linux-kernel - Re: [GIT PULL] Ceph updates for 5.20-rc1

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Date:   Thu, 11 Aug 2022 23:44:08 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [GIT PULL] Ceph updates for 5.20-rc1

On Thu, Aug 11, 2022 at 03:22:38PM -0700, Linus Torvalds wrote:
> On Thu, Aug 11, 2022 at 3:04 PM Al Viro <viro@...iv.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.

Looks sane at the first pass, but right now I'm really half-asleep - 5 hours
of sleep tonight, and about the same yesterday ;-/

I'll try to grab at least an hour or two and reread it once I'm more or less
awake...