linux-kernel - Re: [git pull] vfs part 2

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Message-ID: <CA+55aFy03Xz3GMkCR=R9QPxASs95SLTWPcTrcNqcHASTEytgKQ@mail.gmail.com>
Date:	Thu, 2 Jul 2015 12:16:14 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Jeff Layton <jlayton@...chiereds.net>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] vfs part 2

On Thu, Jul 2, 2015 at 11:40 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> All they are used for is matching response to request.  Basically, you
> can have up to 65535 pending requests.  Reusing it right after getting
> the response is fine.

Reusing a tag right after getting the completion may be fine in
theory, but it still sounds like a bad idea. Sure, it's used to match
the command with the reply, but using those kinds of things for
matching re-sends and to index into various "current data structures"
is also very common (not having looked at p9 I don't know how much it
does), and basically reusing tags "soon" tends to make those kidns of
things fragile.

Which can easily turn a "this _should_ work" into "it doesn't
_actually_ work" just because it ends up making things like race
conditions and re-ordering of replies trigger worse behavior. For
example, things like "done with previous command X" and "now starting
new command X" - if the tag is the same and those *independent*
messages get re-ordered, the tag just failed in what it was supposed
to do.

So circular allocators are likely a good idea even if there are other
layers that should handle retransmits etc.

So it does sound like it would be better to use a circular tag
allocator rather than a "lowest tag first" allocator.

            Linus
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