Messages in this thread | | | From | John Ogness <> | Subject | Re: [PATCH next v1 1/3] printk: track/limit recursion | Date | Tue, 23 Mar 2021 22:32:00 +0100 |
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On 2021-03-22, Petr Mladek <pmladek@suse.com> wrote: > On Wed 2021-03-17 00:33:24, John Ogness wrote: >> Track printk() recursion and limit it to 3 levels per-CPU and per-context. > > Please, explain why it is added. I mean that it will > allow remove printk_safe that provides recursion protection at the > moment.
OK.
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c >> index 2f829fbf0a13..c666e3e43f0c 100644 >> --- a/kernel/printk/printk.c >> +++ b/kernel/printk/printk.c >> @@ -1940,6 +1940,71 @@ static void call_console_drivers(const char *ext_text, size_t ext_len, >> } >> } >> >> +/* >> + * Recursion is tracked separately on each CPU. If NMIs are supported, an >> + * additional NMI context per CPU is also separately tracked. Until per-CPU >> + * is available, a separate "early tracking" is performed. >> + */ >> +#ifdef CONFIG_PRINTK_NMI > > CONFIG_PRINTK_NMI is a shortcut for CONFIG_PRINTK && CONFIG_HAVE_NMI. > It should be possible to use CONFIG_HAVE_NMI here because this should > be in section where CONFIG_PRINTK is defined. > > This would make sense if it allows to remove CONFIG_PRINTK_NMI > entirely. IMHO, it would be nice to remove one layer in the > config options of possible.
OK. I will remove CONFIG_PRINTK_NMI for v2.
>> +#define PRINTK_CTX_NUM 2 >> +#else >> +#define PRINTK_CTX_NUM 1 >> +#endif >> +static DEFINE_PER_CPU(char [PRINTK_CTX_NUM], printk_count); >> +static char printk_count_early[PRINTK_CTX_NUM]; >> + >> +/* >> + * Recursion is limited to keep the output sane. printk() should not require >> + * more than 1 level of recursion (allowing, for example, printk() to trigger >> + * a WARN), but a higher value is used in case some printk-internal errors >> + * exist, such as the ringbuffer validation checks failing. >> + */ >> +#define PRINTK_MAX_RECURSION 3 >> + >> +/* Return a pointer to the dedicated counter for the CPU+context of the caller. */ >> +static char *printk_recursion_counter(void) >> +{ >> + int ctx = 0; >> + >> +#ifdef CONFIG_PRINTK_NMI >> + if (in_nmi()) >> + ctx = 1; >> +#endif >> + if (!printk_percpu_data_ready()) >> + return &printk_count_early[ctx]; >> + return &((*this_cpu_ptr(&printk_count))[ctx]); >> +} > > It is not a big deal. But using an array for two contexts looks strange > especially when only one is used on some architectures. > Also &((*this_cpu_ptr(&printk_count))[ctx]) is quite tricky ;-) > > What do you think about the following, please? > > static DEFINE_PER_CPU(u8 printk_count); > static u8 printk_count_early; > > #ifdef CONFIG_HAVE_NMI > static DEFINE_PER_CPU(u8 printk_count_nmi); > static u8 printk_count_nmi_early; > #endif > > static u8 *printk_recursion_counter(void) > { > if (IS_ENABLED(CONFIG_HAVE_NMI) && in_nmi()) { > if (printk_cpu_data_ready()) > return this_cpu_ptr(&printk_count_nmi); > return printk_count_nmi_early; > } > > if (printk_cpu_data_ready()) > return this_cpu_ptr(&printk_count); > return printk_count_early; > }
I can split it into explicit variables. But is the use of the IS_ENABLED macro preferred over ifdef? I would prefer:
static u8 *printk_recursion_counter(void) { #ifdef CONFIG_HAVE_NMI if (in_nmi()) { if (printk_cpu_data_ready()) return this_cpu_ptr(&printk_count_nmi); return printk_count_nmi_early; } #endif if (printk_cpu_data_ready()) return this_cpu_ptr(&printk_count); return printk_count_early; }
Since @printk_count_nmi and @printk_count_nmi_early would not exist, I would prefer the pre-processor removes that code block rather than relying on compiler optimization.
John Ogness
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