Re: [PATCH v3 1/4] locking: Add rwsem_assert_held() and rwsem_assert_held_write()

[Waiman Long <longman@xxxxxxxxxx>]

On 11/10/23 15:41, Matthew Wilcox (Oracle) wrote:
> Modelled after lockdep_assert_held() and lockdep_assert_held_write(),
> but are always active, even when lockdep is disabled.  Of course, they
> don't test that _this_ thread is the owner, but it's sufficient to catch
> many bugs and doesn't incur the same performance penalty as lockdep.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
>   include/linux/rwbase_rt.h |  9 ++++++--
>   include/linux/rwsem.h     | 46 ++++++++++++++++++++++++++++++++++-----
>   2 files changed, 48 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/rwbase_rt.h b/include/linux/rwbase_rt.h
> index 1d264dd08625..a04acd85705b 100644
> --- a/include/linux/rwbase_rt.h
> +++ b/include/linux/rwbase_rt.h
> @@ -26,12 +26,17 @@ struct rwbase_rt {
>   	} while (0)
>   
>   
> -static __always_inline bool rw_base_is_locked(struct rwbase_rt *rwb)
> +static __always_inline bool rw_base_is_locked(const struct rwbase_rt *rwb)
>   {
>   	return atomic_read(&rwb->readers) != READER_BIAS;
>   }
>   
> -static __always_inline bool rw_base_is_contended(struct rwbase_rt *rwb)
> +static inline void rw_base_assert_held_write(const struct rwbase_rt *rwb)
> +{
> +	BUG_ON(atomic_read(&rwb->readers) != WRITER_BIAS);
> +}
> +
> +static __always_inline bool rw_base_is_contended(const struct rwbase_rt *rwb)
>   {
>   	return atomic_read(&rwb->readers) > 0;
>   }
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index 1dd530ce8b45..b5b34cca86f3 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -66,14 +66,24 @@ struct rw_semaphore {
>   #endif
>   };
>   
> -/* In all implementations count != 0 means locked */
> +#define RWSEM_UNLOCKED_VALUE		0UL
> +#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)
>   {
> -	return atomic_long_read(&sem->count) != 0;
> +	return atomic_long_read(&sem->count) != RWSEM_UNLOCKED_VALUE;
>   }
>   
> -#define RWSEM_UNLOCKED_VALUE		0L
> -#define __RWSEM_COUNT_INIT(name)	.count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
> +static inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
> +{
> +	WARN_ON(atomic_long_read(&sem->count) == RWSEM_UNLOCKED_VALUE);
> +}
That is not correct. You mean "!= RWSEM_UNLOCKED_VALUE". Right?
> +
> +static inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
> +{
> +	WARN_ON(!(atomic_long_read(&sem->count) & RWSEM_WRITER_LOCKED));
> +}
>   
>   /* Common initializer macros and functions */
>   
> @@ -152,11 +162,21 @@ do {								\
>   	__init_rwsem((sem), #sem, &__key);			\
>   } while (0)
>   
> -static __always_inline int rwsem_is_locked(struct rw_semaphore *sem)
> +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)
> +{
> +	BUG_ON(!rwsem_is_locked(sem));
> +}
> +

There are some inconsistency in the use of WARN_ON() and BUG_ON() in the 
assertions. For PREEMPT_RT, held_write is a BUG_ON. For non-PREEMPT_RT, 
held is a BUG_ON. It is not clear why one is BUG_ON and other one is 
WARN_ON. Is there a rationale for that?

BTW, we can actually check if the current process is the write-lock 
owner of a rwsem, but not for a reader-owned rwsem.

Cheers,
Longman