From: Thomas Gleixner <tglx@linutronix.de>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
Roman Pen <roman.penyaev@profitbricks.com>,
Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Chunming Zhou <David1.Zhou@amd.com>,
Alex Deucher <alexander.deucher@amd.com>
Subject: Re: [PATCH 2/2] kthread: don't use to_live_kthread() in kthread_park() and kthread_unpark()
Date: Wed, 9 Nov 2016 09:45:08 +0100 (CET) [thread overview]
Message-ID: <alpine.DEB.2.20.1611090908270.3501@nanos> (raw)
In-Reply-To: <20161031200823.GC19430@redhat.com>
On Mon, 31 Oct 2016, Oleg Nesterov wrote:
> I think we need to unexport kthread_park/unpark, and either make it return
> "void" or actually fix the race with kthred_stop/exit. This patch just adds
> WARN_ON(PF_EXITING) for now.
I'll have a look.
> The usage of kthread_park() in cpuhp code (cpu.c, smpboot.c, stop_machine.c)
> is fine. It can never see an exiting/exited kthread, smpboot_destroy_threads()
> clears *ht->store, smpboot_park_thread() checks it is not NULL under the same
> smpboot_threads_lock. cpuhp_threads and cpu_stop_threads never exit, so other
> callers are fine too.
>
> But it has two more users:
>
> - watchdog_park_threads() and it does not look nice. The code is actually
> correct, get_online_cpus() ensures that kthread_park() can't race with
> itself (note that kthread_park() can't handle this race correctly), but
> imo it should not use kthread_park() directly.
Should we provide an interface through the smpboot thread infrastructure for
this?
> - drivers/gpu/drm/amd/scheduler/gpu_scheduler.c and I think it should not
> use kthread_park() too.
>
> But this patch should not break this code. kthread_park() must not be
> called after amd_sched_fini() which does kthread_stop(), otherwise even
> to_live_kthread() is not safe because task_struct can be already freed
> and sched->thread can point to nowhere.
Right. That's why the smpboot thread code holds a task ref which is only
released after the thread has been stopped.
I can see why that gpu driver wants to use the park mechanism and I guess
there are other legitimate use cases as well. I prefer to implement a
park/unpark variant which is safe to use on arbitrary kthreads over forcing
driver writers to come up with even more broken open coded implementations
of that.
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
next prev parent reply other threads:[~2016-11-09 8:47 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-25 11:05 [PATCH v3 1/1] kthread: allocate kthread structure using kmalloc Roman Pen
2016-10-25 14:03 ` Oleg Nesterov
2016-10-25 15:43 ` Oleg Nesterov
2016-10-25 16:08 ` Roman Penyaev
2016-10-25 16:17 ` Oleg Nesterov
2016-10-25 16:52 ` Andy Lutomirski
2016-10-26 14:14 ` Oleg Nesterov
2016-10-26 14:57 ` Thomas Gleixner
2016-10-26 15:51 ` Oleg Nesterov
2016-10-26 18:31 ` Thomas Gleixner
2016-10-28 16:11 ` [PATCH 0/2] kthread: make struct kthread kmalloc'ed Oleg Nesterov
2016-10-28 16:11 ` [PATCH 1/2] " Oleg Nesterov
2016-10-28 18:48 ` Thomas Gleixner
2016-10-28 16:12 ` [PATCH 2/2] Revert "kthread: Pin the stack via try_get_task_stack()/put_task_stack() in to_live_kthread() function" Oleg Nesterov
2016-10-28 18:49 ` Thomas Gleixner
2016-10-28 18:44 ` [PATCH 0/2] kthread: make struct kthread kmalloc'ed Thomas Gleixner
2016-10-31 20:07 ` [PATCH 0/2] kthread: kill to_live_kthread() Oleg Nesterov
2016-10-31 20:07 ` [PATCH 1/2] kthread: don't use to_live_kthread() in kthread_stop() Oleg Nesterov
2016-11-09 7:58 ` Thomas Gleixner
2016-10-31 20:08 ` [PATCH 2/2] kthread: don't use to_live_kthread() in kthread_park() and kthread_unpark() Oleg Nesterov
2016-11-09 8:45 ` Thomas Gleixner [this message]
2016-11-09 17:27 ` Oleg Nesterov
2016-11-10 17:19 ` [PATCH 0/1] kthread: don't abuse kthread_create_on_cpu() in __kthread_create_worker() Oleg Nesterov
2016-11-10 17:20 ` [PATCH 1/1] " Oleg Nesterov
2016-11-14 11:12 ` Petr Mladek
2016-11-14 11:09 ` [PATCH 0/1] " Petr Mladek
2016-11-07 18:23 ` [PATCH 0/2] kthread: kill to_live_kthread() Andy Lutomirski
2016-10-26 16:13 ` [PATCH v3 1/1] kthread: allocate kthread structure using kmalloc Oleg Nesterov
2016-10-27 2:56 ` Josh Poimboeuf
2016-10-27 13:10 ` Oleg Nesterov
2016-10-25 15:46 ` Roman Penyaev
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