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

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Date:   Sun, 04 Dec 2016 21:42:14 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     viro@...IV.linux.org.uk
Cc:     netdev@...r.kernel.org, xiyou.wangcong@...il.com
Subject: Re: "af_unix: conditionally use freezable blocking calls in read"
 is wrong

From: Al Viro <viro@...IV.linux.org.uk>
Date: Sun, 4 Dec 2016 21:04:55 +0000

> 	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...

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.