Re: [PATCH 02/10] printk: Try harder to get logbuf_lock on NMI

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 25 May 2015 14:46:25 +0200 Petr Mladek <pmladek@suse.cz> wrote:

> If the logbuf_lock is not available immediately, it does not mean
> that there is a deadlock. We should try harder and wait a bit.
> 
> On the other hand, we must not forget that we are in NMI and the timeout
> has to be rather small. It must not cause dangerous stalls.
> 
> I even got full system freeze when the timeout was 10ms and I printed
> backtraces from all CPUs. In this case, all CPUs were blocked for
> too long.
> 
> ...
>
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -231,6 +231,8 @@ static DEFINE_RAW_SPINLOCK(logbuf_lock);
>  
>  #ifdef CONFIG_PRINTK
>  DECLARE_WAIT_QUEUE_HEAD(log_wait);
> +/* cpu currently holding logbuf_lock */
> +static unsigned int logbuf_cpu = UINT_MAX;
>  /* the next printk record to read by syslog(READ) or /proc/kmsg */
>  static u64 syslog_seq;
>  static u32 syslog_idx;
> @@ -1610,6 +1612,38 @@ static size_t cont_print_text(char *text, size_t size)
>  	return textlen;
>  }
>  
> +/*
> + * This value defines the maximum delay that we spend waiting for logbuf_lock
> + * in NMI context. 100us looks like a good compromise. Note that, for example,
> + * syslog_print_all() might hold the lock for quite some time. On the other
> + * hand, waiting 10ms caused system freeze when many backtraces were printed
> + * in NMI.
> + */
> +#define TRY_LOCKBUF_LOCK_MAX_DELAY_NS 100000UL
> +
> +/* We must be careful in NMI when we managed to preempt a running printk */
> +static int try_logbuf_lock_in_nmi(void)
> +{
> +	u64 start_time, current_time;
> +	int this_cpu = smp_processor_id();
> +
> +	/* no way if we are already locked on this CPU */
> +	if (logbuf_cpu == this_cpu)
> +		return 0;
> +
> +	/* try hard to get the lock but do not wait forever */
> +	start_time = cpu_clock(this_cpu);
> +	current_time = start_time;
> +	while (current_time - start_time < TRY_LOCKBUF_LOCK_MAX_DELAY_NS) {
> +		if (raw_spin_trylock(&logbuf_lock))
> +			return 1;
> +		cpu_relax();
> +		current_time = cpu_clock(this_cpu);
> +	}

(Looks at the read_seqcount_retry() in
kernel/time/sched_clock.c:sched_clock())

Running cpu_clock() in NMI context seems a generally bad idea.  Using
an ndelay/udelay wait loop here would be safer?


There are many sites in kernel/printk/printk.c which take logbuf_lock,
but this patch only sets logbuf_cpu in one of those cases:
vprintk_emit().  I suggest adding helper functions to take/release
logbuf_lock.  And rename logbuf_lock to something else to ensure that
nobody accidentally takes the lock directly.