Re: [PATCH, RFC 1/3] Add sem_getcount() to arches that lack it

From: Jody McIntyre
Date: Fri Mar 11 2005 - 00:42:04 EST

On Thu, Mar 10, 2005 at 08:55:03PM -0800, Andrew Morton wrote:
> Jody McIntyre <scjody@xxxxxxxxxxxxxx> wrote:
> >
> > parisc and frv define sem_getcount() in semaphore.h, which returns the
> > current semaphore value. This is cleaner than doing
> > atomic_read(&semaphore.count), currently done in
> > drivers/ieee1394/nodemgr.c and fs/xfs/linux-2.6/xfs_buf.c, and will work
> > on all architectures if sem_getcount() is added.
> That's a fairly bizarre thing to want to do. Would it be hard to modify
> xfs and 1394 to stop wanting to read a semaphore's up() count?

The count is the number of free transaction labels (1394 async is
transaction-based) and is initialized to 64. When a new transaction label
is needed, the requestor does a down(), then locks the tlabel variables
and allocates a new one. When a transaction label is freed, an up()
occurs. The semaphore's up() count is therefore the number of free
tlabels, and the number of outstanding transactions is (64 - count). I
can imagine situations in which this would be a useful statistic, but
I'm not sure any of them actually exist.

I haven't investigated xfs, but modifying 1394 would be fairly easy. I
could add a second variable that tracks the up() count, or just drop the
sysfs attribute that reports the number. The first option seems a bit
wasteful, but only slightly. I thought this patch was worthwhile based
on xfs wanting to do this and 3 arches already having (unused)
implementations of sem_getcount/sema_count.

If this patch isn't accepted, we should get rid of the xfs and 1394
hacks and delete sem_getcount (parisc, frv) and sema_count (arm) as they
are unused.


> (Why do they want to do this anyway?)

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