[PATCH] events: Annotate parent_ctx with __rcu

[PATCH] events: Annotate parent_ctx with __rcu

[Amol Grover]

parent_ctx is used under RCU context in kernel/events/core.c,
tell sparse about it aswell.

Fixes the following instances of sparse error:
kernel/events/core.c:3221:18: error: incompatible types in comparison
kernel/events/core.c:3222:23: error: incompatible types in comparison

This introduces the following two sparse errors:
kernel/events/core.c:3117:18: error: incompatible types in comparison
kernel/events/core.c:3121:30: error: incompatible types in comparison

Hence, use rcu_dereference() to access parent_ctx and silence
the newly introduced errors.

Signed-off-by: Amol Grover <frextrite@gmail.com>
---
 include/linux/perf_event.h |  2 +-
 kernel/events/core.c       | 11 ++++++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 6d4c22aee384..e18a7bdc6f98 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -810,7 +810,7 @@ struct perf_event_context {
 	 * These fields let us detect when two contexts have both
 	 * been cloned (inherited) from a common ancestor.
 	 */
-	struct perf_event_context	*parent_ctx;
+	struct perf_event_context __rcu	*parent_ctx;
 	u64				parent_gen;
 	u64				generation;
 	int				pin_count;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2173c23c25b4..2a8c5670b254 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
 static int context_equiv(struct perf_event_context *ctx1,
 			 struct perf_event_context *ctx2)
 {
+	struct perf_event_context *parent_ctx1, *parent_ctx2;
+
 	lockdep_assert_held(&ctx1->lock);
 	lockdep_assert_held(&ctx2->lock);
 
+	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
+	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);
+
 	/* Pinning disables the swap optimization */
 	if (ctx1->pin_count || ctx2->pin_count)
 		return 0;
 
 	/* If ctx1 is the parent of ctx2 */
-	if (ctx1 == ctx2->parent_ctx && ctx1->generation == ctx2->parent_gen)
+	if (ctx1 == parent_ctx2 && ctx1->generation == ctx2->parent_gen)
 		return 1;
 
 	/* If ctx2 is the parent of ctx1 */
-	if (ctx1->parent_ctx == ctx2 && ctx1->parent_gen == ctx2->generation)
+	if (parent_ctx1 == ctx2 && ctx1->parent_gen == ctx2->generation)
 		return 1;
 
 	/*
 	 * If ctx1 and ctx2 have the same parent; we flatten the parent
 	 * hierarchy, see perf_event_init_context().
 	 */
-	if (ctx1->parent_ctx && ctx1->parent_ctx == ctx2->parent_ctx &&
+	if (ctx1->parent_ctx && parent_ctx1 == parent_ctx2 &&
 			ctx1->parent_gen == ctx2->parent_gen)
 		return 1;
 
-- 
2.24.1

Re: [PATCH] events: Annotate parent_ctx with __rcu

[Peter Zijlstra]

On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:
> parent_ctx is used under RCU context in kernel/events/core.c,
> tell sparse about it aswell.
> 
> Fixes the following instances of sparse error:
> kernel/events/core.c:3221:18: error: incompatible types in comparison
> kernel/events/core.c:3222:23: error: incompatible types in comparison
> 
> This introduces the following two sparse errors:
> kernel/events/core.c:3117:18: error: incompatible types in comparison
> kernel/events/core.c:3121:30: error: incompatible types in comparison
> 
> Hence, use rcu_dereference() to access parent_ctx and silence
> the newly introduced errors.
> 
> Signed-off-by: Amol Grover <frextrite@gmail.com>
> ---
>  include/linux/perf_event.h |  2 +-
>  kernel/events/core.c       | 11 ++++++++---
>  2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 6d4c22aee384..e18a7bdc6f98 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -810,7 +810,7 @@ struct perf_event_context {
>  	 * These fields let us detect when two contexts have both
>  	 * been cloned (inherited) from a common ancestor.
>  	 */
> -	struct perf_event_context	*parent_ctx;
> +	struct perf_event_context __rcu	*parent_ctx;
>  	u64				parent_gen;
>  	u64				generation;
>  	int				pin_count;
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 2173c23c25b4..2a8c5670b254 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
>  static int context_equiv(struct perf_event_context *ctx1,
>  			 struct perf_event_context *ctx2)
>  {
> +	struct perf_event_context *parent_ctx1, *parent_ctx2;
> +
>  	lockdep_assert_held(&ctx1->lock);
>  	lockdep_assert_held(&ctx2->lock);
>  
> +	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> +	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);

Bah.

Why are you  fixing all this sparse crap and making the code worse?

