Re: [PATCH] __cfq_get_queue() fix for 2.6.12-rc5

From: Jens Axboe
Date: Fri Jun 17 2005 - 09:13:07 EST

On Wed, Jun 08 2005, Kiyoshi Ueda wrote:
> Hello,
> I resend this e-mail, as it may not be sent properly.
> Please excuse me, if you receive same one.
> I found a possible bug by which the first get_request() for a process
> fails in cfq I/O scheduler.
> If it's a bug, please consider to apply the patch for 2.6.12-rc5 below.
> When cfq I/O scheduler is selected, get_request() in __make_request()
> calls __cfq_get_queue().
> __cfq_get_queue() finds an existing queue (struct cfq_queue) of the
> current process for the device and returns it. If it's not found,
> __cfq_get_queue() creates and returns a new one if __cfq_get_queue()
> is called with __GFP_WAIT flag, or __cfq_get_queue() returns NULL
> (this means that get_request() fails) if no __GFP_WAIT flag.
> On the other hand, in __make_request(), get_request() is called
> without __GFP_WAIT flag at the first time.
> Thus, the get_request() fails when there is no existing queue,
> typically when it's called for the first I/O request of the process
> to the device.
> Though it will be followed by get_request_wait() for general case,
> __make_request() will just end the I/O with an error (EWOULDBLOCK)
> when the request was for read-ahead.

Good analysis, the patch looks correct. I've applied it, thanks.

Jens Axboe

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