linux-kernel - Re: [RFC][PATCH] sched/wait_bit: Introduce wait_var_event()/wake_up

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Message-ID: <CAPcyv4h3SPqx879nnvQt0rvwanKksxLTqChCQa9ZQH8hsd_7yg@mail.gmail.com>
Date:   Tue, 13 Mar 2018 21:12:58 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Ingo Molnar <mingo@...hat.com>, Christoph Hellwig <hch@....de>,
        david <david@...morbit.com>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [RFC][PATCH] sched/wait_bit: Introduce wait_var_event()/wake_up_var()

On Tue, Mar 13, 2018 at 3:20 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sun, Mar 11, 2018 at 10:15:55AM -0700, Dan Williams wrote:
>> On Sun, Mar 11, 2018 at 4:27 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Fri, Mar 09, 2018 at 10:55:32PM -0800, Dan Williams wrote:
>> >> Add a generic facility for awaiting an atomic_t to reach a value of 1.
>> >>
>> >> Page reference counts typically need to reach 0 to be considered a
>> >> free / inactive page. However, ZONE_DEVICE pages allocated via
>> >> devm_memremap_pages() are never 'onlined', i.e. the put_page() typically
>> >> done at init time to assign pages to the page allocator is skipped.
>> >>
>> >> These pages will have their reference count elevated > 1 by
>> >> get_user_pages() when they are under DMA. In order to coordinate DMA to
>> >> these pages vs filesytem operations like hole-punch and truncate the
>> >> filesystem-dax implementation needs to capture the DMA-idle event i.e.
>> >> the 2 to 1 count transition).
>> >>
>> >> For now, this implementation does not have functional behavior change,
>> >> follow-on patches will add waiters for these page-idle events.
>> >
>> > Argh, no no no.. That whole wait_for_atomic_t thing is a giant
>> > trainwreck already and now you're making it worse still.
>> >
>> > Please have a look here:
>> >
>> >   https://lkml.kernel.org/r/20171101190644.chwhfpoz3ywxx2m7@hirez.programming.kicks-ass.net
>>
>> That thread seems to be worried about the object disappearing the
>> moment it's reference count reaches a target. That isn't the case with
>> the memmap / struct page objects for ZONE_DEVICE pages. I understand
>> wait_for_atomic_one() is broken in the general case, but as far as I
>> can see it works fine specifically for ZONE_DEVICE page busy tracking,
>> just not generic object lifetime.
>
> How's this, compile tested (x86_64-allmodconfig) only.
>
> This allows you to write:
>
>         wait_var_event(&your_atomic, atomic_read(&your_atomic) == 1);

Nice!

I'll give this a shot. I will need to add
wait_var_event_interruptible(), but other than that it looks workable
to me.