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

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Date:	Sun, 15 May 2011 19:15:12 +0200
From:	Jan Kratochvil <jan.kratochvil@...hat.com>
To:	Tejun Heo <tj@...nel.org>
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]

On Sun, 15 May 2011 18:26:30 +0200, Tejun Heo wrote:
> the code to SEIZE and establish initial state would be simple.

In normal case yes; but one needs to handle all the corner cases when the
first signal is not INTERRUPT; which one usually does not handle as during
development (=in normal cases) it is always INTERRUPT.

Such thing is even difficult to test in QA testcases as in some cases one just
cannot reproduce the (in current case) non-SIGSTOP signal arriving as first
one.

> How long does it take to attach to / detach from 10000+ threads?  If
> you don't do it serially, it shouldn't take that long.

It is not (such) a problem it takes time.  It is a problem it stops the tracee
for a moment which completely changes the tracee's racy behavior one tries to
debug.

> You can tell them apart from userland and it doesn't matter which order or
> how many times INTERRUPT occurs.

I must know in which order they come to know when the tracee is still stopped
and I collect the signals to be displayed to the user and at which moment
there are no more signals in the queue and I start waiting on the debuggee
which started running.

Otherwise I can workaround it by various waitpid(NOHANG)s but it is better if
the ordering and when INTERRUPT is / is not reported is well defined.

Thanks,
Jan
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