linux-kernel - Re: Ctrl+C doesn't interrupt process waiting for I/O

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Date:	Sun, 29 Jun 2008 19:57:58 -0400
From:	Bill Davidsen <davidsen@....com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Avi Kivity <avi@...ranet.com>,
	Török Edwin <edwintorok@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Ctrl+C doesn't interrupt process waiting for I/O

Jeremy Fitzhardinge wrote:
> Avi Kivity wrote:
>> Applications should not assume that write() (or other syscalls) can't 
>> return EINTR.  Not all filesystems have a bounded-time backing store.
> 
> The distinction between 'fast' (filesystem) and 'slow' (terminals and 
> pipes) blocking syscalls goes back to the earliest days of Unix, and is 
> part of the ABI.  Most filesystem syscalls are not documented to ever 
> return EINTR.
> 
>> 'soft' has its own problems; namely false positives when someone steps 
>> on the network cable, temporarily blocking packet flow, or when using 
>> a clustered server which may take some time to recover from a fault.
> 
> Sure.  It's the basic problem of trying to make network access 
> transparent by hiding the failure modes.  You either need to put up with 
> spurious timeouts caused by transient failures, or unbounded blocking on 
> real failures.
> 
Basic problem is that you can get a process which you can't interrupt 
(in in most cases can't kill) which has resources tied up. Given the 
choice between surprising a process with an EINTR or killing it during a 
reboot to get the system usable again, I would rather surprise.

The current situation is infrequent but not unheard of. And the causes 
are not all rooted in NFS, I used to see this 4-5 times a year when I 
was running nntp clusters with heavily threaded applications, every once 
in a while some thread would hang in a waiting for i/o state and could 
be killed or fixed. I can't see that an application error would result 
in a thread being left waiting i/o and uninterruptable, that's a kernel 
state.

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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