Re: [PATCH] events: Annotate parent_ctx with __rcu

[Amol Grover]

On Mon, Feb 10, 2020 at 10:36:24AM +0100, Peter Zijlstra wrote:
> On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:
> > parent_ctx is used under RCU context in kernel/events/core.c,
> > tell sparse about it aswell.
> > 
> > Fixes the following instances of sparse error:
> > kernel/events/core.c:3221:18: error: incompatible types in comparison
> > kernel/events/core.c:3222:23: error: incompatible types in comparison
> > 
> > This introduces the following two sparse errors:
> > kernel/events/core.c:3117:18: error: incompatible types in comparison
> > kernel/events/core.c:3121:30: error: incompatible types in comparison
> > 
> > Hence, use rcu_dereference() to access parent_ctx and silence
> > the newly introduced errors.
> > 
> > Signed-off-by: Amol Grover <frextrite@gmail.com>
> > ---
> >  include/linux/perf_event.h |  2 +-
> >  kernel/events/core.c       | 11 ++++++++---
> >  2 files changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 6d4c22aee384..e18a7bdc6f98 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -810,7 +810,7 @@ struct perf_event_context {
> >  	 * These fields let us detect when two contexts have both
> >  	 * been cloned (inherited) from a common ancestor.
> >  	 */
> > -	struct perf_event_context	*parent_ctx;
> > +	struct perf_event_context __rcu	*parent_ctx;
> >  	u64				parent_gen;
> >  	u64				generation;
> >  	int				pin_count;
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 2173c23c25b4..2a8c5670b254 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> >  static int context_equiv(struct perf_event_context *ctx1,
> >  			 struct perf_event_context *ctx2)
> >  {
> > +	struct perf_event_context *parent_ctx1, *parent_ctx2;
> > +
> >  	lockdep_assert_held(&ctx1->lock);
> >  	lockdep_assert_held(&ctx2->lock);
> >  
> > +	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> > +	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);
> 
> Bah.
> 
> Why are you  fixing all this sparse crap and making the code worse?

Hi Peter,

Sparse is quite noisy and we need to eliminate false-positives, right?
__rcu will tell the developer, this pointer could change and he needs to
take the required steps to make sure the code doesn't break.

Thanks
Amol

Re: [PATCH] events: Annotate parent_ctx with __rcu

[Peter Zijlstra]

On Mon, Feb 10, 2020 at 06:29:48PM +0530, Amol Grover wrote:
> On Mon, Feb 10, 2020 at 10:36:24AM +0100, Peter Zijlstra wrote:
> > On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:

> > > @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> > >  static int context_equiv(struct perf_event_context *ctx1,
> > >  			 struct perf_event_context *ctx2)
> > >  {
> > > +	struct perf_event_context *parent_ctx1, *parent_ctx2;
> > > +
> > >  	lockdep_assert_held(&ctx1->lock);
> > >  	lockdep_assert_held(&ctx2->lock);
> > >  
> > > +	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> > > +	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);
> > 
> > Bah.
> > 
> > Why are you  fixing all this sparse crap and making the code worse?
> 
> Hi Peter,
> 
> Sparse is quite noisy and we need to eliminate false-positives, right?

Dunno, I've been happy just ignoring it all.

> __rcu will tell the developer, this pointer could change and he needs to
> take the required steps to make sure the code doesn't break.

I know what it does; what I don't know is why you need to make the code
worse. In paricular, __rcu doesn't mandate rcu_dereference(), esp. not
when you're actually holding the write side lock.

Re: [PATCH] events: Annotate parent_ctx with __rcu

[Amol Grover]

On Mon, Feb 10, 2020 at 02:34:59PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 10, 2020 at 06:29:48PM +0530, Amol Grover wrote:
> > On Mon, Feb 10, 2020 at 10:36:24AM +0100, Peter Zijlstra wrote:
> > > On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:
> 
> > > > @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> > > >  static int context_equiv(struct perf_event_context *ctx1,
> > > >  			 struct perf_event_context *ctx2)
> > > >  {
> > > > +	struct perf_event_context *parent_ctx1, *parent_ctx2;
> > > > +
> > > >  	lockdep_assert_held(&ctx1->lock);
> > > >  	lockdep_assert_held(&ctx2->lock);
> > > >  
> > > > +	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> > > > +	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);
> > > 
> > > Bah.
> > > 
> > > Why are you  fixing all this sparse crap and making the code worse?
> > 
> > Hi Peter,
> > 
> > Sparse is quite noisy and we need to eliminate false-positives, right?
> 
> Dunno, I've been happy just ignoring it all.
> 
> > __rcu will tell the developer, this pointer could change and he needs to
> > take the required steps to make sure the code doesn't break.
> 
> I know what it does; what I don't know is why you need to make the code
> worse. In paricular, __rcu doesn't mandate rcu_dereference(), esp. not
> when you're actually holding the write side lock.

I might've misinterpreted the code. How does replacing rcu_dereference()
with
parent_ctx1 = rcu_dereference_protected(ctx1->parent_ctx,
					lockdep_is_held(&ctx1->lock));
sound?

