Re: [PATCH v4 1/5] printk/nmi: generic solution for safe printk in NMI

Sergey Senozhatsky
2016-04-04T04:48:11Z

Hello,

On (03/30/16 17:53), Petr Mladek wrote:
[..]
> @@ -67,10 +67,12 @@ extern void irq_exit(void);
> preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET); \
> rcu_nmi_enter(); \
> trace_hardirq_enter(); \
> + printk_nmi_enter(); \
> } while (0)
>
> #define nmi_exit() \
> do { \
> + printk_nmi_exit(); \
> trace_hardirq_exit(); \
> rcu_nmi_exit(); \
> BUG_ON(!in_nmi()); \

isn't it a bit too early to printk_nmi_exit()? rcu_nmi_exit() can
WARN_ON_ONCE() in 3 places.

the same goes for printk_nmi_enter(). rcu_nmi_enter() can WARN_ON_ONCE().

seems that in both cases we can endup having WARN_ON_ONCE() from nmi,
but with default printk function.

> +/*
> + * Flush data from the associated per_CPU buffer. The function
> + * can be called either via IRQ work or independently.
> + */
> +static void __printk_nmi_flush(struct irq_work *work)
> +{
> + static raw_spinlock_t read_lock =
> + __RAW_SPIN_LOCK_INITIALIZER(read_lock);
> + struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
> + unsigned long flags;
> + size_t len, size;
> + int i, last_i;
> +
> + /*
> + * The lock has two functions. First, one reader has to flush all
> + * available message to make the lockless synchronization with
> + * writers easier. Second, we do not want to mix messages from
> + * different CPUs. This is especially important when printing
> + * a backtrace.
> + */
> + raw_spin_lock_irqsave(&read_lock, flags);
> +

hm... so here we have
for (; i < size; i++)
printk()

under the spinlock. the thing is that one of printk() can end up
in console_unlock()->call_console_drivers() loop, iterating there
long enough to spinlock lockup other CPUs that might want to flush
NMI buffers (if any), assuming that there are enough printk() (or
may be a slow serial console) happening concurrently on other CPUs
to keep the current ->read_lock busy. async printk can help here,
but user can request sync version of printk.

how about using deferred printk for nmi flush?
print_nmi_seq_line()->printk_deferred() ?

-ss

> + i = 0;
> +more:
> + len = atomic_read(&s->len);
> +
> + /*
> + * This is just a paranoid check that nobody has manipulated
> + * the buffer an unexpected way. If we printed something then
> + * @len must only increase.
> + */
> + if (i && i >= len)
> + pr_err("printk_nmi_flush: internal error: i=%d >= len=%zu\n",
> + i, len);
> +
> + if (!len)
> + goto out; /* Someone else has already flushed the buffer. */
> +
> + /* Make sure that data has been written up to the @len */
> + smp_rmb();
> +
> + size = min(len, sizeof(s->buffer));
> + last_i = i;
> +
> + /* Print line by line. */
> + for (; i < size; i++) {
> + if (s->buffer[i] == '\n') {
> + print_nmi_seq_line(s, last_i, i);
> + last_i = i + 1;
> + }
> + }
> + /* Check if there was a partial line. */
> + if (last_i < size) {
> + print_nmi_seq_line(s, last_i, size - 1);
> + pr_cont("\n");
> + }
> +
> + /*
> + * Check that nothing has got added in the meantime and truncate
> + * the buffer. Note that atomic_cmpxchg() is an implicit memory
> + * barrier that makes sure that the data were copied before
> + * updating s->len.
> + */
> + if (atomic_cmpxchg(&s->len, len, 0) != len)
> + goto more;
> +
> +out:
> + raw_spin_unlock_irqrestore(&read_lock, flags);
> +}