linux-kernel - Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

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Date:   Tue, 25 Jun 2019 15:45:43 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer
 implementation

On (06/19/19 00:12), John Ogness wrote:
> On 2019-06-18, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:
> >> +	struct prb_reserved_entry e;
> >> +	char *s;
> >> +
> >> +	s = prb_reserve(&e, &rb, 32);
> >> +	if (s) {
> >> +		sprintf(s, "Hello, world!");
> >> +		prb_commit(&e);
> >> +	}
> >
> > A nit: snprintf().
> >
> > sprintf() is tricky, it may write "slightly more than was
> > anticipated" bytes - all those string_nocheck(" disabled"),
> > error_string("pK-error"), etc.
> 
> Agreed. Documentation should show good examples.

In vprintk_emit(), are we going to always reserve 1024-byte
records, since we don't know the size in advance, e.g.

	printk("%pS %s\n", regs->ip, current->name)
		prb_reserve(&e, &rb, ????);

or are we going to run vscnprintf() on a NULL buffer first,
then reserve the exactly required number of bytes and afterwards
vscnprintf(s) -> prb_commit(&e)?

	-ss