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

[Dmitry Ilvokhin <d@ilvokhin.com>]

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.