locking/spinlocks: Document the semantics of spin_is_locked()

There appeared to be a certain, recurrent uncertainty concerning the
semantics of spin_is_locked(), likely a consequence of the fact that
this semantics remains undocumented or that it has been historically
linked to the (likewise unclear) semantics of spin_unlock_wait().

A recent auditing [1] of the callers of the primitive confirmed that
none of them are relying on particular ordering guarantees; document
this semantics by adding a docbook header to spin_is_locked(). Also,
describe behaviors specific to certain CONFIG_SMP=n builds.

[1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2
    https://marc.info/?l=linux-kernel&m=152042843808540&w=2
    https://marc.info/?l=linux-kernel&m=152043346110262&w=2

Co-Developed-by: Andrea Parri <[email protected]>
Co-Developed-by: Alan Stern <[email protected]>
Co-Developed-by: David Howells <[email protected]>
Signed-off-by: Andrea Parri <[email protected]>
Signed-off-by: Alan Stern <[email protected]>
Signed-off-by: David Howells <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Cc: Akira Yokosawa <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Jade Alglave <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Luc Maranget <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

1 files changed, 18 insertions, 0 deletions

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 4894d322d258..1e8a46435838 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -380,6 +380,24 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock)
 	raw_spin_trylock_irqsave(spinlock_check(lock), flags); \
 })
 
+/**
+ * spin_is_locked() - Check whether a spinlock is locked.
+ * @lock: Pointer to the spinlock.
+ *
+ * This function is NOT required to provide any memory ordering
+ * guarantees; it could be used for debugging purposes or, when
+ * additional synchronization is needed, accompanied with other
+ * constructs (memory barriers) enforcing the synchronization.
+ *
+ * Returns: 1 if @lock is locked, 0 otherwise.
+ *
+ * Note that the function only tells you that the spinlock is
+ * seen to be locked, not that it is locked on your CPU.
+ *
+ * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n,
+ * the return value is always 0 (see include/linux/spinlock_up.h).
+ * Therefore you should not rely heavily on the return value.
+ */
 static __always_inline int spin_is_locked(spinlock_t *lock)
 {
 	return raw_spin_is_locked(&lock->rlock);