linux-kernel - Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with given pids

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Message-Id: <201111221204.39235.pedro@codesourcery.com>
Date:	Tue, 22 Nov 2011 12:04:38 +0000
From:	Pedro Alves <pedro@...esourcery.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...ux.intel.com>,
	Roland McGrath <roland@...k.frob.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	James Bottomley <jbottomley@...allels.com>
Subject: Re: [RFC][PATCH 0/3] fork: Add the ability to create tasks with given pids

On Tuesday 22 November 2011 11:11:02, Pavel Emelyanov wrote:
> > Can't we for now allow this for root and then later allow CAP_CHECKPOINT 
> > that Cyrill suggested?  Or do we want to allow setting pids even w/o CR 
> > for NS creator?
> 
> I think that systemd guys can play with it. E.g. respawning daemons with predefined
> pids sounds like an interesting thing to play with.

This whole userspace C/R stuff and being able to set the child's pid has potential
of being very useful for GDB too, allowing a much better reimplementation of its
old checkpointing feature [*], and allowing for a faster reverse debugging
implementation, by being able to do faster rewinding -- restore snapshot and replay
instructions up to N (by single stepping or running to breakpoint), rather than
manually undoing the effects of each instruction, one by one.

IOW, root only would be a shame from GDB's perspective.

[*] GDB has an old checkpointing feature ("checkpoint; info checkpoints" commands)
based on forcing the tracee to fork, and holding on the fork child behind the
scenes as a checkpoint.  To restore the debugging state to a previous checkpoint,
gdb swaps the current debuggee for the fork child as transparently for the user
as was possible. Obviously this has a bunch of limitations and downsides like
only working on non-threaded programs, and the inferior's pid changing...

-- 
Pedro Alves
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