Re: [PATCH v4 3/5] locking: Add contended_release tracepoint to sleepable locks

[Usama Arif <usama.arif@linux.dev>]

On 31/03/2026 15:16, Dmitry Ilvokhin wrote:
> On Tue, Mar 31, 2026 at 03:34:50AM -0700, Usama Arif wrote:
>> On Thu, 26 Mar 2026 15:10:02 +0000 Dmitry Ilvokhin <d@ilvokhin.com> wrote:
>>
>>> Add the contended_release trace event. This tracepoint fires on the
>>> holder side when a contended lock is released, complementing the
>>> existing contention_begin/contention_end tracepoints which fire on the
>>> waiter side.
>>>
>>> This enables correlating lock hold time under contention with waiter
>>> events by lock address.
>>>
>>> Add trace_contended_release() calls to the slowpath unlock paths of
>>> sleepable locks: mutex, rtmutex, semaphore, rwsem, percpu-rwsem, and
>>> RT-specific rwbase locks.
>>>
>>> Where possible, trace_contended_release() fires before the lock is
>>> released and before the waiter is woken. For some lock types, the
>>> tracepoint fires after the release but before the wake. Making the
>>> placement consistent across all lock types is not worth the added
>>> complexity.
>>>
>>> For reader/writer locks, the tracepoint fires for every reader releasing
>>> while a writer is waiting, not only for the last reader.
>>>
>>> Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>
>>> ---
>>>  include/trace/events/lock.h   | 17 +++++++++++++++++
>>>  kernel/locking/mutex.c        |  4 ++++
>>>  kernel/locking/percpu-rwsem.c | 11 +++++++++++
>>>  kernel/locking/rtmutex.c      |  1 +
>>>  kernel/locking/rwbase_rt.c    |  6 ++++++
>>>  kernel/locking/rwsem.c        | 10 ++++++++--
>>>  kernel/locking/semaphore.c    |  4 ++++
>>>  7 files changed, 51 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/trace/events/lock.h b/include/trace/events/lock.h
>>> index da978f2afb45..1ded869cd619 100644
>>> --- a/include/trace/events/lock.h
>>> +++ b/include/trace/events/lock.h
>>> @@ -137,6 +137,23 @@ TRACE_EVENT(contention_end,
>>>  	TP_printk("%p (ret=%d)", __entry->lock_addr, __entry->ret)
>>>  );
>>>  
>>> +TRACE_EVENT(contended_release,
>>> +
>>> +	TP_PROTO(void *lock),
>>> +
>>> +	TP_ARGS(lock),
>>> +
>>> +	TP_STRUCT__entry(
>>> +		__field(void *, lock_addr)
>>> +	),
>>> +
>>> +	TP_fast_assign(
>>> +		__entry->lock_addr = lock;
>>> +	),
>>> +
>>> +	TP_printk("%p", __entry->lock_addr)
>>> +);
>>> +
>>>  #endif /* _TRACE_LOCK_H */
>>>  
>>>  /* This part must be outside protection */
>>> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
>>> index 427187ff02db..6c2c9312eb8f 100644
>>> --- a/kernel/locking/mutex.c
>>> +++ b/kernel/locking/mutex.c
>>> @@ -997,6 +997,9 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
>>>  		wake_q_add(&wake_q, next);
>>>  	}
>>>  
>>> +	if (trace_contended_release_enabled() && waiter)
>>> +		trace_contended_release(lock);
>>> +
>>
>> This won't compile? waiter is declared in the if block, so you are using
>> it outside scope here.
>>
> 
> Thanks for the feedback, Usama.
> 
> waiter is declared at function scope, right on top. It's also assigned
> before the if block, so it's still in scope at the tracepoint.

Ah ok, I was reviewing on top of mm-new branch from today where waiter
is declared in the if block. Probably something changed related to
locking/tracing and its not in mm-new yet.