linux-kernel - Re: [PATCH 10/12] writeback: only allow one inflight and pending full flush

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Message-ID: <a978a692-2e7d-a76f-9653-9df1b63f65de@kernel.dk>
Date:   Tue, 3 Oct 2017 10:11:20 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 10/12] writeback: only allow one inflight and pending full
 flush

On 10/03/2017 10:06 AM, Matthew Wilcox wrote:
> On Fri, Sep 29, 2017 at 02:17:32AM +0200, Jens Axboe wrote:
>> On 09/28/2017 11:44 PM, Linus Torvalds wrote:
>>> On Thu, Sep 28, 2017 at 2:41 PM, Andrew Morton
>>> <akpm@...ux-foundation.org> wrote:
>>>>
>>>> test_and_set_bit()?
>>>
>>> If there aren't any atomicity concerns (either because of higher-level
>>> locking, or because racing and having two people set the bit is fine),
>>> it can be better to do them separately if the test_bit() is the common
>>> case and you can avoid dirtying a cacheline that way.
>>>
>>> But yeah, if that is the case, it might be worth documenting, because
>>> test_and_set_bit() is the more obviously appropriate "there can be
>>> only one" model.
>>
>> It is documented though, but maybe not well enough...
>>
>> I've actually had to document/explain it enough times now, that it
>> might be worth making a general construct. Though it has to be
>> used carefully, so perhaps it's better contained as separate use
>> cases.
> 
> test_and_test_and_set_bit()?  It's an unusual name, so when either
> reading it or writing it, people are going to say "something unusual
> here", rather than "That Jens Axboe is such a n00b, he doesn't know how
> to use test_and_set_bit()".  There are a few references out on the web
> to test-and-test-and-set already, so it's not entirely unique to Linux.
> 
> Plus, some architectures might be able to optimise that, particularly
> those which are ll/sc based.  It might be exactly the same as their
> test_and_set().

I like that suggestion, but would suggest we make it
test_then_test_and_set_bit() since the 'then' naming would work for
having similar test_then_clear_bit() and not clash with
test_and_set_bit().

And yes, some archs would be able to optimize this nicely.

All worth it if I never have to explain it or add special comments
about it again :-)

-- 
Jens Axboe