On Sun, 04 Feb 2007 02:32:41 +0100 Tilman Schmidt [off-list ref] wrote:

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+	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.

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+	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.

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+			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.

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+	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?