Add cond_guard() to conditional guards.

cond_guard() is used for the _interruptible(), _killable(), and _try
versions of locks.

It stores a return value to a variable whose address is given to its
second argument, that is either '-1' on failure or '0' on success to
acquire a lock. The returned value can be checked to act accordingly
(e.g., to call printk() and return -EINTR in case of failure of an
_interruptible() variant)

As the other guards, it avoids to open code the release of the lock
after a goto to an 'out' label.

This remains an RFC because Dan suggested a slightly different syntax.

The changes to the CXL code are provided only to show the use of this
macro. If consensus is gathered on this macro, the cleanup of
show_targetN() will be submitted later in a separate patch.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fabio.maria.de.francesco@linux.intel.com>
---

Changes from v1 and v2:
	I've taken into account Dan's comments (thanks).

 drivers/cxl/core/region.c | 15 +++++----------
 include/linux/cleanup.h   | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index 0f05692bfec3..560f25bdfd11 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -668,26 +668,21 @@ static size_t show_targetN(struct cxl_region *cxlr, char *buf, int pos)
 	struct cxl_endpoint_decoder *cxled;
 	int rc;
 
-	rc = down_read_interruptible(&cxl_region_rwsem);
+	cond_guard(rwsem_read_intr, &rc, &cxl_region_rwsem);
 	if (rc)
-		return rc;
+		return -EINTR;
 
 	if (pos >= p->interleave_ways) {
 		dev_dbg(&cxlr->dev, "position %d out of range %d\n", pos,
 			p->interleave_ways);
-		rc = -ENXIO;
-		goto out;
+		return -ENXIO;
 	}
 
 	cxled = p->targets[pos];
 	if (!cxled)
-		rc = sysfs_emit(buf, "\n");
+		return sysfs_emit(buf, "\n");
 	else
-		rc = sysfs_emit(buf, "%s\n", dev_name(&cxled->cxld.dev));
-out:
-	up_read(&cxl_region_rwsem);
-
-	return rc;
+		return sysfs_emit(buf, "%s\n", dev_name(&cxled->cxld.dev));
 }
 
 static int match_free_decoder(struct device *dev, void *data)
diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index c2d09bc4f976..a4b40d511f9e 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -134,6 +134,20 @@ 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, ret, args...):
+ * 	for conditional locks like mutex_trylock() or down_read_interruptible().
+ * 	'ret' is a pointer to a variable where this macro stores 0 on success
+ * 	and -1 on failure to acquire a lock.
+ *
+ * 	Example:
+ *
+ * 	int ret;
+ * 	cond_guard(rwsem_read_try, &ret, &sem);
+ * 	if (ret) {
+ * 		dev_dbg("down_read_trylock() failed to down 'sem')\n");
+ * 		return 0; // down_read_trylock() returns 0 on contention
+ * 	}
+ *
  * 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 +179,11 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 
 #define __guard_ptr(_name) class_##_name##_lock_ptr
 
+#define cond_guard(_name, _ret, args...) \
+	CLASS(_name, scope)(args); \
+	if (!__guard_ptr(_name)(&scope)) *_ret = -1 \
+	else *_ret = 0
+
 #define scoped_guard(_name, args...)					\
 	for (CLASS(_name, scope)(args),					\
 	     *done = NULL; __guard_ptr(_name)(&scope) && !done; done = (void *)1)
-- 
2.43.0