linux-kernel - [PATCH memory-model 1/3] locking: Document the semantics of spin_is

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Date:   Mon, 14 May 2018 16:01:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org
Cc:     stern@...land.harvard.edu, parri.andrea@...il.com,
        will.deacon@....com, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH memory-model 1/3] locking: Document the semantics of spin_is_locked()

From: Andrea Parri <andrea.parri@...rulasolutions.com>

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 <andrea.parri@...rulasolutions.com>
Co-Developed-by: Alan Stern <stern@...land.harvard.edu>
Co-Developed-by: David Howells <dhowells@...hat.com>
Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
Signed-off-by: Alan Stern <stern@...land.harvard.edu>
Signed-off-by: David Howells <dhowells@...hat.com>
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: 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>
Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Randy Dunlap <rdunlap@...radead.org>
---
 include/linux/spinlock.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

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);
-- 
2.5.2