[PATCH v2] arm_mpam: make the mon_sel guard conditional only

[Sasha Levin]

Building arm64 allmodconfig with clang fails:

  drivers/resctrl/mpam_internal.h:207:7: error: ignoring return value
  of function declared with 'warn_unused_result' attribute
  [-Werror,-Wunused-result]
    207 |              mpam_mon_sel_lock(_T), mpam_mon_sel_unlock(_T));

mpam_mon_sel_lock() is __must_check and can fail: for an MSC accessed
through the MPAM-Fb firmware interface it returns false when called
from a context that cannot sleep. The mon_sel guard was defined with
DEFINE_GUARD(), which calls it unconditionally and discards the
result, so a guard(mon_sel) user would carry on without the lock and
release it on scope exit. The unconditional guard only exists as the
base for the conditional mon_sel_lock variant, which is the one
actually used.

Define mon_sel_lock directly as a conditional guard class instead, as
posix-timers does for lock_timer: the constructor returns NULL when
the lock cannot be taken, and the destructor only releases a lock
that was taken. ACQUIRE(mon_sel_lock, ...) works as before, including
ACQUIRE_ERR() returning -EBUSY on failure, and there is no
unconditional variant left to misuse.

Reported-by: kernelci.org bot <bot at kernelci.org>
Closes: https://d.kernelci.org/i/maestro:66f869ecbbb5b8592562ffd89f049c5ddf682547
Closes: https://d.kernelci.org/i/maestro:647269216758644837afcaac8e67d8dbe713a0e2
Fixes: 0db1715e4c9c ("arm_mpam: propagate MSC access errors for hw_probe functions")
Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
Changes in v2:
- Use an if statement in the destructor, as suggested by Ben Horgan.

 drivers/resctrl/mpam_internal.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index ee7c3953a9b6c..1f86cc158100b 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h
@@ -203,9 +203,10 @@ static inline int mpam_mon_sel_lock_init(struct device *dev,
 	return devm_mutex_init(dev, &msc->mon_sel_mutex);
 }
 
-DEFINE_GUARD(mon_sel, struct mpam_msc *,
-	     mpam_mon_sel_lock(_T), mpam_mon_sel_unlock(_T));
-DEFINE_GUARD_COND(mon_sel, _lock, mpam_mon_sel_lock(_T), _RET);
+DEFINE_CLASS(mon_sel_lock, struct mpam_msc *,
+	     if (_T) mpam_mon_sel_unlock(_T),
+	     mpam_mon_sel_lock(msc) ? msc : NULL, struct mpam_msc *msc);
+DEFINE_CLASS_IS_COND_GUARD(mon_sel_lock);
 
 /* Bits for mpam features bitmaps */
 enum mpam_device_features {
-- 
2.53.0