Re: [PATCH printk v4 4/6] printk: remove NMI tracking

From: Petr Mladek
Date: Wed Jul 21 2021 - 08:04:27 EST

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On Thu 2021-07-15 21:39:57, John Ogness wrote:
> All NMI contexts are handled the same as the safe context: store the
> message and defer printing. There is no need to have special NMI
> context tracking for this. Using in_nmi() is enough.
>
> There are several parts of the kernel that are manually calling into
> the printk NMI context tracking in order to cause general printk
> deferred printing:
>
> arch/arm/kernel/smp.c
> arch/powerpc/kexec/crash.c
> kernel/trace/trace.c
>
> For these users, provide a new function pair
> printk_deferred_enter/exit that explicitly achieves the same
> objective.
>
> Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
> ---
> arch/arm/kernel/smp.c | 4 ++--
> arch/powerpc/kexec/crash.c | 2 +-
> include/linux/hardirq.h | 2 --
> include/linux/printk.h | 31 +++++++++++++++++++------------
> init/Kconfig | 5 -----
> kernel/printk/internal.h | 8 --------
> kernel/printk/printk_safe.c | 37 +------------------------------------
> kernel/trace/trace.c | 4 ++--
> 8 files changed, 25 insertions(+), 68 deletions(-)
>
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index c7bb168b0d97..842427ff2b3c 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -667,9 +667,9 @@ static void do_handle_IPI(int ipinr)
> break;
>
> case IPI_CPU_BACKTRACE:
> - printk_nmi_enter();
> + printk_deferred_enter();
> nmi_cpu_backtrace(get_irq_regs());
> - printk_nmi_exit();
> + printk_deferred_exit();
> break;
>
> default:
> diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
> index 0196d0c211ac..1070378c8e35 100644
> --- a/arch/powerpc/kexec/crash.c
> +++ b/arch/powerpc/kexec/crash.c
> @@ -313,7 +313,7 @@ void default_machine_crash_shutdown(struct pt_regs *regs)
> int (*old_handler)(struct pt_regs *regs);
>
> /* Avoid hardlocking with irresponsive CPU holding logbuf_lock */
> - printk_nmi_enter();
> + printk_deferred_enter();
>
> /*
> * This function is only called after the system
> diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> index 69bc86ea382c..76878b357ffa 100644
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -116,7 +116,6 @@ extern void rcu_nmi_exit(void);
> do { \
> lockdep_off(); \
> arch_nmi_enter(); \
> - printk_nmi_enter(); \
> BUG_ON(in_nmi() == NMI_MASK); \
> __preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET); \
> } while (0)
> @@ -135,7 +134,6 @@ extern void rcu_nmi_exit(void);
> do { \
> BUG_ON(!in_nmi()); \
> __preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET); \
> - printk_nmi_exit(); \
> arch_nmi_exit(); \
> lockdep_on(); \
> } while (0)
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 664612f75dac..03f7ccedaf18 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -149,18 +149,6 @@ static inline __printf(1, 2) __cold
> void early_printk(const char *s, ...) { }
> #endif
>
> -#ifdef CONFIG_PRINTK_NMI
> -extern void printk_nmi_enter(void);
> -extern void printk_nmi_exit(void);
> -extern void printk_nmi_direct_enter(void);
> -extern void printk_nmi_direct_exit(void);
> -#else
> -static inline void printk_nmi_enter(void) { }
> -static inline void printk_nmi_exit(void) { }
> -static inline void printk_nmi_direct_enter(void) { }
> -static inline void printk_nmi_direct_exit(void) { }
> -#endif /* PRINTK_NMI */
> -
> struct dev_printk_info;
>
> #ifdef CONFIG_PRINTK
> @@ -180,6 +168,16 @@ int printk(const char *fmt, ...);
> */
> __printf(1, 2) __cold int printk_deferred(const char *fmt, ...);
>
> +extern void __printk_safe_enter(void);
> +extern void __printk_safe_exit(void);
> +/*
> + * The printk_deferred_enter/exit macros are available only as a hack for
> + * some code paths that need to defer all printk console printing. Interrupts
> + * must be disabled for the deferred duration.
> + */
> +#define printk_deferred_enter __printk_safe_enter
> +#define printk_deferred_exit __printk_safe_exit
> +
> /*
> * Please don't use printk_ratelimit(), because it shares ratelimiting state
> * with all other unrelated printk_ratelimit() callsites. Instead use
> @@ -223,6 +221,15 @@ int printk_deferred(const char *s, ...)
> {
> return 0;
> }
> +
> +static inline void printk_deferred_enter(void)
> +{
> +}
> +
> +static inline void printk_deferred_exit(void)
> +{
> +}
> +
> static inline int printk_ratelimit(void)
> {
> return 0;
> diff --git a/init/Kconfig b/init/Kconfig
> index a61c92066c2e..9c0510693543 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1506,11 +1506,6 @@ config PRINTK
> very difficult to diagnose system problems, saying N here is
> strongly discouraged.
>
> -config PRINTK_NMI
> - def_bool y
> - depends on PRINTK
> - depends on HAVE_NMI
> -
> config BUG
> bool "BUG() support" if EXPERT
> default y
> diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
> index 6cc35c5de890..b6d310c72fc9 100644
> --- a/kernel/printk/internal.h
> +++ b/kernel/printk/internal.h
> @@ -6,12 +6,6 @@
>
> #ifdef CONFIG_PRINTK
>
> -#define PRINTK_SAFE_CONTEXT_MASK 0x007ffffff
> -#define PRINTK_NMI_DIRECT_CONTEXT_MASK 0x008000000
> -#define PRINTK_NMI_CONTEXT_MASK 0xff0000000
> -
> -#define PRINTK_NMI_CONTEXT_OFFSET 0x010000000
> -
> __printf(4, 0)
> int vprintk_store(int facility, int level,
> const struct dev_printk_info *dev_info,
> @@ -19,8 +13,6 @@ int vprintk_store(int facility, int level,
>
> __printf(1, 0) int vprintk_default(const char *fmt, va_list args);
> __printf(1, 0) int vprintk_deferred(const char *fmt, va_list args);
> -void __printk_safe_enter(void);
> -void __printk_safe_exit(void);
>
> bool printk_percpu_data_ready(void);
>
> diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
> index 29c580dac93d..ef0f9a2044da 100644
> --- a/kernel/printk/printk_safe.c
> +++ b/kernel/printk/printk_safe.c
> @@ -4,12 +4,9 @@
> */
>
> #include <linux/preempt.h>
> -#include <linux/spinlock.h>
> -#include <linux/debug_locks.h>
> #include <linux/kdb.h>
> #include <linux/smp.h>
> #include <linux/cpumask.h>
> -#include <linux/irq_work.h>
> #include <linux/printk.h>
> #include <linux/kprobes.h>
>
> @@ -17,35 +14,6 @@
>
> static DEFINE_PER_CPU(int, printk_context);
>
> -#ifdef CONFIG_PRINTK_NMI
> -void noinstr printk_nmi_enter(void)
> -{
> - this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
> -}
> -
> -void noinstr printk_nmi_exit(void)
> -{
> - this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
> -}
> -
> -/*
> - * Marks a code that might produce many messages in NMI context
> - * and the risk of losing them is more critical than eventual
> - * reordering.
> - */
> -void printk_nmi_direct_enter(void)
> -{
> - if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
> - this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
> -}
> -
> -void printk_nmi_direct_exit(void)
> -{
> - this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
> -}
> -
> -#endif /* CONFIG_PRINTK_NMI */
> -
> /* Can be preempted by NMI. */
> void __printk_safe_enter(void)
> {
> @@ -70,10 +38,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
> * Use the main logbuf even in NMI. But avoid calling console
> * drivers that might have their own locks.
> */
> - if (this_cpu_read(printk_context) &
> - (PRINTK_NMI_DIRECT_CONTEXT_MASK |
> - PRINTK_NMI_CONTEXT_MASK |
> - PRINTK_SAFE_CONTEXT_MASK)) {
> + if (this_cpu_read(printk_context) || in_nmi()) {
> int len;
>
> len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index d23a09d3eb37..b30ad20d251f 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -9647,7 +9647,7 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
> tracing_off();
>
> local_irq_save(flags);
> - printk_nmi_direct_enter();
> + printk_deferred_enter();

