From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Eric W. Biederman" Date: Mon, 06 Jun 2022 22:10:02 +0000 Subject: Re: [PATCH 07/16] signal: Wake up the designated parent Message-Id: <87a6ap30lh.fsf@email.froward.int.ebiederm.org> List-Id: References: <871qwq5ucx.fsf_-_@email.froward.int.ebiederm.org> <20220518225355.784371-7-ebiederm@xmission.com> <20220524132553.GD14347@redhat.com> <20220524162808.GF14347@redhat.com> <20220525142845.GA2687@redhat.com> In-Reply-To: <20220525142845.GA2687@redhat.com> (Oleg Nesterov's message of "Wed, 25 May 2022 16:28:46 +0200") MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Oleg Nesterov Cc: linux-kernel@vger.kernel.org, rjw@rjwysocki.net, mingo@kernel.org, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de, bigeasy@linutronix.de, Will Deacon , tj@kernel.org, linux-pm@vger.kernel.org, Peter Zijlstra , Richard Weinberger , Anton Ivanov , Johannes Berg , linux-um@lists.infradead.org, Chris Zankel , Max Filippov , linux-xtensa@linux-xtensa.org, Kees Cook , Jann Horn , linux-ia64@vger.kernel.org, Robert OCallahan , Kyle Huey , Richard Henderson , Ivan Kokshaysky , Matt Turner , Jason Wessel , Daniel Thompson , Douglas Anderson , Douglas Miller , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras Oleg Nesterov writes: > On 05/24, Oleg Nesterov wrote: >> >> On 05/24, Oleg Nesterov wrote: >> > >> > I fail to understand this patch... >> > >> > On 05/18, Eric W. Biederman wrote: >> > > >> > > Today if a process is ptraced only the ptracer will ever be woken up in >> > > wait >> > >> > and why is this wrong? >> > >> > > Fixes: 75b95953a569 ("job control: Add @for_ptrace to do_notify_parent_cldstop()") >> > >> > how does this change fix 75b95953a569? >> >> OK, I guess you mean the 2nd do_notify_parent_cldstop() in ptrace_stop(), >> the problematic case is current->ptrace = T. Right? >> >> I dislike this patch anyway, but let me think more about it. > > OK, now that I understand the problem, the patch doesn't look bad to me, > although I'd ask to make the changelog more clear. I will see what I can do. > After this change __wake_up_parent() can't accept any "parent" from > p->parent thread group, but all callers look fine except > ptrace_detach(). Having looked at it a little more I think the change was too restrictive. For the !ptrace_reparented case there are possibly two threads of the parent process that wait_consider_task will allow to wait even with __WNOTHREAD specified. It is desirable to wake them both up. Which if I have had enough sleep reduces this patch to just: diff --git a/kernel/exit.c b/kernel/exit.c index f072959fcab7..c8156366b722 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1431,8 +1431,10 @@ static int child_wait_callback(wait_queue_entry_t *wait, unsigned mode, if (!eligible_pid(wo, p)) return 0; - if ((wo->wo_flags & __WNOTHREAD) && wait->private != p->parent) - return 0; + if ((wo->wo_flags & __WNOTHREAD) && + (wait->private != p->parent) && + (wait->private != p->real_parent)) + return 0; return default_wake_function(wait, mode, sync, key); } I think that solves the issue without missing wake-ups without adding any more. For the same set of reasons it looks like the __wake_up_parent in __ptrace_detach is just simply dead code. I don't think there is a case where when !ptrace_reparented the thread that is the real_parent can sleep in do_wait when the thread that was calling ptrace could not. That needs a very close look to confirm. Eric