linux-kernel - Re: [PATCH] Replace completions with semaphores

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Message-ID: <adaskxqrgsl.fsf@cisco.com>
Date:	Sat, 12 Apr 2008 12:53:30 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Oeser <ioe-lkml@...eria.de>,
	Daniel Walker <dwalker@...sta.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...l.org>
Subject: Re: [PATCH] Replace completions with semaphores

 > Arnd contacted me off-list and made the very sensible suggestion of:
 > 
 > struct completion {
 > 	struct semaphore sem;
 > }
 > 
 > That lets us eliminate the duplicate code since all the completion
 > functions become very thin wrappers around semaphore operations.

Just make sure you don't forget the history of completions...  As
Linus said long ago (http://lwn.net/2001/0802/a/lt-completions.php3):

  In case anybody cares, the race was that Linux semaphores only protect the
  accesses _inside_ the semaphore, while the accesses by the semaphores
  themselves can "race" in the internal implementation. That helps make an
  efficient implementation, but it means that the race was:
  
            cpu #1                       cpu #2
  
            DECLARE_MUTEX_LOCKED(sem);
            ..
            down(&sem);                   up(&sem);
            return;
                                          wake_up(&sem.wait) /*BOOM*/
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