I would prefer to do not manipulate the printk context here anymore,
as it was done in v3.

printk_nmi_direct_enter() was added here by the commit the commit
03fc7f9c99c1e7ae2925d4 ("printk/nmi: Prevent deadlock when
accessing the main log buffer in NMI"). It was _not_ about console
handling. The reason was to modify the default behavior under NMI
and store the messages directly into the main log buffer.

When I think about it. The original fix was not correct. We should have
modified the context only when ftrace_dump() was really called under NMI:

if (in_nmi())
printk_nmi_direct_enter();

By other words. We should try to show the messages on the console
when ftrace_dump()/panic() is not called from NMI. It will help
to see all messages even when the ftrace buffers are bigger
than printk() ones.

And we do not need any special handling here for NMI. vprintk()
in printk/printk_safe.c will do the right thing for us.

Best Regards,
Petr

PS: There is no need to re-send the entire patchset if this was
the only problem. Feel free to send v4.1 version of this
patch only.

That said, I still have to look at 5th and 6th patch.

Next message: Johan Hovold: "Re: [PATCH v2] tty: nozomi: tty_unregister_device -> tty_port_unregister_device"
Previous message: Gavin Shan: "Re: [PATCH v3 02/12] mm/debug_vm_pgtable: Use struct pgtable_debug_args in basic tests"
Next in thread: John Ogness: "Re: [PATCH printk v4 4/6] printk: remove NMI tracking"
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