netdev - Re: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect for sockets in accept(3)

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Date:	Tue, 27 Oct 2015 10:08:07 +0100
From:	Casper.Dik@...cle.com
To:	Al Viro <viro@...IV.linux.org.uk>
cc:	Alan Burlison <Alan.Burlison@...cle.com>,
	David Miller <davem@...emloft.net>, eric.dumazet@...il.com,
	stephen@...workplumber.org, netdev@...r.kernel.org,
	dholland-tech@...bsd.org
Subject: Re: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect for sockets in accept(3) 



>And no, I'm not fond of such irregular ways to pass file descriptors, but
>we can't kill ioctl(2) with all weirdness hiding behind it, more's the pity...

Yeah, there are a number of calls which supposed work on one but have a 
second argument which is also a file descriptor; mostly part of ioctl().

>> In those specific cases where a system call needs to convert a file 
>> descriptor to a file pointer, there is only one routines which can be used.
>
>Obviously, but the problem is deadlock avoidance using it.

The Solaris algorithm is quite different and as such there is no chance of 
having a deadlock using that function (there is a bunch of functions)


>The memory footprint is really scary.  Bitmaps are pretty much noise, but
>blowing it by factor of 8 on normal 64bit (or 16 on something like Itanic -
>or Venus for that matter, which is more relevant for you guys)

Fair enough.  I think we have some systems with a larger cache line.

>Said that, what's the point of "close won't return until..."?  After all,
>you can't guarantee that thread with cancelled syscall won't lose CPU
>immediately upon return to userland, so it *can't* make any assumptions
>about the descriptor not having been already reused.  I don't get it - what
>does that buy for userland code?

Generally I wouldn't see that as a problem, but in the case of a socket 
blocking on accept indefinitely, I do see it as a problem especially as 
the thread actually wants to stop listening.

But in general, this is basically a problem with the application: the file 
descriptor space is shared between threads and having one thread sniping 
at open files, you do have a problem and whatever the kernel does in that 
case perhaps doesn't matter all that much: the application needs to be 
fixed anyway.

Casper

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