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SubjectRe: [PATCH] Replace completions with semaphores
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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



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