Re: [PATCH] printk/nmi: avoid direct printk()-s from __printk_nmi_flush()

From	Petr Mladek <pmladek@suse.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] printk/nmi: avoid direct printk()-s from __printk_nmi_flush()
Date	2016-09-01 10:20 +0200
Message-ID	<scvo5-6SP-5@gated-at.bofh.it> (permalink)
References	(2 earlier) <sbM7D-1Ul-7@gated-at.bofh.it> <sbNdo-2Ps-23@gated-at.bofh.it> <sbNGp-2Zk-1@gated-at.bofh.it> <sbPfb-44D-5@gated-at.bofh.it> <scv4J-6oI-1@gated-at.bofh.it>
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On Thu 2016-09-01 16:55:07, Sergey Senozhatsky wrote:
> On (08/30/16 13:19), Petr Mladek wrote:
> > 
> > I see. But then we will need to be more careful because printk_func
> > and printk_func_saved will be manipulated in different contexts:
> > normal, irq, nmi. A solution might be using an atomic counter
> > and selecting the right vprintk_func according to the value.
> 
> alt_printk_enter() must be done with local IRQs disabled. so IRQ cannot
> race with `normal' alt_printk. other IRQs cannot race with the current IRQ,
> because we have local IRQs disabled. the only thing that can race here is - NMI.
> both `normal' and IRQ alt_printk can use the same per-CPU buffer, they never
> race. NMI needs to have its own.

Yes. Well, my concern was how to atomically change the printk_func
pointer and save the previous value at the same time. You could not
use locks because NMIs are involved.

Best Regards,
Petr

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[PATCH] printk/nmi: avoid direct printk()-s from __printk_nmi_flush() Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2016-08-29 14:40 +0200
  Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Petr Mladek <pmladek@suse.com> - 2016-08-29 17:20 +0200
    Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-08-30 03:10 +0200
    Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-08-30 10:00 +0200
      Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Petr Mladek <pmladek@suse.com> - 2016-08-30 11:10 +0200
        Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-08-30 11:40 +0200
          Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Petr Mladek <pmladek@suse.com> - 2016-08-30 13:20 +0200
            Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-08-31 06:10 +0200
            Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-09-01 10:00 +0200
              Re: [PATCH] printk/nmi: avoid direct printk()-s from  __printk_nmi_flush() Petr Mladek <pmladek@suse.com> - 2016-09-01 10:20 +0200

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