Messages in this thread | | | Date | Mon, 12 Dec 2016 23:12:30 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [RFC][PATCHv5 3/7] printk: introduce per-cpu safe_print seq buffer |
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On (12/12/16 14:54), Petr Mladek wrote: > On Sat 2016-12-10 12:10:22, Sergey Senozhatsky wrote: > > On (12/09/16 17:46), Petr Mladek wrote: > > > > -/* > > > > - * Safe printk() for NMI context. It uses a per-CPU buffer to > > > > - * store the message. NMIs are not nested, so there is always only > > > > - * one writer running. But the buffer might get flushed from another > > > > - * CPU, so we need to be careful. > > > > - */ > > > > > > We should keep/create a good description here because the function > > > has a non-trivial code. What about something like? > > > > > > > which is really not related to this patch set. > > I am sorry but I do not understand. This patch removes description > that explained constrains of a rather complex code. In fact, the > constrains has changed because we started using the function also > in other context. When will be the right time/patchset to explain > it?
but I didn't remove it.
$ grep -A3 -B3 'But the buffer might get flushed from another' kernel/printk/printk_safe.c
/* * Safe printk() for NMI context. It uses a per-CPU buffer to * store the message. NMIs are not nested, so there is always only * one writer running. But the buffer might get flushed from another * CPU, so we need to be careful. */ static int vprintk_safe_nmi(const char *fmt, va_list args)
> > > > +#ifdef CONFIG_PRINTK_NMI > > > > +/* > > > > + * Safe printk() for NMI context. It uses a per-CPU buffer to > > > > + * store the message. NMIs are not nested, so there is always only > > > > + * one writer running. But the buffer might get flushed from another > > > > + * CPU, so we need to be careful. > > > > + */ > > > > > > Hmm, I wanted to describe why we need another per-CPU buffer in NMI > > > and I am not sure that we really need it. > > > > NMI-printk can interrupt safe-printk's vsnprintf() in the middle of > > the "while (*fmt)" loop: safe-priNMI-PRINTK > > But this already happens when any of the WARNs is triggered > inside vsnprintf(). Either this is safe or we are in > trouble.
the point was that when printk-safe resumes after being interrupted by NMI-printk it continues printing from the offset at which it has been interrupted, writing over the lines that were sprintf-d by NMI printk; because NMI-printk used the same buffer offset `s->len'. so at least part of NMI-printk message will be lost.
-ss
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