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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 11 Aug 2022 15:22:38 -0700
Message-ID: <CAHk-=wjCa=Xf=pA2Z844WnwEeYgy9OPoB2kWphvg7PVn3ohScw@mail.gmail.com>
Subject: Re: [GIT PULL] Ceph updates for 5.20-rc1
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Jeff Layton <jlayton@kernel.org>,
        Ilya Dryomov <idryomov@gmail.com>, ceph-devel@vger.kernel.org,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>
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On Thu, Aug 11, 2022 at 3:04 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> FWIW, I wonder if we should do
>         if (READ_ONCE(dentry->d_parent) != parent)
>                 continue;
> before grabbing ->d_lock (and repeat the check after grabbing it,

It kind of makes sense. We already do that d_name.hash check outside
of the lock, so we already have that "we might race with a rename"
situation.

That said, I do think __d_lookup_rcu() is the more important of the two.

Here's a recreation of that patch I mentioned where the OP_COMPARE is
moved out of the loop. Just for fun, look at how much better the code
generation is for the common case when you don't have the call messing
up the clobbered registers etc.

Entirely untested, and I might have messed something up, but I suspect
this is a much bigger deal than whether d_same_name() is inlined or
not in the non-RCU path.

              Linus

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