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

[Sergey Senozhatsky]

On (06/26/19 09:16), John Ogness wrote:
> On 2019-06-26, Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx> wrote:
> > [..]
> >> 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 ;)
> 
> Actually, no. I made use of a printk_ringbuffer (which is global). It
> was used for temporary memory allocation for sprintf, but the result was
> immediately written into the printk buffer from the same context. In
> contrast, printk_safe triggers a different context to handle the
> insertion.

I agree that's not relevant to your patch. But let me explain what I
meant. printk_safe has many faces. The NMI part of printk_safe has
the PRINTK_NMI_DIRECT_CONTEXT_MASK bufferring bypass - when we know
that we are in NMI and printk logbuf is unlocked then we can do the
normal logbuf_store() from NMI, avoiding irq flush because the data
is already in the main log buffer. We also can do the same buffering
bypass for non-NMI part of printk_safe, but just sometimes.
PRINTK_SAFE_CONTEXT_MASK most of the times indicates that logbuf is
locked, but not always - e.g. we call console_drivers under
PRINTK_SAFE_CONTEXT_MASK.

But like I said, not relevant to your patch. The relevant part is the
possibility of race conditions.

> It is still my intention to eliminate the buffering component of
> printk_safe.

That's understandable.

> After we get a lockless ringbuffer that we are happy with, my next
> series to integrate the buffer into printk will again use the sprint_rb
> solution to avoid the issue discussed in this thread.

Yes, I agree that either sprint_rb or just 2 LOG_LINE_MAX per-CPU
buffers looks safer. This basically means that printk cannot use
printk_ringbuffer as is and needs some sort of extra layer next to
(or atop of) printk_ringbuffer, but we have the same thing in printk
right now, basically. static char textbuf[LOG_LINE_MAX] -> logbuf.

	-ss