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SubjectRe: [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



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