date:   Thu, 8 Sep 2022 14:30:09 -0400
From:   Waiman Long <longman@...hat.com>
To:     Mukesh Ojha <quic_mojha@...cinc.com>, peterz@...radead.org,
        mingo@...hat.com, will@...nel.org, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org,
        Gokul krishna Krishnakumar <quic_gokukris@...cinc.com>
Subject: Re: [PATCH v2] locking/rwsem: Disable preemption while trying for
 rwsem lock

On 9/8/22 14:24, Mukesh Ojha wrote:
> From: Gokul krishna Krishnakumar <quic_gokukris@...cinc.com>
>
> Make the region inside the rwsem_write_trylock non preemptible.
>
> We observe RT task is hogging CPU when trying to acquire rwsem lock
> which was acquired by a kworker task but before the rwsem owner was set.
>
> Here is the scenario:
> 1. CFS task (affined to a particular CPU) takes rwsem lock.
>
> 2. CFS task gets preempted by a RT task before setting owner.
>
> 3. RT task (FIFO) is trying to acquire the lock, but spinning until
> RT throttling happens for the lock as the lock was taken by CFS task.
>
> This patch attempts to fix the above issue by disabling preemption
> until owner is set for the lock. While at it also fix the issues
> at the places where rwsem_{set,clear}_owner() are called.
>
> This also adds lockdep annotation of preemption disable in
> rwsem_{set,clear}_owner() on Peter Z. suggestion.
>
> Signed-off-by: Gokul krishna Krishnakumar <quic_gokukris@...cinc.com>
> Signed-off-by: Mukesh Ojha <quic_mojha@...cinc.com>
> ---
> Changes in v2:
>   - Remove preempt disable code in rwsem_try_write_lock_unqueued()
>   - Addressed suggestion from Peter Z.
>   - Modified commit text
>   kernel/locking/rwsem.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 65f0262..4487359 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -133,14 +133,19 @@
>    * the owner value concurrently without lock. Read from owner, however,
>    * may not need READ_ONCE() as long as the pointer value is only used
>    * for comparison and isn't being dereferenced.
> + *
> + * Both rwsem_{set,clear}_owner() functions should be in the same
> + * preempt disable section as the atomic op that changes sem->count.
>    */
>   static inline void rwsem_set_owner(struct rw_semaphore *sem)
>   {
> +	lockdep_assert_preemption_disabled();
>   	atomic_long_set(&sem->owner, (long)current);
>   }
>   
>   static inline void rwsem_clear_owner(struct rw_semaphore *sem)
>   {
> +	lockdep_assert_preemption_disabled();
>   	atomic_long_set(&sem->owner, 0);
>   }
>   
> @@ -251,13 +256,16 @@ static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cntp)
>   static inline bool rwsem_write_trylock(struct rw_semaphore *sem)
>   {
>   	long tmp = RWSEM_UNLOCKED_VALUE;
> +	bool ret = false;
>   
> +	preempt_disable();
>   	if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, RWSEM_WRITER_LOCKED)) {
>   		rwsem_set_owner(sem);
> -		return true;
> +		ret = true;
>   	}
>   
> -	return false;
> +	preempt_enable();
> +	return ret;
>   }
>   
>   /*
> @@ -1352,8 +1360,10 @@ static inline void __up_write(struct rw_semaphore *sem)
>   	DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
>   			    !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
>   
> +	preempt_disable();
>   	rwsem_clear_owner(sem);
>   	tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
> +	preempt_enable();
>   	if (unlikely(tmp & RWSEM_FLAG_WAITERS))
>   		rwsem_wake(sem);
>   }
Reviewed-by: Waiman Long <longman@...hat.com>

Cheers,
Longman