ipc: remove bogus lock comment for ipc_checkid

This series makes the sysv semaphore code more scalable, by reducing the
time the semaphore lock is held, and making the locking more scalable for
semaphore arrays with multiple semaphores.

The first four patches were written by Davidlohr Buesso, and reduce the
hold time of the semaphore lock.

The last three patches change the sysv semaphore code locking to be more
fine grained, providing a performance boost when multiple semaphores in a
semaphore array are being manipulated simultaneously.

On a 24 CPU system, performance numbers with the semop-multi
test with N threads and N semaphores, look like this:

	vanilla		Davidlohr's	Davidlohr's +	Davidlohr's +
	threads			patches		rwlock patches	v3 patches
	10	610652		726325		1783589		2142206
	20	341570		365699		1520453		1977878
	30	288102		307037		1498167		2037995
	40	290714		305955		1612665		2256484
	50	288620		312890		1733453		2650292
	60	289987		306043		1649360		2388008
	70	291298		306347		1723167		2717486
	80	290948		305662		1729545		2763582
	90	290996		306680		1736021		2757524
	100	292243		306700		1773700		3059159

This patch:

There is no reason to be holding the ipc lock while reading ipcp->seq,
hence remove misleading comment.

Also simplify the return value for the function.

Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Chegu Vinod <chegu_vinod@hp.com>
Cc: Emmanuel Benisty <benisty.e@gmail.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Stanislav Kinsbursky <skinsbursky@parallels.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/ipc/util.h b/ipc/util.h
index eeb79a1fbd834eda23027624824faf94bfb2f459..ac1480a4efd1499c5bb72bd85c99902a3ef693a2 100644 (file)
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -150,14 +150,9 @@ static inline int ipc_buildid(int id, int seq)
        return SEQ_MULTIPLIER * seq + id;
 }
 
-/*
- * Must be called with ipcp locked
- */
 static inline int ipc_checkid(struct kern_ipc_perm *ipcp, int uid)
 {
-       if (uid / SEQ_MULTIPLIER != ipcp->seq)
-               return 1;
-       return 0;
+       return uid / SEQ_MULTIPLIER != ipcp->seq;
 }
 
 static inline void ipc_lock_by_ptr(struct kern_ipc_perm *perm)