Re: [RFC -v2] kfifo writer side lock-less support

[Stefani Seibold]

Hi Huang,

Am Dienstag, den 24.08.2010, 09:42 +0800 schrieb Huang Ying:
> Hi, Stefani,
> 
> The sample code is attached with the mail too. If it is necessary, I
> can put the sample code into samples/kfifo directory if the basic
> idea of the patch is accepted.
> 

The samples use a own implementation of a record handling. Please use
the one provided by the kfifo API.

> This patch add lock-less support for kfifo writer side amongst
> different contexts on one CPU, such as NMI, IRQ, soft_irq, process,
> etc. This makes kfifo can be used to implement per-CPU lock-less data
> structure.
> 
> The different contexts on one CPU have some kind of preemption
> priority as follow:
> 
> process -> soft_irq -> IRQ -> NMI
> 
> Where preemption priority increases from left to right. That is, the
> right side context can preempt left side context, but not in the
> reverse direction, that means the right side context will run to the
> end before return to the left side context. The lock-less algorithm
> used in the patch take advantage of this.
> 
> The algorithm works in reserve/commit style, where "reserve" only
> allocate the space, while "commit" really makes the data into buffer,
> data is prepared in between. cmpxchg is used in "reserve", this
> guarantees different spaces will be allocated for different
> contexts. Only the "commit" in lowest context will commit all
> allocated spaces. Because all contexts preempting lowest context
> between "reserve" and "commit" will run to the end, all data put into
> buffer are valid.
> 

I don't like it, because it handles a very rare use case. The batch is
bloating the code of the fifo API and the macros, so user of this get
also an increase of the code size. most and increase the size of the
kfifo structure. Most of the user, i think more than 99 percent will not
need it.
 
And there a lot of bugs on a first small review.

Your kfifo_skip_len does not handle record fifos. User of kfifo_put,
kfifo_avail will get an increased code size. Your kfifo_is_full isn't
longer working correctly. You did not fixed kfifo_in(), so this function
will not work. And so on....

> Some idea of the lock-less algorithm in the patch comes from Steven
> Rostedt's trace ring buffer implementation.
> 
> The new xxx_ptr interface and kfifo_iter makes it possible to
> write/read content of kfifo in-place in addition to copying out/in.
> 

The regular use case of a fifo is that there is one write and one
reader. In this case, the current implementation of the kfifo structure
is lock less.

So i you need this, than it would be better to use the ring buffer.

> Signed-off-by: Huang Ying <ying.huang@xxxxxxxxx>
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>

I was never CC by a mail where Andi has signed off this patch. It would
be great if i will get some infos why he like it.

But i think you will never get an ACK for this by me, because it bloats
the most-used functions of the hand optimized kfifo API.

Rule number one: do not increase the code size if the functionality is
not needed and used!

BTW: I am in vacation, so it could take some time answering Emails.

- Stefani


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