Thanks
Amol

Re: [PATCH] events: Annotate parent_ctx with __rcu

[Joel Fernandes]

On Mon, Feb 10, 2020 at 10:17:27PM +0530, Amol Grover wrote:
> On Mon, Feb 10, 2020 at 02:34:59PM +0100, Peter Zijlstra wrote:
> > On Mon, Feb 10, 2020 at 06:29:48PM +0530, Amol Grover wrote:
> > > On Mon, Feb 10, 2020 at 10:36:24AM +0100, Peter Zijlstra wrote:
> > > > On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:
> > 
> > > > > @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> > > > >  static int context_equiv(struct perf_event_context *ctx1,
> > > > >  			 struct perf_event_context *ctx2)
> > > > >  {
> > > > > +	struct perf_event_context *parent_ctx1, *parent_ctx2;
> > > > > +
> > > > >  	lockdep_assert_held(&ctx1->lock);
> > > > >  	lockdep_assert_held(&ctx2->lock);
> > > > >  
> > > > > +	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> > > > > +	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);

You can probably remove the earlier lockdep_assert_held(s) if you're going to
use rcu_dereference_protected() here, since that would do the checking anyway.

> > > > 
> > > > Bah.
> > > > 
> > > > Why are you  fixing all this sparse crap and making the code worse?
> > > 
> > > Hi Peter,
> > > 
> > > Sparse is quite noisy and we need to eliminate false-positives, right?
> > 
> > Dunno, I've been happy just ignoring it all.

FWIW some of the sparse fixes Amol made recently did uncover so existing
"bugs" :) (Not in perf but other code).

> > > __rcu will tell the developer, this pointer could change and he needs to
> > > take the required steps to make sure the code doesn't break.
> > 
> > I know what it does; what I don't know is why you need to make the code
> > worse. In paricular, __rcu doesn't mandate rcu_dereference(), esp. not
> > when you're actually holding the write side lock.
> 
> I might've misinterpreted the code. How does replacing rcu_dereference()
> with
> parent_ctx1 = rcu_dereference_protected(ctx1->parent_ctx,
> 					lockdep_is_held(&ctx1->lock));
> sound?

FWIW, some maintainers do hate calling RCU APIs when write side lock is held.
Evidently it does make the code readability a bit worse and I can see Peter's
point of view because the existing code is correct. I leave it to you guys to
decide how you want to handle it.

thanks!

 - Joel

Re: [PATCH] events: Annotate parent_ctx with __rcu

[Amol Grover]

On Mon, Feb 10, 2020 at 12:08:31PM -0500, Joel Fernandes wrote:
> On Mon, Feb 10, 2020 at 10:17:27PM +0530, Amol Grover wrote:
> > On Mon, Feb 10, 2020 at 02:34:59PM +0100, Peter Zijlstra wrote:
> > > On Mon, Feb 10, 2020 at 06:29:48PM +0530, Amol Grover wrote:
> > > > On Mon, Feb 10, 2020 at 10:36:24AM +0100, Peter Zijlstra wrote:
> > > > > On Sat, Feb 08, 2020 at 08:16:49PM +0530, Amol Grover wrote:
> > > 
> > > > > > @@ -3106,26 +3106,31 @@ static void ctx_sched_out(struct perf_event_context *ctx,
> > > > > >  static int context_equiv(struct perf_event_context *ctx1,
> > > > > >  			 struct perf_event_context *ctx2)
> > > > > >  {
> > > > > > +	struct perf_event_context *parent_ctx1, *parent_ctx2;
> > > > > > +
> > > > > >  	lockdep_assert_held(&ctx1->lock);
> > > > > >  	lockdep_assert_held(&ctx2->lock);
> > > > > >  
> > > > > > +	parent_ctx1 = rcu_dereference(ctx1->parent_ctx);
> > > > > > +	parent_ctx2 = rcu_dereference(ctx2->parent_ctx);
> 
> You can probably remove the earlier lockdep_assert_held(s) if you're going to
> use rcu_dereference_protected() here, since that would do the checking anyway.
> 

Ah yes, I was thinking this aswell.

> > > > > 
> > > > > Bah.
> > > > > 
> > > > > Why are you  fixing all this sparse crap and making the code worse?
> > > > 
> > > > Hi Peter,
> > > > 
> > > > Sparse is quite noisy and we need to eliminate false-positives, right?
> > > 
> > > Dunno, I've been happy just ignoring it all.
> 
> FWIW some of the sparse fixes Amol made recently did uncover so existing
> "bugs" :) (Not in perf but other code).
> 
> > > > __rcu will tell the developer, this pointer could change and he needs to
> > > > take the required steps to make sure the code doesn't break.
> > > 
> > > I know what it does; what I don't know is why you need to make the code
> > > worse. In paricular, __rcu doesn't mandate rcu_dereference(), esp. not
> > > when you're actually holding the write side lock.
> > 
> > I might've misinterpreted the code. How does replacing rcu_dereference()
> > with
> > parent_ctx1 = rcu_dereference_protected(ctx1->parent_ctx,
> > 					lockdep_is_held(&ctx1->lock));
> > sound?
> 
> FWIW, some maintainers do hate calling RCU APIs when write side lock is held.
> Evidently it does make the code readability a bit worse and I can see Peter's
> point of view because the existing code is correct. I leave it to you guys to
> decide how you want to handle it.
> 

In that case, I think the code is fine as it is. Thank you for the review both!

Thanks
Amol

> thanks!
> 
>  - Joel
>