On Sun, 20 Jun 2010 21:05:05 +0200
"Henrik Rydberg" [off-list ref] wrote:

In spite of the many lock patterns and fifo helpers in the kernel, the
case of a single writer feeding many readers via a circular event
buffer seems to be uncovered. This patch adds the buflock, a mechanism
for handling multiple concurrent read positions in a shared circular
buffer.  Under normal operation, given adequate buffer size, the
operation is lock-less. The mechanism is given the name buflock to
emphasize that the locking depends on the buffer read/write clashes.

Signed-off-by: Henrik Rydberg <redacted>
---
This is version 2 of the buflock, which was first introduced as a
patch against the input subsystem. In the reviews, it was suggested
the file be placed in include/linux/, which is the patch presented
here. The major changes, taking review comments into account, are:

* The API has been rewritten to better abstract a lock, which
  hopefully provides a clearer reason to leave the actual memory
  handling to the user.

* The number of memory barriers has been reduced.

* Overlap detection now takes write interrupts larger than the buffer
  size into account.

* All methods are now static inlines.

I don't understand why this has "lock" in its name.

The API itself is a mixture of "bufwrite_foo" and "bufread_foo".

It's all a bit chaotic.  I'd suggest picking a sane name for the whole
subsytem - perhaps "mrbuf" for "multi reader buffer"?  Then
consistently name all interface functions as "mrbuf_foo". 
mrbuf.h, mrbuf_write_lock(), etc.

+static __always_inline bool __must_check bufread_retry(struct buflock_reader *br, const struct buflock_writer *bw)
+{
+	smp_rmb();
+	if (unlikely(((br->tail - br->last) & bw->page) < bw->next - br->last))
+		return true;
+	++br->tail;
+	if (unlikely(br->head - br->tail > bw->page))
+		br->tail = br->head;
+	return false;
+}

This looks too large to be inlined.

What's the __always_inline for?  Was gcc uninlining this within
separate compilation units?


Dmitry, if/when this code looks suitable to you and if you think it's
all desirable then please merge the
buflock-aka-bufwrite-aka-bufread-aka-mrbuf code via your tree.