Re: [PATCH] locking/rwsem: Disable preemption while trying for rwsem lock

[Mukesh Ojha]

Hi Peter,

Thanks for your time in reviewing this patch.

On 9/8/2022 8:02 PM, Peter Zijlstra wrote:
> On Thu, Sep 01, 2022 at 03:58:10PM +0530, Mukesh Ojha wrote:
> > From: Gokul krishna Krishnakumar <quic_gokukris@xxxxxxxxxxx>
> >
> > 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 this issue
> > at the place where owner being set/cleared.
> >
> > Signed-off-by: Gokul krishna Krishnakumar <quic_gokukris@xxxxxxxxxxx>
> > Signed-off-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
>
> This is not a valid SoB chain.

Since this patch of adding preempt disable() at rwsem_write_trylock() is 
originated from Gokul.

Would be adding him in
Original-patch-by: Gokul krishna Krishnakumar <quic_gokukris@xxxxxxxxxxx>

Convert myself to the author/SoB.

Would that be fine ?  please suggest.

> > ---
> >   kernel/locking/rwsem.c | 18 ++++++++++++++----
> >   1 file changed, 14 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> > index 65f0262..3b4b32e 100644
> > --- a/kernel/locking/rwsem.c
> > +++ b/kernel/locking/rwsem.c
> > @@ -251,13 +251,16 @@ 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;
> >   }
> >   
> >   /*
>
> Yes, this part looks ok.
>
> > @@ -686,16 +689,21 @@ enum owner_state {
> >   static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> >   {
> >   	long count = atomic_long_read(&sem->count);
> > +	bool ret = false;
> >   
> > +	preempt_disable();
> >   	while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
> >   		if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
> >   					count | RWSEM_WRITER_LOCKED)) {
> >   			rwsem_set_owner(sem);
> >   			lockevent_inc(rwsem_opt_lock);
> > -			return true;
> > +			ret = true;
> > +			break;
> >   		}
> >   	}
> > -	return false;
> > +
> > +	preempt_enable();
> > +	return ret;
> >   }
> >   
>
> This one I can't follow; afaict this is only called with preemption
> already disabled.

Agreed. Will remove it in v2.

> >   static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> > @@ -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);
> >   }
>
> Yep, that looks good again.
>
> Perhaps the thing to do would be to add:
>
>    lockdep_assert_preemption_disabled()
>
> to rwsem_{set,clear}_owner() and expand the comment there to explain
> that these functions should be in the same preempt-disable section as
> the atomic op that changes sem->count.

Thanks for the suggestion, will add it in v2.

-Mukesh