linux-kernel - Re: PTRACE_SEIZE should not stop [Re: [PATCH 02/11] ptrace: implement PTRACE

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Date:	Mon, 16 May 2011 15:51:41 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Jan Kratochvil <jan.kratochvil@...hat.com>
Cc:	oleg@...hat.com, vda.linux@...glemail.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu
Subject: Re: PTRACE_SEIZE should not stop  [Re: [PATCH 02/11] ptrace:
 implement PTRACE_SEIZE]

Hello, Jan.

On Mon, May 16, 2011 at 03:03:39PM +0200, Jan Kratochvil wrote:
> If you check the GDB debugging session transcript I gave GDB stopped in
> a moment when all the threads already returned from tkill()s sending SIGUSR1s
> and SIGUSR2.

Yeap.

> All threads are stopped, user is investigating the situation.  And GDB tells
> the user (only) SIGUSR1 was delivered.  The user has no chance to find out
> SIGUSR2 is already pending / to be delivered.  This is one of the many reasons
> why debugging various racy cases is a nightmare.
> 
> I was trying to suggest some ways how to give user the complete overview of
> the debuggee situation - where both SIGUSR1 and SIGUSR2 would be reported on
> the first stop.
> 
> You are right GDB could examine SigCgt, SigBlk (not sure if others) and report
> those signals.  Maybe it is right that way and we can forget about it.

Yes, I think this is the correct way to deal with it.  Multiple
signals can be pending and/or blocked but a single thread can only
deliver a single signal at any given time, which may involve userland
execution.  Parallel delivery simply isn't defined, so I think what
you want here is the list of pending signals, not deliveries, and then
consulting the pending mask is the obvious thing to do.

> There is (was) a larger problem of signals reordering which I fixed in
> 	[patch 3/4]#3 linux-nat: Do not respawn signals
> 	http://sourceware.org/ml/gdb-patches/2010-09/msg00360.html
> but that mess you should have fixed by PTRACE_INTERRUPT which no longer
> interacts with signals.  But it gave me the idea of another situation above
> where the debugger may want to know all the currently pending signals at once.

Yeap, I agree that it would be nice if gdb informs the user of the
pending signals when it stops for signal delivery.

Thank you.

-- 
tejun
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