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Subject: Re: [PATCH 2/2] fs/dcache: Make negative dentries easier to be
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From: Waiman Long <longman@redhat.com>
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Date: Tue, 28 Aug 2018 21:18:56 -0400
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To: Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>, Al Viro <viro@zeniv.linux.org.uk>, Jonathan Corbet <corbet@lwn.net>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>, "Luis R. Rodriguez" <mcgrof@kernel.org>, Kees Cook <keescook@chromium.org>, Jan Kara <jack@suse.cz>, Paul McKenney <paulmck@linux.vnet.ibm.com>, Ingo Molnar <mingo@kernel.org>, Miklos Szeredi <mszeredi@redhat.com>, Larry Woodman <lwoodman@redhat.com>, James Bottomley <James.Bottomley@hansenpartnership.com>, "Wangkai (Kevin,C)" <wangkai86@huawei.com>, Michal Hocko <mhocko@kernel.org>

On 08/28/2018 07:22 PM, Andrew Morton wrote:
> On Tue, 28 Aug 2018 16:10:24 -0700 Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
>> On Tue, Aug 28, 2018 at 3:29 PM Waiman Long <longman@redhat.com> wrote:
>>> Yes, I can rewrite it. What is the problem with the abbreviated form?
>> Either gcc rewrites it for you, or you end up _actually_ using a
>> function pointer and calling through it.
>>
>> The latter would be absolutely horribly bad for something like
>> "list_add()", which should expand to just a couple of instructions.
>>
>> And the former would be ok, except for the "you wrote code the garbage
>> way, and then depended on the compiler fixing it up". Which we
>> generally try to avoid in the kernel.
>>
>> (Don't get me wrong - we definitely depend on the compiler doing a
>> good job at CSE and dead code elimination etc, but generally we try to
>> avoid the whole "compiler has to rewrite code to be good" model).
>>
> And the "abbreviated form" will surely explode if one or both of those
> "functions" happens to be implemented (or later reimplemented) as a macro.
> It's best not to unnecessarily make such assumptions.
>
Yes,  that is true.

Thanks,
Longman