Messages in this thread | | | Date | Wed, 26 Jun 2019 16:45:46 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation |
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On (06/26/19 09:16), John Ogness wrote: > On 2019-06-26, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> 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
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