[2/2] x86/mm/kmmio: Remove rcu_read_lock()

Commit Message

Steven Rostedt Dec. 6, 2022, 7:12 p.m. UTC

From: Steven Rostedt <[e-mail]>

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 it
take the rcu_read_lock() when the fault occurs, and then release it in the
breakpoint that in the 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.

As it also disables preemption everywhere rcu_read_lock() is taken, and
enables preemption everywhere rcu_read_unlock(), and does not enable
preemption in between, it is the same as synchronize_rcu_sched(). But as
the RCU sched variant has the same grace period as normal RCU, there's no
reason to take the rcu_read_lock(). Simply remove it.

Cc: "Paul E. McKenney" <[e-mail]>
Signed-off-by: Steven Rostedt <[e-mail]>
---
 arch/x86/mm/kmmio.c | 3 ---
 1 file changed, 3 deletions(-)

Comments

Paul E. McKenney Dec. 7, 2022, 5:36 p.m. UTC | #1

On Tue, Dec 06, 2022 at 02:12:03PM -0500, Steven Rostedt wrote:
> From: Steven Rostedt <[e-mail]>
> 
> 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 it
> take the rcu_read_lock() when the fault occurs, and then release it in the
> breakpoint that in the 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.
> 
> As it also disables preemption everywhere rcu_read_lock() is taken, and
> enables preemption everywhere rcu_read_unlock(), and does not enable
> preemption in between, it is the same as synchronize_rcu_sched(). But as
> the RCU sched variant has the same grace period as normal RCU, there's no
> reason to take the rcu_read_lock(). Simply remove it.
> 
> Cc: "Paul E. McKenney" <[e-mail]>
> Signed-off-by: Steven Rostedt <[e-mail]>

Might be worth adding a comment saying that others are using this
preempt_disable() to block an RCU grace period, but that is up to
you guys.  I will let you and your future selves be the judges.

Acked-by: Paul E. McKenney <[e-mail]>

> ---
>  arch/x86/mm/kmmio.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/arch/x86/mm/kmmio.c b/arch/x86/mm/kmmio.c
> index edb486450158..e15e3aaaf94c 100644
> --- a/arch/x86/mm/kmmio.c
> +++ b/arch/x86/mm/kmmio.c
> @@ -254,7 +254,6 @@ int kmmio_handler(struct pt_regs *regs, unsigned long addr)
>  	 * again.
>  	 */
>  	preempt_disable();
> -	rcu_read_lock();
>  
>  	faultpage = get_kmmio_fault_page(page_base);
>  	if (!faultpage) {
> @@ -323,7 +322,6 @@ int kmmio_handler(struct pt_regs *regs, unsigned long addr)
>  	return 1; /* fault handled */
>  
>  no_kmmio:
> -	rcu_read_unlock();
>  	preempt_enable_no_resched();
>  	return ret;
>  }
> @@ -363,7 +361,6 @@ 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();
>  	preempt_enable_no_resched();
>  
>  	/*
> -- 
> 2.35.1
> 
>

Steven Rostedt Dec. 9, 2022, 6:03 p.m. UTC | #2

On Wed, 7 Dec 2022 09:36:21 -0800
"Paul E. McKenney" <[e-mail]> wrote:

> > Cc: "Paul E. McKenney" <[e-mail]>
> > Signed-off-by: Steven Rostedt <[e-mail]>  
> 
> Might be worth adding a comment saying that others are using this
> preempt_disable() to block an RCU grace period, but that is up to
> you guys.  I will let you and your future selves be the judges.

Good point. I'll add a comment in v2.

> 
> Acked-by: Paul E. McKenney <[e-mail]>

Thanks!

-- Steve

Steven Rostedt Dec. 9, 2022, 6:09 p.m. UTC | #3

On Fri, 9 Dec 2022 13:03:34 -0500
Steven Rostedt <[e-mail]> wrote:

> On Wed, 7 Dec 2022 09:36:21 -0800
> "Paul E. McKenney" <[e-mail]> wrote:
> 
> > > Cc: "Paul E. McKenney" <[e-mail]>
> > > Signed-off-by: Steven Rostedt <[e-mail]>    
> > 
> > Might be worth adding a comment saying that others are using this
> > preempt_disable() to block an RCU grace period, but that is up to
> > you guys.  I will let you and your future selves be the judges.  
> 
> Good point. I'll add a comment in v2.

Actually, rcu_read_lock_sched_notrace() may work instead. Let me test it.

-- Steve

Patch

diff --git a/arch/x86/mm/kmmio.c b/arch/x86/mm/kmmio.c
index edb486450158..e15e3aaaf94c 100644
--- a/arch/x86/mm/kmmio.c
+++ b/arch/x86/mm/kmmio.c
@@ -254,7 +254,6 @@  int kmmio_handler(struct pt_regs *regs, unsigned long addr)
 	 * again.
 	 */
 	preempt_disable();
-	rcu_read_lock();
 
 	faultpage = get_kmmio_fault_page(page_base);
 	if (!faultpage) {
@@ -323,7 +322,6 @@  int kmmio_handler(struct pt_regs *regs, unsigned long addr)
 	return 1; /* fault handled */
 
 no_kmmio:
-	rcu_read_unlock();
 	preempt_enable_no_resched();
 	return ret;
 }
@@ -363,7 +361,6 @@  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();
 	preempt_enable_no_resched();
 
 	/*