Messages in this thread | | | Date | Mon, 16 Oct 2017 10:12:52 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH] printk: Remove superfluous memory barriers from printk_safe |
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On Sun 2017-10-15 20:27:15, Steven Rostedt wrote: > On Sat, 14 Oct 2017 18:21:29 +0900 > Sergey Senozhatsky <sergey.senozhatsky@gmail.com> wrote: > > > On (10/11/17 12:46), Steven Rostedt wrote: > > > From: Steven Rostedt (VMware) <rostedt@goodmis.org> > > > > > > The variable printk_safe_irq_ready is set and never cleared at system > > > boot up, when there's only one CPU active. It is set before other > > > CPUs come on line. Also, it is extremely unlikely that an NMI would > > > trigger this early in boot up (which I wonder why we even have this > > > variable at all). > > > > it's not only NMI related, printk() recursion can happen at any stages, > > including... um... wait a second. ... including the "before we set up > > per-CPU areas" stage? hmm... smells like a bug? > > I think this was just being overly paranoid.
I was curious because it was not only about reading the per-CPU variables. We set and clear the printk_context per-CPU variable in every printk() call. I wondered if we accessed some non-initialized stuff.
Fortunately, it seems that we are on the safe side.
If I get it correctly, the per-CPU variables are set up in setup_per_cpu_areas(). But some per-CPU variables are used even before, see
boot_cpu_init() smp_processor_id() raw_smp_processor_id() this_cpu_read(cpu_number)
IMHO, the trick is the following code in setup_per_cpu_areas() from arch/x86/kernel/setup_percpu.c:
/* * Up to this point, the boot CPU has been using .init.data * area. Reload any changed state for the boot CPU. */ if (!cpu) switch_to_new_gdt(cpu);
IMHO, this means that per-CPU variable for the first boot-CPU can be used at any time. And all the interesting functions: boot_cpu_init(), setup_per_cpu_areas(), printk_safe_init() are still called in the single-CPU mode.
Best Regards, Petr
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