linux-kernel - [PATCH] spinlock: clarify doc for raw_spin_unlock

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Date:	Tue, 28 Apr 2015 12:44:45 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Chris Metcalf <cmetcalf@...hip.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	"Oleg Nesterov" <oleg@...hat.com>,
	Kirill Tkhai <ktkhai@...allels.com>,
	<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
	"Josh Poimboeuf" <jpoimboe@...hat.com>
Subject: [PATCH] spinlock: clarify doc for raw_spin_unlock_wait()

The intent of the function is to wait for the current locker.
Thus, we don't have to worry about potential later lockers,
but we also have to ensure that the arch implementation
doesn't return a false positive for the current locker.

Signed-off-by: Chris Metcalf <cmetcalf@...hip.com>
---
On 04/28/2015 12:24 PM, Peter Zijlstra wrote:
> I think it must not return before the lock holder that is current at the
> time of calling releases. Anything thereafter is indeed fair game as per
> your logic above.

Great, that seems like a workable definition.  How does this modified
language seem?  With this definition I can actually modify the
implementation of tile's arch_raw_spin_unlock_wait() to just read
current_ticket once and just wait until it changes (assuming the
lock is, in fact, locked).

Not sure whose tree this should go through; any takers?

 include/linux/spinlock.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
index 3e18379dfa6f..36de5fc86647 100644
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -141,7 +141,8 @@ do {								\
 #endif
 
 /**
- * raw_spin_unlock_wait - wait until the spinlock gets unlocked
+ * raw_spin_unlock_wait - wait until the lock holder that is current at the
+ * time of calling releases the lock (or return immediately if unlocked).
  * @lock: the spinlock in question.
  */
 #define raw_spin_unlock_wait(lock)	arch_spin_unlock_wait(&(lock)->raw_lock)
-- 
2.1.2

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