linux-kernel - Re: [GIT PULL] dax fixes for 4.20-rc6

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Message-ID: <CAPcyv4jznHRB=BCsm8Y3d-uqDy_OWHSgTsPZoPK-L80FiQftFg@mail.gmail.com>
Date:   Sun, 9 Dec 2018 10:26:54 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>
Subject: Re: [GIT PULL] dax fixes for 4.20-rc6

[ add Willy and Jan ]

On Sun, Dec 9, 2018 at 10:02 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sat, Dec 8, 2018 at 10:26 PM Williams, Dan J
> <dan.j.williams@...el.com> wrote:
> >
> >   git://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm tags/dax-fixes-4.20-rc6
>
> What's going on with the odd non-exclusive exclusive wait?
>
>         prepare_to_wait_exclusive(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
>         ...
>         /*
>          * Entry lock waits are exclusive. Wake up the next waiter since
>          * we aren't sure we will acquire the entry lock and thus wake
>          * the next waiter up on unlock.
>          */
>         if (waitqueue_active(wq))
>                 __wake_up(wq, TASK_NORMAL, 1, &ewait.key);
>
> that seems to make little or no sense.
>
> Why isn't that prepare_to_wait_exclusive() just a regular
> prepare_to_wait(), and then the whole "let's wake up anybody else" can
> be removed?
>
> I've pulled it, but am awaiting explanation of what looks like some
> pretty crazy code. I *suspect* it's a copy-and-paste situation where
> you took the exclusive wait from somewhere else.

Yes, I believe that's true. In the other instances of waiting for an
entry to be in unlocked there is a guarantee that the waiter will
attain the lock and perform an unlock + wakeup. In the dax_lock_page()
path there is the possibility that the inode dies before the lock is
attained and a subsequent unlock sequence is not guaranteed. So, I
believe the intent, Willy correct me if I am wrong, was to keep all
waits "exclusive" for some sense of symmetry, but this one can and
should be a non-exclusive wait.

I can send a cleanup, do you want one immediately, or is post -rc6 ok?