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From: Frederic Weisbecker <frederic@kernel.org>
To: Petr Mladek <pmladek@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	rostedt@goodmis.org, mingo@kernel.org, joel@joelfernandes.org,
	gregkh@linuxfoundation.org, gustavo@embeddedor.com,
	tglx@linutronix.de, paulmck@kernel.org, josh@joshtriplett.org,
	mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
	luto@kernel.org, tony.luck@intel.com, dan.carpenter@oracle.com,
	mhiramat@kernel.org, Will Deacon <will@kernel.org>,
	Marc Zyngier <maz@kernel.org>
Subject: Re: [PATCH v4 02/27] hardirq/nmi: Allow nested nmi_enter()
Date: Tue, 25 Feb 2020 02:30:14 +0100	[thread overview]
Message-ID: <20200225013013.GA28329@lenoir> (raw)
In-Reply-To: <20200224121331.tnkvp6mmjchm4s2i@pathway.suse.cz>

On Mon, Feb 24, 2020 at 01:13:31PM +0100, Petr Mladek wrote:
> On Fri 2020-02-21 23:21:30, Frederic Weisbecker wrote:
> > 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.

Cool, because it's sad that we have to mimic/mirror the preempt count
with printk_context just for the sake of debugging preempt_count_add()
and other early code in nmi_enter().

The diff below looks good, thanks!

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>


> 
> 
> 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

  reply	other threads:[~2020-02-25  1:30 UTC|newest]

Thread overview: 84+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-21 13:34 [PATCH v4 00/27] tracing vs world Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 01/27] lockdep: Teach lockdep about "USED" <- "IN-NMI" inversions Peter Zijlstra
2020-02-21 15:08   ` Steven Rostedt
2020-02-21 20:25     ` Peter Zijlstra
2020-02-21 20:28       ` Steven Rostedt
2020-02-21 22:01       ` Frederic Weisbecker
2020-02-22  3:08   ` Joel Fernandes
2020-02-24 10:10     ` Peter Zijlstra
2020-02-25  2:12       ` Joel Fernandes
2020-03-20 12:58   ` [tip: locking/core] " tip-bot2 for Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 02/27] hardirq/nmi: Allow nested nmi_enter() Peter Zijlstra
2020-02-21 22:21   ` Frederic Weisbecker
2020-02-24 12:13     ` Petr Mladek
2020-02-25  1:30       ` Frederic Weisbecker [this message]
2020-02-24 16:13     ` Peter Zijlstra
2020-02-25  3:09       ` Frederic Weisbecker
2020-02-25 15:41         ` Peter Zijlstra
2020-02-25 16:21           ` Frederic Weisbecker
2020-02-25 22:10           ` Frederic Weisbecker
2020-02-27  9:10             ` Peter Zijlstra
2020-02-27 13:34               ` Frederic Weisbecker
2020-02-21 13:34 ` [PATCH v4 03/27] x86/entry: Flip _TIF_SIGPENDING and _TIF_NOTIFY_RESUME handling Peter Zijlstra
2020-02-21 16:14   ` Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 04/27] x86/mce: Delete ist_begin_non_atomic() Peter Zijlstra
2020-02-21 19:07   ` Andy Lutomirski
2020-02-21 23:40   ` Frederic Weisbecker
2020-02-21 13:34 ` [PATCH v4 05/27] x86: Replace ist_enter() with nmi_enter() Peter Zijlstra
2020-02-21 19:05   ` Andy Lutomirski
2020-02-21 20:22     ` Peter Zijlstra
2020-02-24 10:43       ` Peter Zijlstra
2020-02-24 16:27         ` Steven Rostedt
2020-02-24 16:34           ` Peter Zijlstra
2020-02-24 16:47             ` Steven Rostedt
2020-02-24 21:31               ` Peter Zijlstra
2020-02-24 22:02                 ` Steven Rostedt
2020-02-26 10:25                   ` Peter Zijlstra
2020-02-26 13:16                     ` Peter Zijlstra
2020-02-26 10:27                   ` Peter Zijlstra
2020-02-26 15:20                     ` Steven Rostedt
2020-03-07  1:53                     ` Masami Hiramatsu
2020-02-21 13:34 ` [PATCH v4 06/27] x86/doublefault: Remove memmove() call Peter Zijlstra
2020-02-21 19:10   ` Andy Lutomirski
2020-02-21 13:34 ` [PATCH v4 07/27] rcu: Make RCU IRQ enter/exit functions rely on in_nmi() Peter Zijlstra
2020-02-26  0:23   ` Frederic Weisbecker
2020-02-21 13:34 ` [PATCH v4 08/27] rcu/kprobes: Comment why rcu_nmi_enter() is marked NOKPROBE Peter Zijlstra
2020-02-26  0:27   ` Frederic Weisbecker
2020-02-21 13:34 ` [PATCH v4 09/27] rcu: Rename rcu_irq_{enter,exit}_irqson() Peter Zijlstra
2020-02-21 20:21   ` Steven Rostedt
2020-02-24 10:24     ` Peter Zijlstra
2020-02-26  0:35   ` Frederic Weisbecker
2020-02-21 13:34 ` [PATCH v4 10/27] rcu: Mark rcu_dynticks_curr_cpu_in_eqs() inline Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 11/27] rcu,tracing: Create trace_rcu_{enter,exit}() Peter Zijlstra
2020-03-06 11:50   ` Peter Zijlstra
2020-03-06 12:40     ` Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 12/27] sched,rcu,tracing: Avoid tracing before in_nmi() is correct Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 13/27] x86,tracing: Add comments to do_nmi() Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 14/27] perf,tracing: Prepare the perf-trace interface for RCU changes Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 15/27] tracing: Employ trace_rcu_{enter,exit}() Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 16/27] tracing: Remove regular RCU context for _rcuidle tracepoints (again) Peter Zijlstra
2020-03-06 10:43   ` Peter Zijlstra
2020-03-06 11:31     ` Peter Zijlstra
2020-03-06 15:51       ` Alexei Starovoitov
2020-03-06 16:04         ` Mathieu Desnoyers
2020-03-06 17:55           ` Steven Rostedt
2020-03-06 18:45             ` Joel Fernandes
2020-03-06 18:59               ` Steven Rostedt
2020-03-06 19:14                 ` Joel Fernandes
2020-03-06 20:22             ` Mathieu Desnoyers
2020-03-06 20:45               ` Steven Rostedt
2020-03-06 20:55                 ` Mathieu Desnoyers
2020-03-06 21:06                   ` Steven Rostedt
2020-03-06 23:10                   ` Alexei Starovoitov
2020-03-06 17:21         ` Joel Fernandes
2020-02-21 13:34 ` [PATCH v4 17/27] perf,tracing: Allow function tracing when !RCU Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 18/27] x86/int3: Ensure that poke_int3_handler() is not traced Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 19/27] locking/atomics, kcsan: Add KCSAN instrumentation Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 20/27] asm-generic/atomic: Use __always_inline for pure wrappers Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 21/27] asm-generic/atomic: Use __always_inline for fallback wrappers Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 22/27] compiler: Simple READ/WRITE_ONCE() implementations Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 23/27] locking/atomics: Flip fallbacks and instrumentation Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 24/27] x86/int3: Avoid atomic instrumentation Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 25/27] lib/bsearch: Provide __always_inline variant Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 26/27] x86/int3: Inline bsearch() Peter Zijlstra
2020-02-21 13:34 ` [PATCH v4 27/27] x86/int3: Ensure that poke_int3_handler() is not sanitized Peter Zijlstra

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