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Date: Mon, 16 May 2011 11:13:18 +0200
From: Tejun Heo <tj@kernel.org>
To: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>, vda.linux@googlemail.com,
        linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
        akpm@linux-foundation.org, indan@nul.nu
Subject: Re: waitpid(WNOHANG) should report SIGCHLD-notified signals  [Re:
 [PATCH 09/11] job control: reorganize wait_task_stopped()]
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Hey, again.

On Sun, May 15, 2011 at 07:47:22PM +0200, Jan Kratochvil wrote:
> > But, the current WNOHANG wait is racy.  It's unlikely but definitely
> > possible for WNOHANG to fail when it's expected to succeed (not the
> > above case but more convoluted ones).
> 
> OK, so FYI it breaks current GDB.

That might be true but the race cases are very obscure.  Not sure
whether the race conditions could actually affect ptracer.  With
SEIZE, it won't, I think.

> > But, just out of curiosity, is there any reason the ptracer itself
> > should be doing something other than waitpid() while tracee is
> > running?  It's not like ptrace requests can be issued during that time
> > and sleeping waitpid() is way saner mechanism to wait for tracee
> > events than signal.
> 
> If the debugger wants to be single-threaded ("poll() model", not "threads
> model") and it wants to communicate with user and examine debuggee symbols and
> memory data it cannot use sleeping wait.  GDB is single-threaded and it
> supports `set target-async 1': info '(gdb)Background Execution'

I don't think target-async is necessarily related.  It doesn't really
matter whether the execution per-se is async or not.  The ptracer can
be a separate thread regardless and the interlocking can be added on
top (or not).

Anyways, I would recommend using sleeping wait(2)'s for ptrace event
tracking.  Well, signal notification would work but I think that would
be much more error-prone and has much higher chance of being fragile.

Thanks.

-- 
tejun
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