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

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Message-ID: <4866F6FE.9000503@goop.org>
Date:	Sat, 28 Jun 2008 19:44:14 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Török Edwin <edwintorok@...il.com>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Ctrl+C doesn't interrupt process waiting for I/O

Török Edwin wrote:
> Hi,
>
> I have encountered the following situation several times, but I've been
> unable to come up with a way to reproduce this until now:
>  - some process is keeping the disk busy (some cron job for example:
> updatedb, chkrootkit, ...)
>  - other processes that want to do I/O have to wait (this is normal)
>  - I have a (I/O bound) process running in my terminal, and I want to
> interrupt it with Ctrl+C
>  - I type Ctrl+C several times, and the process is not interrupted for
> several seconds (10-30 secs)
>  - if I type Ctrl+Z, and use kill %1 the process dies faster than
> waiting for it to react to Ctrl+C
>
> This issue occurs both on my x86-64 machine that uses reiserfs, and on
> my x86 machine that uses XFS, so it doesn't seem related to the
> underlying FS.
> I use 2.6.25-2 and 2.6.26-rc8 now; I don't recall seeing this behaviour
> with old kernels (IIRC I see this since 2.6.21 or 2.6.23).
>
> Is this intended behaviour, or should I report a bug?
>   

Yes, it's intended behaviour.  Filesystem IO syscalls are considered 
"fast" and are interruptible.  Usermode code can reasonably expect that 
file IO will never return EINTR.

That said, if a program is blocking for tens of seconds in block IO, 
then that could be a problem in itself.

    J
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