Messages in this thread | | | Date | Mon, 5 Jan 2015 10:19:25 -0500 | From | Steven Rostedt <> | 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@linaro.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
| |