From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sebastian Andrzej Siewior Subject: Re: [PATCH RT 3.18] ring-buffer: Mark irq_work as HARD_IRQ to prevent deadlocks Date: Thu, 16 Apr 2015 16:26:57 +0200 Message-ID: <552FC6B1.1040000@linutronix.de> References: <552FC1FE.4020406@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Transfer-Encoding: 7bit Cc: Linux Kernel Mailing List , Steven Rostedt To: Jan Kiszka , RT Return-path: Received: from www.linutronix.de ([62.245.132.108]:44239 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753303AbbDPO1A (ORCPT ); Thu, 16 Apr 2015 10:27:00 -0400 In-Reply-To: <552FC1FE.4020406@siemens.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On 04/16/2015 04:06 PM, Jan Kiszka wrote: > ftrace may trigger rb_wakeups while holding pi_lock which will also be > requested via trace_...->...->ring_buffer_unlock_commit->...-> > irq_work_queue->raise_softirq->try_to_wake_up. This quickly causes > deadlocks when trying to use ftrace under -rt. > > Resolve this by marking the ring buffer's irq_work as HARD_IRQ. > > Signed-off-by: Jan Kiszka > --- > > I'm not yet sure if this doesn't push work into hard-irq context that > is better not done there on -rt. everything should be done in the soft-irq. > > I'm also not sure if there aren't more such cases, given that -rt turns > the default irq_work wakeup policy around. But maybe we are lucky. The only thing that is getting done in the hardirq is the FULL_NO_HZ thingy. I would be _very_ glad if we could keep it that way. > kernel/trace/ring_buffer.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index f4fbbfc..6a1939e 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -1252,6 +1252,7 @@ rb_allocate_cpu_buffer(struct ring_buffer *buffer, int nr_pages, int cpu) > init_irq_work(&cpu_buffer->irq_work.work, rb_wake_up_waiters); > init_waitqueue_head(&cpu_buffer->irq_work.waiters); > init_waitqueue_head(&cpu_buffer->irq_work.full_waiters); > + cpu_buffer->irq_work.work.flags = IRQ_WORK_HARD_IRQ; > > bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()), > GFP_KERNEL, cpu_to_node(cpu)); > Sebastian