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

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Date:   Mon, 5 Dec 2016 20:24:36 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Colin Cross <ccross@...roid.com>
Subject: Re: "af_unix: conditionally use freezable blocking calls in read" is wrong

On Sun, Dec 4, 2016 at 7:52 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> 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?

I suppose this question is for Colin. ;)