Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use rcu_read_lock_sched_notrace()

["Paul E. McKenney" <paulmck@kernel.org>]

On Sat, Dec 10, 2022 at 08:58:03AM -0500, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@goodmis.org>
> 
> The mmiotrace tracer is "special". The purpose is to help reverse engineer
> binary drivers by removing the memory allocated by the driver and when the
> driver goes to access it, a fault occurs, the mmiotracer will record what
> the driver was doing and then do the work on its behalf by single stepping
> through the process.
> 
> But to achieve this ability, it must do some special things. One is to
> take the rcu_read_lock() when the fault occurs, and then release it in the
> breakpoint that is single stepping. This makes lockdep unhappy, as it
> changes the state of RCU from within an exception that is not contained in
> that exception, and we get a nasty splat from lockdep.
> 
> Instead, switch to rcu_read_lock_sched_notrace() as the RCU sched variant
> has the same grace period as normal RCU. This is basically the same as
> rcu_read_lock() but does not make lockdep complain about it.
> 
> Note, the preempt_disable() is still needed as it uses preempt_enable_no_resched().
> 
> Link: https://lore.kernel.org/linux-trace-kernel/20221209134144.04f33626@gandalf.local.home
> 
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Karol Herbst <karolherbst@gmail.com>
> Cc: Pekka Paalanen <ppaalanen@gmail.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Executable code can be the best form of comment.  ;-)

This does mess with preempt_count() redundantly, but the overhead from
that should be way down in the noise.

Acked-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  arch/x86/mm/kmmio.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/kmmio.c b/arch/x86/mm/kmmio.c
> index edb486450158..853c49877c16 100644
> --- a/arch/x86/mm/kmmio.c
> +++ b/arch/x86/mm/kmmio.c
> @@ -254,7 +254,7 @@ int kmmio_handler(struct pt_regs *regs, unsigned long addr)
>  	 * again.
>  	 */
>  	preempt_disable();
> -	rcu_read_lock();
> +	rcu_read_lock_sched_notrace();
>  
>  	faultpage = get_kmmio_fault_page(page_base);
>  	if (!faultpage) {
> @@ -323,7 +323,7 @@ int kmmio_handler(struct pt_regs *regs, unsigned long addr)
>  	return 1; /* fault handled */
>  
>  no_kmmio:
> -	rcu_read_unlock();
> +	rcu_read_unlock_sched_notrace();
>  	preempt_enable_no_resched();
>  	return ret;
>  }
> @@ -363,7 +363,7 @@ static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
>  	/* These were acquired in kmmio_handler(). */
>  	ctx->active--;
>  	BUG_ON(ctx->active);
> -	rcu_read_unlock();
> +	rcu_read_unlock_sched_notrace();
>  	preempt_enable_no_resched();
>  
>  	/*
> -- 
> 2.35.1
> 
>