linux-kernel - Re: [PATCH RESEND 1/3] locking: add rwsem_is_write

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Message-ID: <c9f8bed8-c0d9-4fc2-ac9d-4e7b78418438@lucifer.local>
Date: Fri, 16 Jan 2026 16:29:25 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Zi Yan <ziy@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...nel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeel.butt@...ux.dev>, Jann Horn <jannh@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-rt-devel@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Clark Williams <clrkwllms@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RESEND 1/3] locking: add rwsem_is_write_locked(), update
 non-lockdep asserts

On Fri, Jan 16, 2026 at 10:08:00AM -0500, Zi Yan wrote:
> On 16 Jan 2026, at 8:36, Lorenzo Stoakes wrote:
>
> > As part of adding some additional lock asserts in mm, we wish to be able to
> > determine if a read/write semaphore is write-locked, so add
> > rwsem_is_write_locked() to do the write-lock equivalent of
> > rwsem_is_locked().
> >
> > While we're here, update rwsem_assert_[write_]held_nolockdep() to utilise
> > the rwsem_is_[write_]locked() helpers directly to reduce code duplication,
> > and also update rwsem_is_locked() to take a const rwsem and return a
> > boolean.
> >
> > This patch also updates the CONFIG_PREEMPT_RT helpers to do the same thing
> > there.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > ---
> >  include/linux/rwsem.h | 20 +++++++++++++++-----
> >  1 file changed, 15 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> > index f1aaf676a874..b25b7944ad99 100644
> > --- a/include/linux/rwsem.h
> > +++ b/include/linux/rwsem.h
> > @@ -70,19 +70,24 @@ struct rw_semaphore {
> >  #define RWSEM_WRITER_LOCKED		(1UL << 0)
> >  #define __RWSEM_COUNT_INIT(name)	.count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
> >
> > -static inline int rwsem_is_locked(struct rw_semaphore *sem)
> > +static inline bool rwsem_is_locked(const struct rw_semaphore *sem)
> >  {
> >  	return atomic_long_read(&sem->count) != RWSEM_UNLOCKED_VALUE;
> >  }
> >
> > +static inline bool rwsem_is_write_locked(const struct rw_semaphore *sem)
> > +{
> > +	return atomic_long_read(&sem->count) & RWSEM_WRITER_LOCKED;
> > +}
> > +
> >  static inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
> >  {
> > -	WARN_ON(atomic_long_read(&sem->count) == RWSEM_UNLOCKED_VALUE);
> > +	WARN_ON(!rwsem_is_locked(sem));
> >  }
> >
> >  static inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
> >  {
> > -	WARN_ON(!(atomic_long_read(&sem->count) & RWSEM_WRITER_LOCKED));
> > +	WARN_ON(!rwsem_is_write_locked(sem));
> >  }
> >
> >  /* Common initializer macros and functions */
> > @@ -174,11 +179,16 @@ do {								\
> >  	__init_rwsem((sem), #sem, &__key);			\
> >  } while (0)
> >
> > -static __always_inline int rwsem_is_locked(const struct rw_semaphore *sem)
> > +static __always_inline bool rwsem_is_locked(const struct rw_semaphore *sem)
> >  {
> >  	return rw_base_is_locked(&sem->rwbase);
> >  }
> >
> > +static __always_inline bool rwsem_is_write_locked(const struct rw_semaphore *sem)
> > +{
> > +	return rw_base_is_write_locked(&sem->rwbase);
> > +}
> > +
> >  static __always_inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
> >  {
> >  	WARN_ON(!rwsem_is_locked(sem));
> > @@ -186,7 +196,7 @@ static __always_inline void rwsem_assert_held_nolockdep(const struct rw_semaphor
> >
> >  static __always_inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
> >  {
> > -	WARN_ON(!rw_base_is_write_locked(&sem->rwbase));
> > +	WARN_ON(!rwsem_is_write_locked(sem));
>
> I thought it was wrong since rwsem_is_write_locked() at the top reads ->count
> instead of ->rwbase until I see there is another rwsem_is_write_locked() above.

:)

>
>
> >  }
> >
> >  static __always_inline int rwsem_is_contended(struct rw_semaphore *sem)
> > --
>
> Reviewed-by: Zi Yan <ziy@...dia.com>

Thanks!

>
>
> --
> Best Regards,
> Yan, Zi