linux-kernel - RE: [PATCH 1/2 v2] cleanup: Add cond

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Date: Mon, 5 Feb 2024 11:02:47 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>, "Peter
 Zijlstra" <peterz@...radead.org>, Dan Williams <dan.j.williams@...el.com>,
	<linux-kernel@...r.kernel.org>
CC: <linux-cxl@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, "Jonathan
 Cameron" <Jonathan.Cameron@...wei.com>, "Fabio M. De Francesco"
	<fabio.maria.de.francesco@...ux.intel.com>, Ira Weiny <ira.weiny@...el.com>
Subject: RE: [PATCH 1/2 v2] cleanup: Add cond_guard() to conditional guards

Fabio M. De Francesco wrote:
> Add cond_guard() macro to conditional guards.
> 
> cond_guard() is a guard to be used with the conditional variants of locks,
> like down_read_trylock() or mutex_lock_interruptible().
> 
> It takes a statement (or more statements in a block) that is passed to its
> second argument. That statement (or block) is executed if waiting for a
> lock is interrupted or if a _trylock() fails in case of contention.
> 
> Usage example:
> 
> 	cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
> 
> Consistently with the other guards, locks are unlocked at the exit of the
> scope where cond_guard() is called.
> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Suggested-by: Dan Williams <dan.j.williams@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@...ux.intel.com>
> ---
>  include/linux/cleanup.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index c2d09bc4f976..88af56600325 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -134,6 +134,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>   *	an anonymous instance of the (guard) class, not recommended for
>   *	conditional locks.
>   *
> + * cond_guard(name, fail, args...):
> + *	a guard to be used with the conditional variants of locks, like
> + *	down_read_trylock() or mutex_lock_interruptible. 'fail' are one or more
> + *	statements that are executed when waiting for a lock is interrupted or
> + *	when a _trylock() fails in case of contention.
> + *
> + *	Example:
> + *
> + *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);

That _fail argument likely needs to be a statement expression for the
multi-statement case.

> + *
>   * 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
> @@ -165,6 +175,10 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
>  
>  #define __guard_ptr(_name) class_##_name##_lock_ptr
>  
> +#define cond_guard(_name, _fail, args...) \
> +	CLASS(_name, scope)(args); \
> +	if (!__guard_ptr(_name)(&scope)) _fail

No, as I stated before this is broken for usages of:

    if () cond_guard() else if ()

The 'else' in the definition is critical, this builds for me (untested):

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 88af56600325..665407498781 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -142,7 +142,7 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
  *
  *	Example:
  *
- *		cond_guard(rwsem_read_try, { printk(...); return 0; }, &semaphore);
+ *		cond_guard(rwsem_read_try, ({ printk(...); return 0; }), &semaphore);
  *
  * scoped_guard (name, args...) { }:
  *	similar to CLASS(name, scope)(args), except the variable (with the
@@ -177,7 +177,8 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 
 #define cond_guard(_name, _fail, args...) \
 	CLASS(_name, scope)(args); \
-	if (!__guard_ptr(_name)(&scope)) _fail
+	if (!__guard_ptr(_name)(&scope)) _fail; \
+	else
 
 #define scoped_guard(_name, args...)					\
 	for (CLASS(_name, scope)(args),					\