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SubjectRe: [PATCH] locking/rwsem: Disable preemption while trying for rwsem lock
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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@quicinc.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 this issue
>> at the place where owner being set/cleared.
>>
>> Signed-off-by: Gokul krishna Krishnakumar <quic_gokukris@quicinc.com>
>> Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
>
> 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@quicinc.com>

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

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