netdev - Re: [PATCH v4 net-next 00/15] af

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Message-ID: <20240304173104.45591-1-kuniyu@amazon.com>
Date: Mon, 4 Mar 2024 09:31:04 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <pabeni@...hat.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<kuni1840@...il.com>, <kuniyu@...zon.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 net-next 00/15] af_unix: Rework GC.

From: Paolo Abeni <pabeni@...hat.com>
Date: Mon, 04 Mar 2024 17:18:32 +0100
> On Thu, 2024-02-29 at 18:22 -0800, Kuniyuki Iwashima wrote:
> > When we pass a file descriptor to an AF_UNIX socket via SCM_RIGTHS,
> > the underlying struct file of the inflight fd gets its refcount bumped.
> > If the fd is of an AF_UNIX socket, we need to track it in case it forms
> > cyclic references.
> > 
> > Let's say we send a fd of AF_UNIX socket A to B and vice versa and
> > close() both sockets.
> > 
> > When created, each socket's struct file initially has one reference.
> > After the fd exchange, both refcounts are bumped up to 2.  Then, close()
> > decreases both to 1.  From this point on, no one can touch the file/socket.
> > 
> > However, the struct file has one refcount and thus never calls the
> > release() function of the AF_UNIX socket.
> > 
> > That's why we need to track all inflight AF_UNIX sockets and run garbage
> > collection.
> > 
> > This series replaces the current GC implementation that locks each inflight
> > socket's receive queue and requires trickiness in other places.
> > 
> > The new GC does not lock each socket's queue to minimise its effect and
> > tries to be lightweight if there is no cyclic reference or no update in
> > the shape of the inflight fd graph.
> > 
> > The new implementation is based on Tarjan's Strongly Connected Components
> > algorithm, and we will consider each inflight AF_UNIX socket as a vertex
> > and its file descriptor as an edge in a directed graph.
> > 
> > For the details, please see each patch.
> > 
> >   patch 1  -  3 : Add struct to express inflight socket graphs
> >   patch       4 : Optimse inflight fd counting
> >   patch 5  -  6 : Group SCC possibly forming a cycle
> >   patch 7  -  8 : Support embryo socket
> >   patch 9  - 11 : Make GC lightweight
> >   patch 12 - 13 : Detect dead cycle references
> >   patch      14 : Replace GC algorithm
> >   patch      15 : selftest
> > 
> > After this series is applied, we can remove the two ugly tricks for race,
> > scm_fp_dup() in unix_attach_fds() and spin_lock dance in unix_peek_fds()
> > as done in patch 14/15 of v1.
> 
> I plan to have a better look tomorrow.

Thanks for your time!

> 
> Generally speaking we have a timing problem. This looks great, but also
> quite complex, and thus the potential for untrivial regressions. 
> 
> We are very late in this cycle and likely there will not be rc8, would
> you be ok to eventually postpone this to the next cycle?

No problem at all, I was also thinking about that after reading the
announcement :)