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Subject: Re: [PATCH] Replace completions with semaphores
From: Daniel Walker <dwalker@mvista.com>
To: Matthew Wilcox <matthew@wil.cx>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Oeser <ioe-lkml@rameria.de>,
       linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@osdl.org>
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Date: Sat, 12 Apr 2008 11:01:35 -0700
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On Sat, 2008-04-12 at 11:26 -0600, Matthew Wilcox wrote:

> It would look bloody odd to write (code taken from megasas_mgmt_ioctl_fw() in
> drivers/scsi/megaraid/megaraid_sas.c):
> 
>         if (wait_for_completion_interruptible(&instance->ioctl_completion)) {
>                 error = -ERESTARTSYS;
>                 goto out_kfree_ioc;
>         }
>         error = megasas_mgmt_fw_ioctl(instance, user_ioc, ioc);
>         complete(&instance->ioctl_sem);
> 
> What I'm trying to get a feeling for is whether people find it similarly
> odd to use semaphores where we currently use completions.  We *used*
> to, but I don't find that a compelling reason.

The above doesn't look all that odd to me. It may be that you've seen
semaphores in that position in the past and just expect to see them. 

> 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.
> 
> I'll note that the semaphore code I hae queued for 2.6.26 is slightly
> more efficient than the current implementation of completions because
> completions use the generic waitqueue code and thus do an indirect
> function call per wakeup.  Of course, there's no reason completions
> couldn't use the same technique as my semaphore code ... but then they
> would be identical to semaphores ;-)

I would just re-write completions keeping the name and API in tact, make
them better and just leave semaphores alone..

Daniel