linux-kernel - Re: [PATCH 1/3] cleanup: add conditional guard helper

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Message-ID: <20241018111503.GC36494@noisy.programming.kicks-ass.net>
Date: Fri, 18 Oct 2024 13:15:03 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: David Lechner <dlechner@...libre.com>
Cc: Dan Williams <dan.j.williams@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Cameron <jic23@...nel.org>,
	Nuno Sá <nuno.sa@...log.com>,
	Michael Hennerich <michael.hennerich@...log.com>,
	linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
	linux-cxl@...r.kernel.org, przemyslaw.kitszel@...el.com
Subject: Re: [PATCH 1/3] cleanup: add conditional guard helper

On Tue, Oct 01, 2024 at 05:30:18PM -0500, David Lechner wrote:
> Add a new if_not_cond_guard() macro to cleanup.h for handling
> conditional guards such as mutext_trylock().
> 
> This is more ergonomic than scoped_cond_guard() for most use cases.
> Instead of hiding the error handling statement in the macro args, it
> works like a normal if statement and allow the error path to be indented
> while the normal code flow path is not indented. And it avoid unwanted
> side-effect from hidden for loop in scoped_cond_guard().
> 
> Signed-off-by: David Lechner <dlechner@...libre.com>
> ---
>  include/linux/cleanup.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index 038b2d523bf8..682bb3fadfc9 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -273,6 +273,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>   *	an anonymous instance of the (guard) class, not recommended for
>   *	conditional locks.
>   *
> + * if_not_cond_guard(name, args...) { <error handling> }:
> + *	convenience macro for conditional guards that calls the statement that
> + *	follows only if the lock was not acquired (typically an error return).
> + *
>   * scoped_guard (name, args...) { }:
>   *	similar to CLASS(name, scope)(args), except the variable (with the
>   *	explicit name 'scope') is declard in a for-loop such that its scope is
> @@ -304,6 +308,13 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>  
>  #define __guard_ptr(_name) class_##_name##_lock_ptr
>  
> +#define __if_not_cond_guard(_name, _id, args...)	\
> +	CLASS(_name, _id)(args);			\
> +	if (!__guard_ptr(_name)(&_id))
> +
> +#define if_not_cond_guard(_name, args...) \
> +	__if_not_cond_guard(_name, __UNIQUE_ID(guard), args)
> +
>  #define scoped_guard(_name, args...)					\
>  	for (CLASS(_name, scope)(args),					\
>  	     *done = NULL; __guard_ptr(_name)(&scope) && !done; done = (void *)1)


So if I stick this on top of:

  https://lkml.kernel.org/r/20241011121535.28049-1-przemyslaw.kitszel@intel.com

then I can add the below:

--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -277,6 +277,8 @@ static inline class_##_name##_t class_##
  *	convenience macro for conditional guards that calls the statement that
  *	follows only if the lock was not acquired (typically an error return).
  *
+ *	Only for conditional locks.
+ *
  * scoped_guard (name, args...) { }:
  *	similar to CLASS(name, scope)(args), except the variable (with the
  *	explicit name 'scope') is declard in a for-loop such that its scope is
@@ -290,7 +292,6 @@ static inline class_##_name##_t class_##
  *      acquire fails.
  *
  *	Only for conditional locks.
- *
  */
 
 #define __DEFINE_CLASS_IS_CONDITIONAL(_name, _is_cond)	\
@@ -342,6 +343,7 @@ _label:										\
 		       __UNIQUE_ID(label), args)
 
 #define __if_not_guard(_name, _id, args...)		\
+	BUILD_BUG_ON(!__is_cond_ptr(_name));		\
 	CLASS(_name, _id)(args);			\
 	if (!__guard_ptr(_name)(&_id))
 

That make sense to people?

I've queued these two patches:

  git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git locking/core

But lacking if_not_guard() users, the robot isn't really going to give
me much feedback there, I suppose...