Andrew Morton wrote:
[...]

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.

Point taken.

quoted

+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?

As you say, the function is large, and I am uncertain about the rules regarding
compiler reordering across general function calls. Starting a general function
with a memory barrier feels weird. Perhaps the function should be split?

Thanks,
Henrik