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

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Date:	Thu, 22 Oct 2015 20:24:51 +0200
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) 


>On Thu, Oct 22, 2015 at 08:34:19AM +0200, Casper.Dik@...cle.com wrote:
>> 
>> 
>> >And I'm really curious about the things Solaris would do with dup2() there.
>> >Does it take into account the possibility of new accept() coming just as
>> >dup2() is trying to terminate the ongoing ones?  Is there a window when
>> >descriptor-to-file lookups would fail?  Looks like a race/deadlock country...
>> 
>> Solaris does not "terminate" threads, instead it tells them that the
>> file descriptor information used is stale and wkae's up the thread.
>
>Sorry, lousy wording - I meant "terminate syscall in another thread".
>Better yet, make that "what happens if new accept(newfd) comes while dup2()
>waits for affected syscalls in other threads to finish"?  Assuming it
>does wait, that is..

No there is no such window; the accept() call either returns EBADF
(dup2()) wins the race or it returns a new file descriptor (and dup2()
then closes the listening descriptor).

One or the other.

>While we are at it, what's the relative order of record locks removal
>and switching the meaning of newfd?  In our kernel it happens *after*
>the switchover (i.e. if another thread is waiting for a record lock held on
>any alias of newfd and we do dup2(oldfd, newfd), the waiter will not see
>the state with newfd still refering to what it used to; note that waiter
>might be using any descriptor refering to the file newfd used to refer
>to, so it won't be affected by the "wake those who had the meaning of
>their arguments change" side of things).

The external behaviour atomic; you cannot distinguish the order
between the closing of the original file (and waking up other threads
waiting for a record lock) or changing the file referenced by that newfd.

But this not include a global or process specific lock.

Casper

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