On Tue 2020-12-01 21:59:41, John Ogness wrote:
> Since the ringbuffer is lockless, there is no need for it to be
> protected by @logbuf_lock. Remove @logbuf_lock.
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1847,6 +1811,65 @@ static void call_console_drivers(const char *ext_text,
> size_t ext_len,
> }
> }
The recursion-related code needs some explanation or we should do it
another way. I spent quite some time on it and I am still not sure
that I understand it. Let me describe how I understand it.
> +#ifdef CONFIG_PRINTK_NMI
> +#define NUM_RECURSION_CTX 2
> +#else
> +#define NUM_RECURSION_CTX 1
> +#endif
OK, the number of context is limited because interrupts are disabled inside
print_enter()/printk_exit(). It is basically the same reason why
we have only two printk_safe buffers (NNI + other contexts).
What is the exact reason to disable interrupts around the entire
vprintk_store(), please? It should get documented.
One reason is the use of per-cpu variables. Alternative solution would
be to store printk_context into task_struct. Well, I am not sure if
"current" task is available during early boot. But it might solve
problems with per-cpu variables that are not working during early boot.
That said, I am not sure if it is worth it.
> +
> +struct printk_recursion {
> + char count[NUM_RECURSION_CTX];
> +};
>
> +static DEFINE_PER_CPU(struct printk_recursion, percpu_printk_recursion);
> +static char printk_recursion_count[NUM_RECURSION_CTX];
This is pretty confusing. The array is hidden in a struct when per-cpu
variables are used. And a naked array is used for early boot.
Is the structure really needed? What about?
static DEFINE_PER_CPU(char [PRINTK_CTX_NUM], printk_count);
static char printk_count_early[NUM_RECURSION_CTX];
> +
> +static char *get_printk_count(void)
> +{
> + struct printk_recursion *rec;
> + char *count;
> +
> + if (!printk_percpu_data_ready()) {
> + count = &printk_recursion_count[0];
I see why you avoided per-cpu variables for early boot. I am just
curious how printk_context variable works these days. It is used by
any printk(), including early code, see vprintk_func().
> + } else {
> + rec = this_cpu_ptr(&percpu_printk_recursion);
> +
> + count = &rec->count[0];
> + }
> +
> +#ifdef CONFIG_PRINTK_NMI
> + if (in_nmi())
> + count++;
> +#endif
This is extremely confusing. It is far from obvious that
the pointer and not the value is incremented.
If we really need this to avoid per-cpu variables during early boot
then a more clear implementation would be:
char *get_printk_counter_by_ctx()
{
int ctx = 0;
if (in_nmi)
ctx = 1;
if (!printk_percpu_data_ready())
return &printk_count_early[ctx];
return this_cpu_ptr(printk_count[ctx]);
}
> +
> + return count;
> +}
> +
> +static bool printk_enter(unsigned long *flags)
> +{
> + char *count;
> +
> + local_irq_save(*flags);
> + count = get_printk_count();
> + /* Only 1 level of recursion allowed. */
We should allow at least some level of recursion. Otherwise, we would
not see warnings printed from vsprintf code.
> + if (*count > 1) {
> + local_irq_restore(*flags);
> + return false;
> + }
> + (*count)++;
> +
> + return true;
> +}
This should be unified with printk_context, printk_nmi_enter(),
printk_nmi_exit(). It does not make sense to have two separate
printk context counters.
Or is there any plan to remove printk_safe and printk_context?
BTW: I prefer to use the bitmask approach. It allows to check
the normal context by a single operation (no bit is set).
There is no need to go through all counters.
Note that we need at least one more context for gdb.
Best Regards,
Petr
