linux-kernel - Re: [PATCH] fix race in AF

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Date:	Wed, 06 Jun 2007 07:26:52 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	davem@...emloft.net
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix race in AF_UNIX

> > From: Miklos Szeredi <miklos@...redi.hu>
> > Date: Mon, 04 Jun 2007 11:45:32 +0200
> > 
> > > > A recv() on an AF_UNIX, SOCK_STREAM socket can race with a
> > > > send()+close() on the peer, causing recv() to return zero, even though
> > > > the sent data should be received.
> > > > 
> > > > This happens if the send() and the close() is performed between
> > > > skb_dequeue() and checking sk->sk_shutdown in unix_stream_recvmsg():
> > > > 
> > > > process A  skb_dequeue() returns NULL, there's no data in the socket queue
> > > > process B  new data is inserted onto the queue by unix_stream_sendmsg()
> > > > process B  sk->sk_shutdown is set to SHUTDOWN_MASK by unix_release_sock()
> > > > process A  sk->sk_shutdown is checked, unix_release_sock() returns zero
> > > 
> > > This is only part of the story.  It turns out, there are other races
> > > involving the garbage collector, that can throw away perfectly good
> > > packets with AF_UNIX sockets in them.
> > > 
> > > The problems arise when a socket goes from installed to in-flight or
> > > vica versa during garbage collection.  Since gc is done with a
> > > spinlock held, this only shows up on SMP.
> > > 
> > > The following patch fixes it for me, but it's possibly the wrong
> > > approach.
> > > 
> > > Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
> 
> Concerning this specific patch I think we need to rethink it
> a bit.
> 
> Holding a global mutex over recvmsg() calls under AF_UNIX is pretty
> much a non-starter, this will kill performance for multi-threaded
> apps.

That's an rwsem held for read.  It's held for write in unix_gc() only
for a short duration, and unix_gc() should only rarely be called.  So
I don't think there's any performance problem here.

> 
> One possible solution is for the garbage collection code to hold the
> u->readlock while processing a socket, but be careful about deadlocks.

That would have exactly the same effect.  Only the code would be more
complicated.

Miklos
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