From: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>

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 statement-expression) that is passed as its
second argument. That statement (or statement-expression) is executed if
waiting for a lock is interrupted or if a _trylock() fails in case of
contention.

Usage example:

	cond_guard(mutex_intr, return -EINTR, &mutex);

Consistent with other usage of _guard(), locks are unlocked at the exit of
the scope where cond_guard() is called. This macro can be called multiple
times in the same scope.

Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 include/linux/cleanup.h |   20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index c2d09bc4f976..602afb85da34 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -134,6 +134,19 @@ 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' is a
+ *	statement or statement-expression that is executed if waiting for a
+ *	lock is interrupted or if a _trylock() fails in case of contention.
+ *
+ *	Example:
+ *
+ *		cond_guard(mutex_intr, return -EINTR, &mutex);
+ *
+ * 	This macro can be called multiple times in the same scope, for it
+ * 	declares unique instances of type 'name'.
+ *
  * 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 +178,13 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 
 #define __guard_ptr(_name) class_##_name##_lock_ptr
 
+#define __cond_guard(__unique, _name, _fail, args...) \
+	CLASS(_name, __unique)(args); \
+	if (!__guard_ptr(_name)(&__unique)) _fail; \
+	else { }
+#define cond_guard(_name, _fail, args...) \
+	__cond_guard(__UNIQUE_ID(scope), _name, _fail, args)
+
 #define scoped_guard(_name, args...)					\
 	for (CLASS(_name, scope)(args),					\
 	     *done = NULL; __guard_ptr(_name)(&scope) && !done; done = (void *)1)

On Tue, 27 Feb 2024 at 08:48, Dan Williams <dan.j.williams@intel.com> wrote:
>
>         cond_guard(mutex_intr, return -EINTR, &mutex);

Again, this is *not* helping make code readable and less likely to have bugs.

The macro has obvious deficiencies, like the "_fail" argument not
being surrounded by  "{ }" (the equivalent of parenthesizing an
expression argument), but even with that trivial fix the syntax is
just too ugly to live, and doesn't match normal C syntax.

And yes, we have other macros that don't have normal C syntax, and
they are ugly too (example: #define CHKINFO(ret) in
drivers/video/fbdev/hgafb.c), but we should have higher standards for
globally visible helpers, and we should have *MUCH* higher standards
for helpers that are supposed to be all about reducing mistakes.

Bad / odd syntax does not reduce mistakes.

If a sane 'guard' model doesn't work for some code, the answer is not
to make an insane guard model. The answer is to not use 'guard' in
code like that.

               Linus