linux-kernel - Re: [PATCH v2 01/11] lockdep: Use raw_cpu

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Date:   Wed, 26 Aug 2020 21:05:43 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org, will@...nel.org,
        npiggin@...il.com, elver@...gle.com, jgross@...e.com,
        paulmck@...nel.org, rostedt@...dmis.org, rjw@...ysocki.net,
        svens@...ux.ibm.com, tglx@...utronix.de
Subject: Re: [PATCH v2 01/11] lockdep: Use raw_cpu_*() for per-cpu variables

On Fri, Aug 21, 2020 at 10:47:39AM +0200, Peter Zijlstra wrote:
> Sven reported that commit a21ee6055c30 ("lockdep: Change
> hardirq{s_enabled,_context} to per-cpu variables") caused trouble on
> s390 because their this_cpu_*() primitives disable preemption which
> then lands back tracing.
> 
> On the one hand, per-cpu ops should use preempt_*able_notrace() and
> raw_local_irq_*(), on the other hand, we can trivialy use raw_cpu_*()
> ops for this.
> 
> Fixes: a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to per-cpu variables")
> Reported-by: Sven Schnelle <svens@...ux.ibm.com>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> Tested-by: Marco Elver <elver@...gle.com>

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel

> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/irqflags.h |    6 +++---
>  include/linux/lockdep.h  |   18 +++++++++++++-----
>  kernel/locking/lockdep.c |    4 ++--
>  3 files changed, 18 insertions(+), 10 deletions(-)
> 
> --- a/include/linux/irqflags.h
> +++ b/include/linux/irqflags.h
> @@ -53,13 +53,13 @@ DECLARE_PER_CPU(int, hardirq_context);
>    extern void trace_hardirqs_off_finish(void);
>    extern void trace_hardirqs_on(void);
>    extern void trace_hardirqs_off(void);
> -# define lockdep_hardirq_context()	(this_cpu_read(hardirq_context))
> +# define lockdep_hardirq_context()	(raw_cpu_read(hardirq_context))
>  # define lockdep_softirq_context(p)	((p)->softirq_context)
>  # define lockdep_hardirqs_enabled()	(this_cpu_read(hardirqs_enabled))
>  # define lockdep_softirqs_enabled(p)	((p)->softirqs_enabled)
>  # define lockdep_hardirq_enter()			\
>  do {							\
> -	if (this_cpu_inc_return(hardirq_context) == 1)	\
> +	if (__this_cpu_inc_return(hardirq_context) == 1)\
>  		current->hardirq_threaded = 0;		\
>  } while (0)
>  # define lockdep_hardirq_threaded()		\
> @@ -68,7 +68,7 @@ do {						\
>  } while (0)
>  # define lockdep_hardirq_exit()			\
>  do {						\
> -	this_cpu_dec(hardirq_context);		\
> +	__this_cpu_dec(hardirq_context);	\
>  } while (0)
>  # define lockdep_softirq_enter()		\
>  do {						\
> --- a/include/linux/lockdep.h
> +++ b/include/linux/lockdep.h
> @@ -535,19 +535,27 @@ do {									\
>  DECLARE_PER_CPU(int, hardirqs_enabled);
>  DECLARE_PER_CPU(int, hardirq_context);
>  
> +/*
> + * The below lockdep_assert_*() macros use raw_cpu_read() to access the above
> + * per-cpu variables. This is required because this_cpu_read() will potentially
> + * call into preempt/irq-disable and that obviously isn't right. This is also
> + * correct because when IRQs are enabled, it doesn't matter if we accidentally
> + * read the value from our previous CPU.
> + */
> +
>  #define lockdep_assert_irqs_enabled()					\
>  do {									\
> -	WARN_ON_ONCE(debug_locks && !this_cpu_read(hardirqs_enabled));	\
> +	WARN_ON_ONCE(debug_locks && !raw_cpu_read(hardirqs_enabled));	\
>  } while (0)
>  
>  #define lockdep_assert_irqs_disabled()					\
>  do {									\
> -	WARN_ON_ONCE(debug_locks && this_cpu_read(hardirqs_enabled));	\
> +	WARN_ON_ONCE(debug_locks && raw_cpu_read(hardirqs_enabled));	\
>  } while (0)
>  
>  #define lockdep_assert_in_irq()						\
>  do {									\
> -	WARN_ON_ONCE(debug_locks && !this_cpu_read(hardirq_context));	\
> +	WARN_ON_ONCE(debug_locks && !raw_cpu_read(hardirq_context));	\
>  } while (0)
>  
>  #define lockdep_assert_preemption_enabled()				\
> @@ -555,7 +563,7 @@ do {									\
>  	WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT)	&&		\
>  		     debug_locks			&&		\
>  		     (preempt_count() != 0		||		\
> -		      !this_cpu_read(hardirqs_enabled)));		\
> +		      !raw_cpu_read(hardirqs_enabled)));		\
>  } while (0)
>  
>  #define lockdep_assert_preemption_disabled()				\
> @@ -563,7 +571,7 @@ do {									\
>  	WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_COUNT)	&&		\
>  		     debug_locks			&&		\
>  		     (preempt_count() == 0		&&		\
> -		      this_cpu_read(hardirqs_enabled)));		\
> +		      raw_cpu_read(hardirqs_enabled)));			\
>  } while (0)
>  
>  #else
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -3756,7 +3756,7 @@ void noinstr lockdep_hardirqs_on(unsigne
>  
>  skip_checks:
>  	/* we'll do an OFF -> ON transition: */
> -	this_cpu_write(hardirqs_enabled, 1);
> +	__this_cpu_write(hardirqs_enabled, 1);
>  	trace->hardirq_enable_ip = ip;
>  	trace->hardirq_enable_event = ++trace->irq_events;
>  	debug_atomic_inc(hardirqs_on_events);
> @@ -3795,7 +3795,7 @@ void noinstr lockdep_hardirqs_off(unsign
>  		/*
>  		 * We have done an ON -> OFF transition:
>  		 */
> -		this_cpu_write(hardirqs_enabled, 0);
> +		__this_cpu_write(hardirqs_enabled, 0);
>  		trace->hardirq_disable_ip = ip;
>  		trace->hardirq_disable_event = ++trace->irq_events;
>  		debug_atomic_inc(hardirqs_off_events);
> 
>