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Date:   Sat, 27 Aug 2022 01:17:01 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     NeilBrown <neilb@suse.de>, Daire Byrne <daire@dneg.com>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Chuck Lever <chuck.lever@oracle.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 01/10] VFS: support parallel updates in the one directory.
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On Fri, Aug 26, 2022 at 12:06:55PM -0700, Linus Torvalds wrote:

> Because right now I think the main reason we cannot move the lock into
> the filesystem is literally that we've made the lock cover not just
> the filesystem part, but the "lookup and create dentry" part too.

How about rename loop prevention?  mount vs. rmdir?  d_splice_alias()
fun on tree reconnects?

> But once you have that "DCACHE_PAR_LOOKUP" bit and the
> d_alloc_parallel() logic to serialize a _particular_ dentry being
> created (as opposed to serializing all the sleeping ops to that
> directly), I really think we should strive to move the locking - that
> no longer helps the VFS dcache layer - closer to the filesystem call
> and eventually into it.

See above.