netdev - Re: "af_unix: conditionally use freezable blocking calls in read" is wrong

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Date:   Mon, 5 Dec 2016 03:52:41 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, xiyou.wangcong@...il.com
Subject: Re: "af_unix: conditionally use freezable blocking calls in read" is
 wrong

On Sun, Dec 04, 2016 at 09:42:14PM -0500, David Miller wrote:
> > 	I've run into that converting AF_UNIX to generic_file_splice_read();
> > I can kludge around that ("freezable unless ->msg_iter is ITER_PIPE"), but
> > that only delays trouble.
> > 
> > 	Note that the only other user of freezable_schedule_timeout() is
> > a very different story - it's a kernel thread, which *does* have a guaranteed
> > locking environment.  Making such assumptions in unix_stream_recvmsg(),
> > OTOH, is insane...
> 
> We have to otherwise Android phones drain their batteries in 10
> minutes.
> 
> I'm not going to revert this and be responsible for that.
> 
> So you have to find a way to make the freezable calls legitimate.

Oh, well...  As I said, I can kludge around that - call from
generic_file_splice_read() can be distinguished by looking at the
->msg_iter->type; it still means unpleasantness for kernel_recvmsg()
users - in effect, it can only be called with locks held if you know that
the socket is not an AF_UNIX one.

BTW, how do they deal with plain pipes?