linux-kernel - [PATCH] Documentation/locking: Document the semantics of spin_is

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Date:   Wed, 28 Feb 2018 11:39:32 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andrea Parri <parri.andrea@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>
Subject: [PATCH] Documentation/locking: 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().

Document this semantics.

Signed-off-by: Andrea Parri <parri.andrea@...il.com>
Cc: Alan Stern <stern@...land.harvard.edu>
Cc: Will Deacon <will.deacon@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: David Howells <dhowells@...hat.com>
Cc: Jade Alglave <j.alglave@....ac.uk>
Cc: Luc Maranget <luc.maranget@...ia.fr>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Akira Yokosawa <akiyks@...il.com>
---
 include/linux/spinlock.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 4894d322d2584..2639fdc9a916c 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -380,6 +380,17 @@ 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.
+ *
+ * Return: 1, if @lock is (found to be) locked; 0, otherwise.
+ */
 static __always_inline int spin_is_locked(spinlock_t *lock)
 {
 	return raw_spin_is_locked(&lock->rlock);
-- 
2.7.4