linux-kernel - Re: Allow signaling a process by all its thread ids?

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Date:	Thu, 21 May 2009 17:11:31 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Simon Holm Thøgersen <odie@...aau.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Jean-Paul Calderone <exarkun@...stedmatrix.com>
Subject: Re: Allow signaling a process by all its thread ids?

Alan Cox wrote:

> You need to show that
> - The current API breaks stuff
> - The current API is absolutely invalid in posix
> - Changing it improves the functionality and power of the kernel

I think an argument could be made for these.  A task is not necessarily
a process, and kill() is only defined to send a signal to one or more
processes.  This introduces potential problems if a) it is possible to
send a signal to an individual tid, and b) a tid can be reused in a
different process.

We already have the tgkill() syscall, which fixes the ambiguity by
specifying both the pid and tid.  This is what pthread_kill() uses under
the hood.  The fact that it was seen as necessary (back in 2.5)
indicates that there are problems with kill() as currently implemented.

> - Changing it doesn't break existing applications

This last one is the kicker.  As I mentioned in my other reply, I
suspect that making such a change would break a lot of (not-quite-POSIX)
applications that assume they can send a signal to particular threads
using kill().

I see analogies to the whole fsync() issue.

Chris
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