From: Manfred Spraul <manfred@colorfullife.com>
Date: Wed, 26 May 2010 21:43:43 +0000 (-0700)
Subject: ipc/sem.c: update description of the implementation
X-Git-Tag: v2.6.35-rc1~133
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c5cf6359ad1d322c16e159011247341849cc0d3a;p=pandora-kernel.git

ipc/sem.c: update description of the implementation

ipc/sem.c begins with a 15 year old description about bugs in the initial
implementation in Linux-1.0.  The patch replaces that with a top level
description of the current code.

A TODO could be derived from this text:

The opengroup man page for semop() does not mandate FIFO.  Thus there is
no need for a semaphore array list of pending operations.

If

- this list is removed
- the per-semaphore array spinlock is removed (possible if there is no
  list to protect)
- sem_otime is moved into the semaphores and calculated on demand during
  semctl()

then the array would be read-mostly - which would significantly improve
scaling for applications that use semaphore arrays with lots of entries.

The price would be expensive semctl() calls:

	for(i=0;i<sma->sem_nsems;i++) spin_lock(sma->sem_lock);
	<do stuff>
	for(i=0;i<sma->sem_nsems;i++) spin_unlock(sma->sem_lock);

I'm not sure if the complexity is worth the effort, thus here is the
documentation of the current behavior first.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Zach Brown <zach.brown@oracle.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---

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