Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 11 Aug 2022 23:44:08 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
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>
Subject: Re: [GIT PULL] Ceph updates for 5.20-rc1
Message-ID: <YvWGOKqmuaGm8yCd@ZenIV>
References: <20220811152446.281723-1-idryomov@gmail.com>
 <CAHk-=wifgq59uru6xDB=nY-1p6aQ-1YB8nVhW7T-N2ctK3m1gw@mail.gmail.com>
 <CAOi1vP9BSi-65of-8D0BA1_DC0eVD_TQcWkhrGJwaXw_skhHFQ@mail.gmail.com>
 <5d0b0367a5e28ec5b1f3b995c7792ff9a5cbcbd4.camel@kernel.org>
 <YvVzHQ5DVaPAvw26@ZenIV>
 <72a93a2c8910c3615bba7c093c66c18b1a6a2696.camel@kernel.org>
 <YvV2zfT0XbgwHGe/@ZenIV>
 <CAHk-=wgYnAPiGsh7H4BS_E1aMM46PdSGg8YqFhi2SpGw+Ac_PQ@mail.gmail.com>
 <YvV86p5DjBLjjXHo@ZenIV>
 <CAHk-=wjCa=Xf=pA2Z844WnwEeYgy9OPoB2kWphvg7PVn3ohScw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=wjCa=Xf=pA2Z844WnwEeYgy9OPoB2kWphvg7PVn3ohScw@mail.gmail.com>
Sender: Al Viro <viro@ftp.linux.org.uk>
Precedence: bulk

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@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.

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...