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Message-ID: <20150105101925.64e8ecec@gandalf.local.home> Date: Mon, 5 Jan 2015 10:19:25 -0500 From: Steven Rostedt <rostedt@...dmis.org> To: Daniel Thompson <daniel.thompson@...aro.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Jason Cooper <jason@...edaemon.net>, Russell King <linux@....linux.org.uk>, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, patches@...aro.org, linaro-kernel@...ts.linaro.org, John Stultz <john.stultz@...aro.org>, Sumit Semwal <sumit.semwal@...aro.org>, Dirk Behme <dirk.behme@...bosch.com>, Daniel Drake <drake@...lessm.com>, Dmitry Pervushin <dpervushin@...il.com>, Tim Sander <tim@...eglstein.org>, Stephen Boyd <sboyd@...eaurora.org> Subject: Re: [PATCH 3.19-rc2 v13 4/5] ARM: Add support for on-demand backtrace of other CPUs On Mon, 5 Jan 2015 14:54:58 +0000 Daniel Thompson <daniel.thompson@...aro.org> wrote: > + > +/* For reliability, we're prepared to waste bits here. */ > +static DECLARE_BITMAP(backtrace_mask, NR_CPUS) __read_mostly; > +static cpumask_t printtrace_mask; > + > +#define NMI_BUF_SIZE 4096 > + > +struct nmi_seq_buf { > + unsigned char buffer[NMI_BUF_SIZE]; > + struct seq_buf seq; > +}; > + > +/* Safe printing in NMI context */ > +static DEFINE_PER_CPU(struct nmi_seq_buf, nmi_print_seq); > + > +/* "in progress" flag of arch_trigger_all_cpu_backtrace */ > +static unsigned long backtrace_flag; > + > +/* > + * It is not safe to call printk() directly from NMI handlers. > + * It may be fine if the NMI detected a lock up and we have no choice > + * but to do so, but doing a NMI on all other CPUs to get a back trace > + * can be done with a sysrq-l. We don't want that to lock up, which > + * can happen if the NMI interrupts a printk in progress. > + * > + * Instead, we redirect the vprintk() to this nmi_vprintk() that writes > + * the content into a per cpu seq_buf buffer. Then when the NMIs are > + * all done, we can safely dump the contents of the seq_buf to a printk() > + * from a non NMI context. > + */ > +static int nmi_vprintk(const char *fmt, va_list args) > +{ > + struct nmi_seq_buf *s = this_cpu_ptr(&nmi_print_seq); > + unsigned int len = seq_buf_used(&s->seq); > + > + seq_buf_vprintf(&s->seq, fmt, args); > + return seq_buf_used(&s->seq) - len; > +} > + This is the same code as in x86. I wonder if we should move the duplicate code into kernel/printk/ and have it compiled if the arch requests it (CONFIG_ARCH_WANT_NMI_PRINTK or something). That way we don't have 20 copies of the same nmi_vprintk() and later find that we need to change it, and have to change it in 20 different archs. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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