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From: Andrew Morton on 21 Jun 2010 19:20
On Sun, 20 Jun 2010 21:05:05 +0200
"Henrik Rydberg" <rydberg(a)euromail.se> 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 <rydberg(a)euromail.se>
> ---
> 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.
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