Re: [PATCH net] af_unix: Guard against other == sk in unix_dgram_sendmsg

From	Rainer Weikusat <rweikusat@mobileactivedefense.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH net] af_unix: Guard against other == sk in unix_dgram_sendmsg
Date	2016-02-12 22:10 +0100
Message-ID	<r1t8t-3TO-7@gated-at.bofh.it> (permalink)
References	(7 earlier) <r1i3o-4XY-17@gated-at.bofh.it> <r1lXl-7yy-19@gated-at.bofh.it> <r1s2K-2YC-17@gated-at.bofh.it> <r1sm5-3kI-9@gated-at.bofh.it> <r1sP8-3ws-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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Ben Hutchings <ben@decadent.org.uk> writes:
> On Fri, 2016-02-12 at 20:17 +0000, Rainer Weikusat wrote:

[...]

>>>> I don't think this should apply when
>>>> receiving and sending sockets are identical. But that's just my
>>>> opinion. The other option would be to avoid the unix_state_double_lock
>>>> for sk == other.
>>> 
>>> Given that unix_state_double_lock() already handles sk == other, I'm
>>> not sure why you think it needs to be avoided.
>> 
>> Because the whole complication of restarting the operation after locking
>> both sk and other because other had to be unlocked before calling
>> unix_state_double_lock is useless for this case:

[...]

> Well of course it's useless, but it's also harmless.  

As is adding a

for (i = 0; i < 1000000; ++i);

between any two statements. And this isn't even entirely true as the
pointless double-lock will then require "did we pointlessly
doube-lock" checks elsewhere. I think it should be possible to do this
in a simpler way by not pointlessly double-locking (this may be
wrong but it's worth a try).

> If we really wanted to optimise this we could also skip unlocking if
> other < sk.

I wouldn't want to hardcode assumptions about the unix_state_double_lock
algorithm in functions using it.

Thread

Re: Bug 4.1.16: self-detected stall in net/unix/? Philipp Hahn <pmhahn@pmhahn.de> - 2016-02-11 14:50 +0100
  Re: Bug 4.1.16: self-detected stall in net/unix/? Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2016-02-11 17:00 +0100
    Re: Bug 4.1.16: self-detected stall in net/unix/? Ben Hutchings <ben@decadent.org.uk> - 2016-02-11 18:10 +0100
      Re: Bug 4.1.16: self-detected stall in net/unix/? Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2016-02-11 18:50 +0100
        Re: Bug 4.1.16: self-detected stall in net/unix/? Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2016-02-11 19:00 +0100
        Re: Bug 4.1.16: self-detected stall in net/unix/? Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2016-02-11 19:40 +0100
          [PATCH net] af_unix: Guard against other == sk in unix_dgram_sendmsg Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2016-02-11 20:40 +0100
            Re: [PATCH net] af_unix: Guard against other == sk in  unix_dgram_sendmsg Philipp Hahn <pmhahn@pmhahn.de> - 2016-02-12 10:20 +0100
              Re: [PATCH net] af_unix: Guard against other == sk in unix_dgram_sendmsg Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2016-02-12 14:30 +0100
                Re: [PATCH net] af_unix: Guard against other == sk in  unix_dgram_sendmsg Ben Hutchings <ben@decadent.org.uk> - 2016-02-12 21:00 +0100
                Re: [PATCH net] af_unix: Guard against other == sk in unix_dgram_sendmsg Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2016-02-12 21:20 +0100
                Re: [PATCH net] af_unix: Guard against other == sk in  unix_dgram_sendmsg Ben Hutchings <ben@decadent.org.uk> - 2016-02-12 21:50 +0100
                Re: [PATCH net] af_unix: Guard against other == sk in unix_dgram_sendmsg Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2016-02-12 22:10 +0100
            Re: [PATCH net] af_unix: Guard against other == sk in  unix_dgram_sendmsg David Miller <davem@davemloft.net> - 2016-02-16 19:00 +0100

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