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

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Date:   Wed, 26 Jun 2019 11:08:37 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        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 (06/25/19 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