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

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Message-ID: <84A469CA-3497-42A3-A3BB-42B7CB1195DC@nvidia.com>
Date: Fri, 16 Jan 2026 10:08:00 -0500
From: Zi Yan <ziy@...dia.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 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>


--
Best Regards,
Yan, Zi