linux-kernel - Re: [PATCH v2] Locking: Let PREEMPT_RT compile again with new rwsem asserts.

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Date: Tue, 19 Mar 2024 20:58:13 -0400
From: Waiman Long <longman@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
 Boqun Feng <boqun.feng@...il.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Will Deacon <will@...nel.org>, Clark Williams <williams@...hat.com>
Subject: Re: [PATCH v2] Locking: Let PREEMPT_RT compile again with new rwsem
 asserts.

On 3/19/24 14:20, Sebastian Andrzej Siewior wrote:
> The commit cited below broke the build for PREEMPT_RT because
> rwsem_assert_held_write_nolockdep() passes a struct rw_semaphore but
> rw_base_assert_held_write() expects struct rwbase_rt. Fixing the type
> alone leads to the problem that WARN_ON() is not found because bug.h is
> missing.
>
> In order to resolve this:
> - Keep the assert (WARN_ON()) in rwsem.h (not rwbase_rt.h)
> - Make rwsem_assert_held_write_nolockdep() do the implementation
>    specific (rw_base) writer check.
> - Replace the "inline" with __always_inline which was used before.
>
> Fixes: f70405afc99b1 ("locking: Add rwsem_assert_held() and rwsem_assert_held_write()")
> Reported-by: Clark Williams <williams@...hat.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> v1…v2:
>    - Merge rwsem_held_write() into rwsem_assert_held_write_nolockdep() as
>      per Waiman Long / Matthew Wilcox.
>
>   include/linux/rwbase_rt.h | 4 ++--
>   include/linux/rwsem.h     | 6 +++---
>   2 files changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/rwbase_rt.h b/include/linux/rwbase_rt.h
> index 29c4e4f243e47..f2394a409c9d5 100644
> --- a/include/linux/rwbase_rt.h
> +++ b/include/linux/rwbase_rt.h
> @@ -31,9 +31,9 @@ static __always_inline bool rw_base_is_locked(const struct rwbase_rt *rwb)
>   	return atomic_read(&rwb->readers) != READER_BIAS;
>   }
>   
> -static inline void rw_base_assert_held_write(const struct rwbase_rt *rwb)
> +static __always_inline bool rw_base_is_write_locked(const struct rwbase_rt *rwb)
>   {
> -	WARN_ON(atomic_read(&rwb->readers) != WRITER_BIAS);
> +	return atomic_read(&rwb->readers) == WRITER_BIAS;
>   }
>   
>   static __always_inline bool rw_base_is_contended(const struct rwbase_rt *rwb)
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 4f1c18992f768..c8b543d428b0a 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -167,14 +167,14 @@ static __always_inline int rwsem_is_locked(const struct rw_semaphore *sem)
>   	return rw_base_is_locked(&sem->rwbase);
>   }
>   
> -static inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
> +static __always_inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
>   {
>   	WARN_ON(!rwsem_is_locked(sem));
>   }
>   
> -static inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
> +static __always_inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
>   {
> -	rw_base_assert_held_write(sem);
> +	WARN_ON(!rw_base_is_write_locked(&sem->rwbase));
>   }
>   
>   static __always_inline int rwsem_is_contended(struct rw_semaphore *sem)
Reviewed-by: Waiman Long <longman@...hat.com>