Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read_lock()

From: Josh Triplett <josh@joshtriplett.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org,
	jiangshanlai@gmail.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
	dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com,
	fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 07/13] rcu: Move preemption disabling out of __srcu_read_lock()
Date: Tue, 6 Oct 2015 10:18:39 -0700	[thread overview]
Message-ID: <20151006171839.GD9600@cloud> (raw)
In-Reply-To: <1444148028-11551-7-git-send-email-paulmck@linux.vnet.ibm.com>

On Tue, Oct 06, 2015 at 09:13:42AM -0700, Paul E. McKenney wrote:
> Currently, __srcu_read_lock() cannot be invoked from restricted
> environments because it contains calls to preempt_disable() and
> preempt_enable(), both of which can invoke lockdep, which is a bad
> idea in some restricted execution modes.  This commit therefore moves
> the preempt_disable() and preempt_enable() from __srcu_read_lock()
> to srcu_read_lock().  It also inserts the preempt_disable() and
> preempt_enable() around the call to __srcu_read_lock() in do_exit().

What restricted environments do you intend to invoke
__srcu_read_lock from?

This change seems fine, but I don't see any change in this patch series
that needs this, hence my curiosity.

> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  include/linux/srcu.h | 5 ++++-
>  kernel/exit.c        | 2 ++
>  kernel/rcu/srcu.c    | 2 --
>  3 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index bdeb4567b71e..f5f80c5643ac 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -215,8 +215,11 @@ static inline int srcu_read_lock_held(struct srcu_struct *sp)
>   */
>  static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
>  {
> -	int retval = __srcu_read_lock(sp);
> +	int retval;
>  
> +	preempt_disable();
> +	retval = __srcu_read_lock(sp);
> +	preempt_enable();
>  	rcu_lock_acquire(&(sp)->dep_map);
>  	return retval;
>  }
> diff --git a/kernel/exit.c b/kernel/exit.c
> index ea95ee1b5ef7..0e93b63bbc59 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -761,7 +761,9 @@ void do_exit(long code)
>  	 */
>  	flush_ptrace_hw_breakpoint(tsk);
>  
> +	TASKS_RCU(preempt_disable());
>  	TASKS_RCU(tasks_rcu_i = __srcu_read_lock(&tasks_rcu_exit_srcu));
> +	TASKS_RCU(preempt_enable());
>  	exit_notify(tsk, group_dead);
>  	proc_exit_connector(tsk);
>  #ifdef CONFIG_NUMA
> diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
> index 9e6122540d28..a63a1ea5a41b 100644
> --- a/kernel/rcu/srcu.c
> +++ b/kernel/rcu/srcu.c
> @@ -298,11 +298,9 @@ int __srcu_read_lock(struct srcu_struct *sp)
>  	int idx;
>  
>  	idx = READ_ONCE(sp->completed) & 0x1;
> -	preempt_disable();
>  	__this_cpu_inc(sp->per_cpu_ref->c[idx]);
>  	smp_mb(); /* B */  /* Avoid leaking the critical section. */
>  	__this_cpu_inc(sp->per_cpu_ref->seq[idx]);
> -	preempt_enable();
>  	return idx;
>  }
>  EXPORT_SYMBOL_GPL(__srcu_read_lock);
> -- 
> 2.5.2
> 