Andrew Morton wrote:
> On Thu, 3 Jun 2010 10:00:59 +0200 "Henrik Rydberg" <rydberg@xxxxxxxxxxx> 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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