Re: [PATCH v4 02/27] hardirq/nmi: Allow nested nmi_enter()

Petr Mladek
2020-02-24T12:13:36Z

On Fri 2020-02-21 23:21:30, Frederic Weisbecker wrote:
> On Fri, Feb 21, 2020 at 02:34:18PM +0100, Peter Zijlstra wrote:
> > Since there are already a number of sites (ARM64, PowerPC) that
> > effectively nest nmi_enter(), lets make the primitive support this
> > before adding even more.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Reviewed-by: Petr Mladek <pmladek@suse.com>
> > Acked-by: Will Deacon <will@kernel.org>
> > Acked-by: Marc Zyngier <maz@kernel.org>
> > ---
> > arch/arm64/include/asm/hardirq.h | 4 ++--
> > arch/arm64/kernel/sdei.c | 14 ++------------
> > arch/arm64/kernel/traps.c | 8 ++------
> > arch/powerpc/kernel/traps.c | 22 ++++++----------------
> > include/linux/hardirq.h | 5 ++++-
> > include/linux/preempt.h | 4 ++--
> > kernel/printk/printk_safe.c | 6 ++++--
> > 7 files changed, 22 insertions(+), 41 deletions(-)
> >
> > --- a/kernel/printk/printk_safe.c
> > +++ b/kernel/printk/printk_safe.c
> > @@ -296,12 +296,14 @@ static __printf(1, 0) int vprintk_nmi(co
> >
> > void notrace printk_nmi_enter(void)
> > {
> > - this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> > + if (!in_nmi())
> > + this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
> > }
> >
> > void notrace printk_nmi_exit(void)
> > {
> > - this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
> > + if (!in_nmi())
> > + this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
> > }
>
> If the outermost NMI is interrupted while between printk_nmi_enter()
> and preempt_count_add(), there is still a risk that we race and clear?

Great catch!

There is plenty of space in the printk_context variable. I would
reserve one byte there for the NMI context to be on the safe side
and be done with it.

It should never overflow. The BUG_ON(in_nmi() == NMI_MASK)
in nmi_enter() will trigger much earlier.

Also I hope that printk_context will get removed with
the lockless printk() implementation soon anyway.

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index c8e6ab689d42..109c5ab70a0c 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -6,9 +6,11 @@

#ifdef CONFIG_PRINTK

-#define PRINTK_SAFE_CONTEXT_MASK 0x3fffffff
-#define PRINTK_NMI_DIRECT_CONTEXT_MASK 0x40000000
-#define PRINTK_NMI_CONTEXT_MASK 0x80000000
+#define PRINTK_SAFE_CONTEXT_MASK 0x007ffffff
+#define PRINTK_NMI_DIRECT_CONTEXT_MASK 0x008000000
+#define PRINTK_NMI_CONTEXT_MASK 0xff0000000
+
+#define PRINTK_NMI_CONTEXT_OFFSET 0x010000000

extern raw_spinlock_t logbuf_lock;

diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index b4045e782743..e8989418a139 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -296,12 +296,12 @@ static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)

void notrace printk_nmi_enter(void)
{
- this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
+ this_cpu_add(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
}

void notrace printk_nmi_exit(void)
{
- this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
+ this_cpu_sub(printk_context, PRINTK_NMI_CONTEXT_OFFSET);
}

/*

Best Regards,
Petr