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

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Message-ID: <20161204210455.GI1555@ZenIV.linux.org.uk>
Date:   Sun, 4 Dec 2016 21:04:55 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     netdev@...r.kernel.org
Cc:     Cong Wang <xiyou.wangcong@...il.com>
Subject: "af_unix: conditionally use freezable blocking calls in read" is
 wrong

	Could we please kill that kludge?  "af_unix: use freezable blocking
calls in read" had been wrong to start with; having a method make assumptions
of that sort ("nobody will call me while holding locks I hadn't thought of")
is asking for serious trouble.  splice is just a place where lockdep has
caught that - we *can't* assume that nobody will ever call kernel_recvmsg()
while holding some locks.

	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...