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

From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Andrea Parri <andrea.parri@amarulasolutions.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation
Date: Wed, 26 Jun 2019 11:08:37 +0900	[thread overview]
Message-ID: <20190626020837.GA25178@jagdpanzerIV> (raw)
In-Reply-To: <87woh9hnxg.fsf@linutronix.de>

On (06/25/19 14:03), John Ogness wrote:
[..]
> > CPU0								CPU1
> > printk(...)
> >  sz = vscprintf(NULL, "Comm %s\n", current->comm);
> > 								ia64_mca_modify_comm()
> > 								  snprintf(comm, sizeof(comm), "%s %d", current->comm, previous_current->pid);
> > 								  memcpy(current->comm, comm, sizeof(current->comm));
> >  if ((buf = prb_reserve(... sz))) {
> >    vscnprintf(buf, "Comm %s\n", current->comm);
> > 				^^^^^^^^^^^^^^ ->comm has changed.
> > 					       Nothing critical, we
> > 					       should not corrupt
> > 					       anything, but we will
> > 					       truncate ->comm if its
> > 					       new size is larger than
> > 					       what it used to be when
> > 					       we did vscprintf(NULL).
> >    prb_commit(...);
> >  }

[..]
> In my v1 rfc series, I avoided this issue by having a separate dedicated
> ringbuffer (rb_sprintf) that was used to allocate a temporary max-size
> (2KB) buffer for sprinting to. Then _that_ was used for the real
> ringbuffer input (strlen, prb_reserve, memcpy, prb_commit). That would
> still be the approach of my choice.

In other words per-CPU buffering, AKA printk_safe ;)

	-ss