Re: [PATCH v2 1/8] cleanup: Introduce ACQUIRE() and ACQUIRE_ERR() for conditional locks

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Wed, 18 Jun 2025 22:04:09 -0700
Dan Williams <dan.j.williams@intel.com> wrote:

> From: Peter Zijlstra <peterz@infradead.org>
> 
> scoped_cond_guard(), automatic cleanup for conditional locks, has a couple
> pain points:
> 
> * It causes existing straight-line code to be re-indented into a new
>   bracketed scope. While this can be mitigated by a new helper function
>   to contain the scope, that is not always a comfortable conversion.
> 
> * The return code from the conditional lock is tossed in favor of a scheme
>   to pass a 'return err;' statement to the macro.
> 
> Other attempts to clean this up, to behave more like guard() [1], got hung
> up trying to both establish and evaluate the conditional lock in one
> statement.
> 
> ACQUIRE() solves this by reflecting the result of the condition in the
> automatic variable established by the lock CLASS(). The result is
> separately retrieved with the ACQUIRE_ERR() helper, effectively a PTR_ERR()
> operation.
> 
> Link: http://lore.kernel.org/all/Z1LBnX9TpZLR5Dkf@gmail.com [1]
> Link: http://patch.msgid.link/20250512105026.GP4439@noisy.programming.kicks-ass.net
> Link: http://patch.msgid.link/20250512185817.GA1808@noisy.programming.kicks-ass.net
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: David Lechner <dlechner@baylibre.com>
> Cc: Fabio M. De Francesco <fabio.m.de.francesco@linux.intel.com>
> Not-yet-signed-off-by: Peter Zijlstra <peterz@infradead.org>
> [djbw: wrap Peter's proposal with changelog and comments]
> Co-developed-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
This looks like a nice solution.  One trivial style thing inline.

> ---
>  include/linux/cleanup.h | 77 ++++++++++++++++++++++++++++++++++-------
>  include/linux/mutex.h   |  2 +-
>  include/linux/rwsem.h   |  2 +-
>  3 files changed, 67 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index 7093e1d08af0..1e1eb35cc225 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> +#define __GUARD_IS_ERR(_ptr) \
> +	({ unsigned long _rc = (__force unsigned long)(_ptr); \
> +	   unlikely((_rc-1) >= -MAX_ERRNO-1); })

Trivial but I'd have added spaces to make this
	   unlikely((_rc - 1) >= -MAX_ERRNO - 1); })

> +
>  #define __DEFINE_GUARD_LOCK_PTR(_name, _exp) \
>  	static inline void * class_##_name##_lock_ptr(class_##_name##_t *_T) \
> -	{ return (void *)(__force unsigned long)*(_exp); }
> +	{ void *_ptr = (void *)(__force unsigned long)*(_exp); \
> +	  if (IS_ERR(_ptr)) { _ptr = NULL; } return _ptr; } \
> +	static inline int class_##_name##_lock_err(class_##_name##_t *_T) \
> +	{ long _rc = (__force unsigned long)*(_exp); \
> +	  if (!_rc) { _rc = -EBUSY; } if (!IS_ERR_VALUE(_rc)) { _rc = 0; } \
> +	  return _rc; }
>