On Thu, Jun 19, 2014 at 05:33:31PM -0400, Steven Rostedt wrote: > From: "Steven Rostedt (Red Hat)" <[email protected]> > > Being able to divert printk to call another function besides the normal > logging is useful for such things like NMI handling. If some functions > are to be called from NMI that does printk() it is possible to lock up > the box if the nmi handler triggers when another printk is happening. > > One example of this use is to perform a stack trace on all CPUs via NMI. > But if the NMI is to do the printk() it can cause the system to lock up. > By allowing the printk to be diverted to another function that can safely > record the printk output and then print it when it in a safe context > then NMIs will be safe to call these functions like show_regs(). > > Signed-off-by: Steven Rostedt <[email protected]>
Acked-by: Paul E. McKenney <[email protected]> > --- > include/linux/percpu.h | 3 +++ > include/linux/printk.h | 2 ++ > kernel/printk/printk.c | 38 +++++++++++++++++++++++++++++--------- > 3 files changed, 34 insertions(+), 9 deletions(-) > > diff --git a/include/linux/percpu.h b/include/linux/percpu.h > index 8419053d0f2e..9997c92ce3bd 100644 > --- a/include/linux/percpu.h > +++ b/include/linux/percpu.h > @@ -802,4 +802,7 @@ do { __this_cpu_preempt_check("or"); > \ > > (__this_cpu_preempt_check("cmpxchg_double"),__pcpu_double_call_return_bool(raw_cpu_cmpxchg_double_, > (pcp1), (pcp2), (oval1), (oval2), (nval1), (nval2))) > #endif > > +/* To avoid include hell, as printk can not declare this, we declare it here > */ > +DECLARE_PER_CPU(printk_func_t, printk_func); > + > #endif /* __LINUX_PERCPU_H */ > diff --git a/include/linux/printk.h b/include/linux/printk.h > index 319ff7e53efb..e26310b2d2fd 100644 > --- a/include/linux/printk.h > +++ b/include/linux/printk.h > @@ -159,6 +159,8 @@ extern int kptr_restrict; > > extern void wake_up_klogd(void); > > +typedef int(*printk_func_t)(const char *fmt, va_list args); > + > void log_buf_kexec_setup(void); > void __init setup_log_buf(int early); > void dump_stack_set_arch_desc(const char *fmt, ...); > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index ea2d5f6962ed..d019ccd10d4c 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -1764,6 +1764,30 @@ asmlinkage int printk_emit(int facility, int level, > } > EXPORT_SYMBOL(printk_emit); > > +int vprintk_default(const char *fmt, va_list args) > +{ > + int r; > + > +#ifdef CONFIG_KGDB_KDB > + if (unlikely(kdb_trap_printk)) { > + r = vkdb_printf(fmt, args); > + return r; > + } > +#endif > + r = vprintk_emit(0, -1, NULL, 0, fmt, args); > + > + return r; > +} > +EXPORT_SYMBOL_GPL(vprintk_default); > + > +/* > + * This allows printk to be diverted to another function per cpu. > + * This is useful for calling printk functions from within NMI > + * without worrying about race conditions that can lock up the > + * box. > + */ > +DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default; > + > /** > * printk - print a kernel message > * @fmt: format string > @@ -1787,19 +1811,15 @@ EXPORT_SYMBOL(printk_emit); > */ > asmlinkage __visible int printk(const char *fmt, ...) > { > + printk_func_t vprintk_func; > va_list args; > int r; > > -#ifdef CONFIG_KGDB_KDB > - if (unlikely(kdb_trap_printk)) { > - va_start(args, fmt); > - r = vkdb_printf(fmt, args); > - va_end(args); > - return r; > - } > -#endif > va_start(args, fmt); > - r = vprintk_emit(0, -1, NULL, 0, fmt, args); > + preempt_disable(); > + vprintk_func = this_cpu_read(printk_func); > + r = vprintk_func(fmt, args); > + preempt_enable(); > va_end(args); > > return r; > -- > 2.0.0.rc2 > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
