On 2021-03-22, Petr Mladek <[email protected]> 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
