linux-kernel - [PATCH v2 tip/core/rcu 01/10] atomics: Revert addition of comment header to spin_unlock

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Date:   Mon, 31 Jul 2017 15:58:08 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH v2 tip/core/rcu 01/10] atomics: Revert addition of comment header to spin_unlock_wait()

There is still considerable confusion as to the semantics of
spin_unlock_wait(), but there seems to be universal agreement that
it is not that of a lock/unlock pair.  This commit therefore removes
the comment added by 6016ffc3874d ("atomics: Add header comment so
spin_unlock_wait()") in order to prevent at least that flavor of
confusion.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 include/linux/spinlock.h | 20 --------------------
 1 file changed, 20 deletions(-)

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index d9510e8522d4..59248dcc6ef3 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -369,26 +369,6 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock)
 	raw_spin_trylock_irqsave(spinlock_check(lock), flags); \
 })
 
-/**
- * spin_unlock_wait - Interpose between successive critical sections
- * @lock: the spinlock whose critical sections are to be interposed.
- *
- * Semantically this is equivalent to a spin_lock() immediately
- * followed by a spin_unlock().  However, most architectures have
- * more efficient implementations in which the spin_unlock_wait()
- * cannot block concurrent lock acquisition, and in some cases
- * where spin_unlock_wait() does not write to the lock variable.
- * Nevertheless, spin_unlock_wait() can have high overhead, so if
- * you feel the need to use it, please check to see if there is
- * a better way to get your job done.
- *
- * The ordering guarantees provided by spin_unlock_wait() are:
- *
- * 1.  All accesses preceding the spin_unlock_wait() happen before
- *     any accesses in later critical sections for this same lock.
- * 2.  All accesses following the spin_unlock_wait() happen after
- *     any accesses in earlier critical sections for this same lock.
- */
 static __always_inline void spin_unlock_wait(spinlock_t *lock)
 {
 	raw_spin_unlock_wait(&lock->rlock);
-- 
2.5.2