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

[Waiman Long <longman@redhat.com>]

On 3/5/24 06:04, Aaro Koskinen wrote:
> Hi,
>
> It seems this patch (commit 48dfb5d2560d) is missing from
> at least 5.15 stable tree.
>
> Based on quick test, it seems to fix an issue where system locks up
> easily when RT throttling is disabled, and also it applies cleanly, so
> I think it should be good to have it 5.15?

You need to cc stable as the locking maintainers are not responsible for 
merging patches to stable trees.

Cheers,
Longman

>
> A.
>
> On Thu, Sep 08, 2022 at 11:54:27PM +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 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@quicinc.com>
>> Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.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);
>>   }
>> -- 
>> 2.7.4
>>
>>