linux-kernel - Re: [PATCH 1/4] input: Introduce buflock, a one-to-many circular buffer mechanism

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Date:	Sat, 05 Jun 2010 13:21:39 +0200
From:	Henrik Rydberg <rydberg@...omail.se>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jiri Kosina <jkosina@...e.cz>,
	Mika Kuoppala <mika.kuoppala@...ia.com>,
	Benjamin Tissoires <tissoire@...a.fr>,
	Rafi Rubin <rafi@...s.upenn.edu>
Subject: Re: [PATCH 1/4] input: Introduce buflock, a one-to-many circular
 buffer mechanism

Andrew Morton wrote:
> On Thu,  3 Jun 2010 10:00:59 +0200 "Henrik Rydberg" <rydberg@...omail.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 buffer
>> seems to be uncovered. This patch adds the buflock, a minimalistic
>> interface implementing SMP-safe locking for such a buffer. Under
>> normal operation, given adequate buffer size, the operation is
>> lock-less. The template is given the name buflock to emphasize that
>> the locking depends on the buffer read/write clashes.
>>
> 
> Seems that reviewers have already covered most of the oddities.
> 
>> +/*
>> + * Write to buffer without locking
>> + *
>> + * bw - the buflock_writer keeping track of the write position
>> + * buf - the buffer to write to (array of item type)
>> + * size - the size of the circular buffer (must be a power of two)
>> + * item - the item to write
>> + *
>> + * There is no locking involved during write, so this method is
>> + * suitable to use in interrupt context.
>> + */
> 
> And if the buffer fills up, it silently overwrites old data?
> 
> There are many options in this sort of thing.  Certain choices have
> been made here and they should be spelled out exhaustively please.
> 
>> +#define buflock_write(bw, buf, size, item)				\
>> +	do {								\
>> +		bw.next_head = (bw.head + 1) & ((size) - 1);		\
>> +		smp_wmb();						\
>> +		buf[bw.head] = item;					\
>> +		smp_wmb();						\
>> +		bw.head = bw.next_head;					\
>> +		smp_wmb();						\
>> +	} while (0)
> 
> I don't think there's a reason why these all had to be implemented as
> bloaty, un-typesafe macros?  Especially as buggy ones which reference
> their arguments multiple times!
> 
> Code it in C if possible, please.

Thanks Andrew, Dmitry, Oleg and Jonathan for your reviews. Next round of the
buflock stuff will be targeting the wider audience in include/linux/.

Henrik

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