linux-kernel - Re: [PATCH v4 3/5] locking/rwsem: Change waiter->hanodff_set to a handoff

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Date:   Mon, 24 Oct 2022 13:25:30 -0500
From:   john.p.donnelly@...cle.com
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, Hillf Danton <hdanton@...a.com>,
        Mukesh Ojha <quic_mojha@...cinc.com>,
        Ting11 Wang 王婷 <wangting11@...omi.com>,
        Jack Vogel <jack.vogel@...cle.com>,
        Jorge Lopez <jorge.jo.lopez@...cle.com>,
        Vijayendra Suman <vijayendra.suman@...cle.com>,
        John Donnelly <john.p.donnelly@...cle.com>
Subject: Re: [PATCH v4 3/5] locking/rwsem: Change waiter->hanodff_set to a
 handoff_state enum

On 10/24/22 12:44 PM, Waiman Long wrote:
> Change the boolean waiter->handoff_set to an enum type so that we can
> have more states in some later patches. Also use READ_ONCE() outside
> wait_lock critical sections for read and WRITE_ONCE() inside wait_lock
> critical sections for write for proper synchronization. There is no
> functional change.

Hi,

Do we need


Fixes: d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling more 
consistent")

Fixes: 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically 
spin on owner")

Cc: stable@...r.kernel.org


clauses added to patches 3,4,5 so they are all picked up in one series ?

Thank you,

John.



> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>   kernel/locking/rwsem.c | 21 +++++++++++++--------
>   1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index c68d76fc8c68..a8bfc905637a 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -338,12 +338,17 @@ enum rwsem_waiter_type {
>   	RWSEM_WAITING_FOR_READ
>   };
>   
> +enum rwsem_handoff_state {
> +	HANDOFF_NONE = 0,
> +	HANDOFF_REQUESTED,
> +};
> +
>   struct rwsem_waiter {
>   	struct list_head list;
>   	struct task_struct *task;
>   	enum rwsem_waiter_type type;
> +	enum rwsem_handoff_state handoff_state;
>   	unsigned long timeout;
> -	bool handoff_set;
>   };
>   #define rwsem_first_waiter(sem) \
>   	list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
> @@ -470,7 +475,7 @@ static void rwsem_mark_wake(struct rw_semaphore *sem,
>   					adjustment -= RWSEM_FLAG_HANDOFF;
>   					lockevent_inc(rwsem_rlock_handoff);
>   				}
> -				waiter->handoff_set = true;
> +				WRITE_ONCE(waiter->handoff_state, HANDOFF_REQUESTED);
>   			}
>   
>   			atomic_long_add(-adjustment, &sem->count);
> @@ -622,7 +627,7 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
>   			 * waiter is the one that set it. Otherwisee, we
>   			 * still try to acquire the rwsem.
>   			 */
> -			if (first->handoff_set && (waiter != first))
> +			if (first->handoff_state && (waiter != first))
>   				return false;
>   		}
>   
> @@ -650,11 +655,11 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
>   
>   	/*
>   	 * We have either acquired the lock with handoff bit cleared or set
> -	 * the handoff bit. Only the first waiter can have its handoff_set
> +	 * the handoff bit. Only the first waiter can have its handoff_state
>   	 * set here to enable optimistic spinning in slowpath loop.
>   	 */
>   	if (new & RWSEM_FLAG_HANDOFF) {
> -		first->handoff_set = true;
> +		WRITE_ONCE(first->handoff_state, HANDOFF_REQUESTED);
>   		lockevent_inc(rwsem_wlock_handoff);
>   		return false;
>   	}
> @@ -1043,7 +1048,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
>   	waiter.task = current;
>   	waiter.type = RWSEM_WAITING_FOR_READ;
>   	waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
> -	waiter.handoff_set = false;
> +	waiter.handoff_state = HANDOFF_NONE;
>   
>   	raw_spin_lock_irq(&sem->wait_lock);
>   	if (list_empty(&sem->wait_list)) {
> @@ -1131,7 +1136,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>   	waiter.task = current;
>   	waiter.type = RWSEM_WAITING_FOR_WRITE;
>   	waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
> -	waiter.handoff_set = false;
> +	waiter.handoff_state = HANDOFF_NONE;
>   
>   	raw_spin_lock_irq(&sem->wait_lock);
>   	rwsem_add_waiter(sem, &waiter);
> @@ -1176,7 +1181,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
>   		 * In this case, we attempt to acquire the lock again
>   		 * without sleeping.
>   		 */
> -		if (waiter.handoff_set) {
> +		if (READ_ONCE(waiter.handoff_state)) {
>   			enum owner_state owner_state;
>   
>   			preempt_disable();