Re: [PATCH tip/core/rcu 05/26] rcu: Allow rcu_user_enter()/exit() to nest

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Aug 30, 2012 at 02:05:22PM -0700, Paul E. McKenney wrote:
> From: Frederic Weisbecker <fweisbec@gmail.com>
> 
> Allow calls to rcu_user_enter() even if we are already
> in userspace (as seen by RCU) and allow calls to rcu_user_exit()
> even if we are already in the kernel.
> 
> This makes the APIs more flexible to be called from architectures.
> Exception entries for example won't need to know if they come from
> userspace before calling rcu_user_exit().
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Alessio Igor Bogani <abogani@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Avi Kivity <avi@redhat.com>
> Cc: Chris Metcalf <cmetcalf@tilera.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Geoff Levand <geoff@infradead.org>
> Cc: Gilad Ben Yossef <gilad@benyossef.com>
> Cc: Hakan Akkan <hakanakkan@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Kevin Hilman <khilman@ti.com>
> Cc: Max Krasnyansky <maxk@qualcomm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Stephen Hemminger <shemminger@vyatta.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  kernel/rcutree.c |   41 +++++++++++++++++++++++++++++++++--------
>  kernel/rcutree.h |    3 +++
>  2 files changed, 36 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index e287c4a..8bbc7fb 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -390,11 +390,9 @@ static void rcu_eqs_enter_common(struct rcu_dynticks *rdtp, long long oldval,
>   */
>  static void rcu_eqs_enter(bool user)
>  {
> -	unsigned long flags;
>  	long long oldval;
>  	struct rcu_dynticks *rdtp;
>  
> -	local_irq_save(flags);
>  	rdtp = &__get_cpu_var(rcu_dynticks);
>  	oldval = rdtp->dynticks_nesting;
>  	WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
> @@ -403,7 +401,6 @@ static void rcu_eqs_enter(bool user)
>  	else
>  		rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
>  	rcu_eqs_enter_common(rdtp, oldval, user);
> -	local_irq_restore(flags);
>  }
>  
>  /**
> @@ -420,7 +417,11 @@ static void rcu_eqs_enter(bool user)
>   */
>  void rcu_idle_enter(void)
>  {
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
>  	rcu_eqs_enter(0);
> +	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(rcu_idle_enter);
>  
> @@ -435,7 +436,18 @@ EXPORT_SYMBOL_GPL(rcu_idle_enter);
>   */
>  void rcu_user_enter(void)
>  {
> -	rcu_eqs_enter(1);
> +	unsigned long flags;
> +	struct rcu_dynticks *rdtp;
> +
> +	WARN_ON_ONCE(!current->mm);
> +
> +	local_irq_save(flags);
> +	rdtp = &__get_cpu_var(rcu_dynticks);
> +	if (!rdtp->in_user) {
> +		rdtp->in_user = true;
> +		rcu_eqs_enter(1);
> +	}
> +	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(rcu_user_enter);
>  
> @@ -530,11 +542,9 @@ static void rcu_eqs_exit_common(struct rcu_dynticks *rdtp, long long oldval,
>   */
>  static void rcu_eqs_exit(bool user)
>  {
> -	unsigned long flags;
>  	struct rcu_dynticks *rdtp;
>  	long long oldval;
>  
> -	local_irq_save(flags);
>  	rdtp = &__get_cpu_var(rcu_dynticks);
>  	oldval = rdtp->dynticks_nesting;
>  	WARN_ON_ONCE(oldval < 0);
> @@ -543,7 +553,6 @@ static void rcu_eqs_exit(bool user)
>  	else
>  		rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
>  	rcu_eqs_exit_common(rdtp, oldval, user);
> -	local_irq_restore(flags);
>  }
>  
>  /**
> @@ -559,7 +568,11 @@ static void rcu_eqs_exit(bool user)
>   */
>  void rcu_idle_exit(void)
>  {
> +	unsigned long flags;
> +
> +	local_irq_save(flags);
>  	rcu_eqs_exit(0);
> +	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(rcu_idle_exit);
>  
> @@ -572,7 +585,16 @@ EXPORT_SYMBOL_GPL(rcu_idle_exit);
>   */
>  void rcu_user_exit(void)
>  {
> -	rcu_eqs_exit(1);
> +	unsigned long flags;
> +	struct rcu_dynticks *rdtp;
> +
> +	local_irq_save(flags);
> +	rdtp = &__get_cpu_var(rcu_dynticks);
> +	if (rdtp->in_user) {
> +		rdtp->in_user = false;
> +		rcu_eqs_exit(1);
> +	}
> +	local_irq_restore(flags);
>  }
>  EXPORT_SYMBOL_GPL(rcu_user_exit);
>  
> @@ -2590,6 +2612,9 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
>  	rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
>  	WARN_ON_ONCE(rdp->dynticks->dynticks_nesting != DYNTICK_TASK_EXIT_IDLE);
>  	WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
> +#ifdef CONFIG_RCU_USER_QS
> +	WARN_ON_ONCE(rdp->dynticks->in_user);
> +#endif
>  	rdp->cpu = cpu;
>  	rdp->rsp = rsp;
>  	raw_spin_unlock_irqrestore(&rnp->lock, flags);
> diff --git a/kernel/rcutree.h b/kernel/rcutree.h
> index 4d29169..0dd5fd6 100644
> --- a/kernel/rcutree.h
> +++ b/kernel/rcutree.h
> @@ -102,6 +102,9 @@ struct rcu_dynticks {
>  				    /* idle-period nonlazy_posted snapshot. */
>  	int tick_nohz_enabled_snap; /* Previously seen value from sysfs. */
>  #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
> +#ifdef CONFIG_RCU_USER_QS
> +	bool in_user;		    /* Is the CPU in userland from RCU POV? */
> +#endif
>  };
>  
>  /* RCU's kthread states for tracing. */
> -- 
> 1.7.8
>