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SubjectRe: [PATCH] drivers/isdn/gigaset: new M101 driver
On Sun, 04 Feb 2007 02:32:41 +0100 Tilman Schmidt <tilman@imap.cc> wrote:

> >> + spin_lock_irqsave(&cs->cmdlock, flags);
> >> + cb = cs->cmdbuf;
> >> + spin_unlock_irqrestore(&cs->cmdlock, flags);
> >
> > It is doubtful if the locking here does anything useful.
>
> It assures atomicity when reading the cs->cmdbuf pointer.

I think it's bogus. If the quantity being copied here is more than 32-bits
then yes, a lock is appropriate. But if it's a single word then it's
unlikely that the locking does anything useful. Or there might be a bug
here.

> >> + spin_lock_irqsave(&cs->cmdlock, flags);
> >> + cb->prev = cs->lastcmdbuf;
> >> + if (cs->lastcmdbuf)
> >> + cs->lastcmdbuf->next = cb;
> >> + else {
> >> + cs->cmdbuf = cb;
> >> + cs->curlen = len;
> >> + }
> >> + cs->cmdbytes += len;
> >> + cs->lastcmdbuf = cb;
> >> + spin_unlock_irqrestore(&cs->cmdlock, flags);
> >
> > Would the use of list_heads simplify things here?
>
> I don't think so. The operations in list.h do not keep track of
> the total byte count, and adding that in a race-free way appears
> non-trivial.

Maintaining a byte count isn't related to maintaining a list.

> >> + down(&cs->hw.ser->dead_sem);
> >
> > Does this actually use the semaphore's counting feature? If not, can we
> > switch it to a mutex?
>
> I stole that code from the PPP line discipline. It is to assure all
> other ldisc methods have completed before the close method proceeds.
> This doesn't look like a case for a mutex to me, but I'm open to
> suggestions if it's important to avoid a semaphore here.

If a sleeping lock is being used as a mutex, please use a mutex. We prefer
that semaphores only be used in those situations where their counting
feature is being used.

Reasons: a) mutexes have better runtime debugging support and b) Ingo had
some plans to reimplement semaphores in an arch-neutral way and for some
reason reducing the number of callers would help that. I forget what the
reason was, actually.

> >> + tail = atomic_read(&inbuf->tail);
> >> + head = atomic_read(&inbuf->head);
> >> + gig_dbg(DEBUG_INTR, "buffer state: %u -> %u, receive %u bytes",
> >> + head, tail, count);
> >> +
> >> + if (head <= tail) {
> >> + n = RBUFSIZE - tail;
> >> + if (count >= n) {
> >> + /* buffer wraparound */
> >> + memcpy(inbuf->data + tail, buf, n);
> >> + tail = 0;
> >> + buf += n;
> >> + count -= n;
> >> + } else {
> >> + memcpy(inbuf->data + tail, buf, count);
> >> + tail += count;
> >> + buf += count;
> >> + count = 0;
> >> + }
> >> + }
> >
> > Perhaps the (fairly revolting) circ_buf.h can be used for this stuff.
>
> It probably could, but IMHO readability would suffer rather than improve.
>

How about kernel/kfifo.c?

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