linux-kernel - Re: Change in register

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Date:   Thu, 1 Feb 2018 18:10:50 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>,
        gregkh@...uxfoundation.org
Cc:     axboe@...nel.dk, jlayton@...chiereds.net, bfields@...ldses.org,
        linux-kernel@...r.kernel.org
Subject: Re: Change in register_blkdev() behavior



On 01/02/18 05:23 PM, Srivatsa S. Bhat wrote:
> static int find_dynamic_major(void)
> {
>          int i;
>          struct char_device_struct *cd;
> 
>          for (i = ARRAY_SIZE(chrdevs)-1; i > CHRDEV_MAJOR_DYN_END; i--) {
>                                           ^^^^
> As far as I can see, _DYN_END is inclusive, so shouldn't this be >= ?

Yes, it looks like _DYN_END should have been inclusive based on the way 
I documented it.

> 
>                  for (cd = chrdevs[major_to_index(i)]; cd; cd = cd->next)
>                          if (cd->major == i)
>                                  break;
> 
>                  if (cd == NULL || cd->major != i)
>                                       ^^^^^^^^
> It seems that this latter condition is unnecessary, as it will never be
> true. (We'll reach here only if cd == NULL or cd->major == i).

Not quite. chrdevs[] may contain majors that also hit on the hash but 
don't equal 'i'. So the for loop will iterate through all hashes 
matching 'i' and if there is one or more and they all don't match 'i', 
it will fall through the loop and cd will be set to something non-null 
and not equal to i.

Thanks for the review!

Logan