linux-kernel - Re: [PATCH 02/10] job control: introduce JOBCTL_TRAP

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Message-ID: <20110518164814.GA28848@redhat.com>
Date:	Wed, 18 May 2011 18:48:14 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	jan.kratochvil@...hat.com, vda.linux@...glemail.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu, bdonlan@...il.com
Subject: Re: [PATCH 02/10] job control: introduce JOBCTL_TRAP_STOP and use
	it for group stop trap

On 05/16, Tejun Heo wrote:
>
> This patch adds a new jobctl flag - JOBCTL_TRAP_STOP.  When set, it
> triggers a trap site, which behaves like group stop trap, in
> get_signal_to_deliver() before checking for pending signals.  While
> ptraced, do_signal_stop() doesn't stop itself.  It initiates group
> stop if requested and schedules JOBCTL_TRAP_STOP and returns, which
> makes its caller - get_signal_to_deliver() - to relock, check and
> enter the trap.

Heh. Starting from this patch, I think I will never understand this
code in details ;)

> Although this adds an unlock-relocking between checking of
> JOBCTL_STOP_PENDING and actually trapping for STOP, this doesn't
> affect correctness.

Well, I think it does affect. Although the problem is minor.

> ptrace_stop() already had conditional
> unlock-relocking

Yes, but ptrace_stop() can't send the CLD_STOPPED notfication before
the tracee is ready for do_wait(WNOHANG). Contrary, get_signal_to_deliver()
can if we race with SIGCONT.

> While at it, add proper function comment to do_signal_stop() and make
> it return bool.
> ...
>
> + * RETURNS:
> + * %false if group stop is already cancelled and nothing happened.  %true
> + * if participated in group stop.

Well, the traced task didn't participate yet... Nevermind.



So far I can't really understand why do we have both JOBCTL_TRAP_STOP and
and JOBCTL_TRAPPING... Please ignore, I didn't read other patches yet.


Hmm. And afaics there is a bug in do_signal_stop(), after
"[PATCH 6/9] job control: make task_clear_jobctl_pending() clear TRAPPING automatically"
->jobctl &= ~JOBCTL_STOP_SIGMASK is no longer safe. We can clear _TRAPPING
without wakeup.

Oleg.

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