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Wed, 12 Feb 2020 15:20:06 -0800 (PST) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id q130sm227784qka.114.2020.02.12.15.20.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Feb 2020 15:20:05 -0800 (PST) Date: Wed, 12 Feb 2020 18:20:05 -0500 From: Joel Fernandes To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, rostedt@goodmis.org, mingo@kernel.org, gregkh@linuxfoundation.org, gustavo@embeddedor.com, tglx@linutronix.de, paulmck@kernel.org, josh@joshtriplett.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com Subject: Re: [PATCH v2 3/9] rcu,tracing: Create trace_rcu_{enter,exit}() Message-ID: <20200212232005.GC115917@google.com> References: <20200212210139.382424693@infradead.org> <20200212210749.971717428@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200212210749.971717428@infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 12, 2020 at 10:01:42PM +0100, Peter Zijlstra wrote: > To facilitate tracers that need RCU, add some helpers to wrap the > magic required. > > The problem is that we can call into tracers (trace events and > function tracing) while RCU isn't watching and this can happen from > any context, including NMI. > > It is this latter that is causing most of the trouble; we must make > sure in_nmi() returns true before we land in anything tracing, > otherwise we cannot recover. > > These helpers are macros because of header-hell; they're placed here > because of the proximity to nmi_{enter,exit{(). > > Signed-off-by: Peter Zijlstra (Intel) > --- > include/linux/hardirq.h | 48 ++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 48 insertions(+) > > --- a/include/linux/hardirq.h > +++ b/include/linux/hardirq.h > @@ -89,4 +89,52 @@ extern void irq_exit(void); > arch_nmi_exit(); \ > } while (0) > > +/* > + * Tracing vs RCU > + * -------------- > + * > + * tracepoints and function-tracing can happen when RCU isn't watching (idle, > + * or early IRQ/NMI entry). > + * > + * When it happens during idle or early during IRQ entry, tracing will have > + * to inform RCU that it ought to pay attention, this is done by calling > + * rcu_irq_enter_irqsave(). > + * > + * On NMI entry, we must be very careful that tracing only happens after we've > + * incremented preempt_count(), otherwise we cannot tell we're in NMI and take > + * the special path. > + */ > + > +#define __TR_IRQ 1 > +#define __TR_NMI 2 > + > +#define trace_rcu_enter() \ > +({ \ > + unsigned long state = 0; \ > + if (!rcu_is_watching()) { \ > + if (in_nmi()) { \ > + state = __TR_NMI; \ > + rcu_nmi_enter(); \ > + } else { \ > + state = __TR_IRQ; \ > + rcu_irq_enter_irqsave(); \ I think this can be simplified. You don't need to rely on in_nmi() here. I believe for NMI's, you can just call rcu_irq_enter_irqsave() and that should be sufficient to get RCU watching. Paul can correct me if I'm wrong, but I am pretty sure that would work. In fact, I think a better naming for rcu_irq_enter_irqsave() pair could be (in the first patch): rcu_ensure_watching_begin(); rcu_ensure_watching_end(); thanks, - Joel > + } \ > + } \ > + state; \ > +}) > + > +#define trace_rcu_exit(state) \ > +do { \ > + switch (state) { \ > + case __TR_IRQ: \ > + rcu_irq_exit_irqsave(); \ > + break; \ > + case __TR_NMI: \ > + rcu_nmi_exit(); \ > + break; \ > + default: \ > + break; \ > + } \ > +} while (0) > + > #endif /* LINUX_HARDIRQ_H */ > >