Re: [PATCH 5/5] kfifo: log based kfifo API

Dmitry Torokhov [off-list ref] wrote:

Hi Yuanhan,

On Tue, Jan 08, 2013 at 10:57:53PM +0800, Yuanhan Liu wrote:

quoted

The current kfifo API take the kfifo size as input, while it rounds
 _down_ the size to power of 2 at __kfifo_alloc. This may introduce
potential issue.

Take the code at drivers/hid/hid-logitech-dj.c as example:

	if (kfifo_alloc(&djrcv_dev->notif_fifo,
                       DJ_MAX_NUMBER_NOTIFICATIONS * sizeof(struct

dj_report),

quoted

                       GFP_KERNEL)) {

Where, DJ_MAX_NUMBER_NOTIFICATIONS is 8, and sizeo of(struct

dj_report)

quoted

is 15.

Which means it wants to allocate a kfifo buffer which can store 8
dj_report entries at once. The expected kfifo buffer size would be
8 * 15 = 120 then. While, in the end, __kfifo_alloc will turn the
size to rounddown_power_of_2(120) =  64, and then allocate a buf
with 64 bytes, which I don't think this is the original author want.

With the new log API, we can do like following:

	int kfifo_size_order = order_base_2(DJ_MAX_NUMBER_NOTIFICATIONS *
					    sizeof(struct dj_report));

	if (kfifo_alloc(&djrcv_dev->notif_fifo, kfifo_size_order,

GFP_KERNEL)) {

quoted

This make sure we will allocate enough kfifo buffer for holding
DJ_MAX_NUMBER_NOTIFICATIONS dj_report entries.

Why don't you simply change __kfifo_alloc to round the allocation up
instead of down?

Thanks.

-- 
Dmitry
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Hi Dmitry,

I agree.   I don't see the benefit in pushing up the change to a kfifo internal decision/problem to many different places in the kernel.

Regards,
Andy

 

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