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Date: Thu, 1 Dec 2016 10:08:48 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Tejun Heo <tj@kernel.org>, Calvin Owens <calvinowens@fb.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mel Gorman <mgorman@techsingularity.net>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Laura Abbott <labbott@redhat.com>, Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>, linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [RFC][PATCHv4 3/6] printk: introduce per-cpu safe_print seq
 buffer
Message-ID: <20161201010848.GD12039@jagdpanzerIV>
References: <20161027154933.1211-1-sergey.senozhatsky@gmail.com>
 <20161027154933.1211-4-sergey.senozhatsky@gmail.com>
 <20161124165821.GG24103@pathway.suse.cz>
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On (11/24/16 17:58), Petr Mladek wrote:
[..]
> > +#ifdef CONFIG_PRINTK
> > +
> > +#define PRINTK_SAFE_CONTEXT_MASK	0x7fffffff
> > +#define PRINTK_SAFE_NMI_CONTEXT_MASK	0x80000000
> 
> What about shorter name PRINTK_NMI_CONTEXT_MASK?

ok.

> > diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
> > index 1f66163..af74d9c 100644
> > --- a/kernel/printk/printk_safe.c
> > +++ b/kernel/printk/printk_safe.c
> > @@ -50,27 +49,26 @@ struct printk_safe_seq_buf {
> >  	struct irq_work		work;	/* IRQ work that flushes the buffer */
> >  	unsigned char		buffer[SAFE_LOG_BUF_LEN];
> >  };
> > +
> > +static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
> > +static DEFINE_PER_CPU(int, printk_safe_context);
> 
> I would personally use the short name "printk_context". It is a generic
> value. Zero value means that it is a normal context. Also there is
> an idea to add KDB context that would use its own vprintk_kdb()
> implementation and will not use the printk_safe buffer.

ok.

> > -	if (len >= sizeof(s->buffer)) {
> > -		atomic_inc(&nmi_message_lost);
> > -		return 0;
> > -	}
> > +	if (len >= sizeof(s->buffer))
> > +		return -E2BIG;
> 
> E2BIG means "argument list too long" and does not fit much here.
> I would suggest to use -ENOSPC. It is not ideal either but it fits
> slightly better.

ok.

> > +/*
> > + * Lockless printk(), to avoid deadlocks should the printk() recurse
> > + * into itself. It uses a per-CPU buffer to store the message, just like
> > + * NMI.
> > + */
> > +static int vprintk_safe(const char *fmt, va_list args)
> > +{
> > +	struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
> > +
> > +	return printk_safe_log_store(s, fmt, args);
> 
> We should return zero if printk_safe_log_store() returns an error.
> I know that it will get fixed in the next patch. But we should do
> some minimum sanity check here because of bisection.

ok.

	-ss