[PATCH 1/2] regulator: dummy: force synchronous probing

[PATCH 1/2] regulator: dummy: force synchronous probing

[Christian Eggers]

Sometimes I get a NULL pointer dereference at boot time in kobject_get()
with the following call stack:

anatop_regulator_probe()
 devm_regulator_register()
  regulator_register()
   regulator_resolve_supply()
    kobject_get()

By placing some extra BUG_ON() statements I could verify that this is
raised because probing of the 'dummy' regulator driver is not completed
('dummy_regulator_rdev' is still NULL).

In the JTAG debugger I can see that dummy_regulator_probe() and
anatop_regulator_probe() can be run by different kernel threads
(kworker/u4:*).  I haven't further investigated whether this can be
changed or if there are other possibilities to force synchronization
between these two probe routines.  On the other hand I don't expect much
boot time penalty by probing the 'dummy' regulator synchronously.

Cc: stable@vger.kernel.org
Fixes: 259b93b21a9f ("regulator: Set PROBE_PREFER_ASYNCHRONOUS for drivers that existed in 4.14")
Signed-off-by: Christian Eggers <ceggers@arri.de>
---
 drivers/regulator/dummy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/regulator/dummy.c b/drivers/regulator/dummy.c
index 5b9b9e4e762d..9f59889129ab 100644
--- a/drivers/regulator/dummy.c
+++ b/drivers/regulator/dummy.c
@@ -60,7 +60,7 @@ static struct platform_driver dummy_regulator_driver = {
 	.probe		= dummy_regulator_probe,
 	.driver		= {
 		.name		= "reg-dummy",
-		.probe_type	= PROBE_PREFER_ASYNCHRONOUS,
+		.probe_type	= PROBE_FORCE_SYNCHRONOUS,
 	},
 };
 
-- 
2.43.0

[PATCH 2/2] regulator: assert that dummy regulator has been probed before using it

[Christian Eggers]

Due to asynchronous driver probing there is a chance that the dummy
regulator hasn't already been probed when first accessing it.

Cc: stable@vger.kernel.org
Signed-off-by: Christian Eggers <ceggers@arri.de>
---
 drivers/regulator/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
index 4ddf0efead68..bb9fe44aea11 100644
--- a/drivers/regulator/core.c
+++ b/drivers/regulator/core.c
@@ -2069,6 +2069,7 @@ static int regulator_resolve_supply(struct regulator_dev *rdev)
 
 		if (have_full_constraints()) {
 			r = dummy_regulator_rdev;
+			BUG_ON(!r);
 			get_device(&r->dev);
 		} else {
 			dev_err(dev, "Failed to resolve %s-supply for %s\n",
@@ -2086,6 +2087,7 @@ static int regulator_resolve_supply(struct regulator_dev *rdev)
 			goto out;
 		}
 		r = dummy_regulator_rdev;
+		BUG_ON(!r);
 		get_device(&r->dev);
 	}
 
@@ -2213,6 +2215,7 @@ struct regulator *_regulator_get_common(struct regulator_dev *rdev, struct devic
 			 */
 			dev_warn(dev, "supply %s not found, using dummy regulator\n", id);
 			rdev = dummy_regulator_rdev;
+			BUG_ON(!rdev);
 			get_device(&rdev->dev);
 			break;
 
-- 
2.43.0

Re: [PATCH 2/2] regulator: assert that dummy regulator has been probed before using it

[Mark Brown]

[-- Attachment #1: Type: text/plain, Size: 628 bytes --]

On Mon, Mar 10, 2025 at 05:33:02PM +0100, Christian Eggers wrote:
> Due to asynchronous driver probing there is a chance that the dummy
> regulator hasn't already been probed when first accessing it.

>  		if (have_full_constraints()) {
>  			r = dummy_regulator_rdev;
> +			BUG_ON(!r);
>  			get_device(&r->dev);
>  		} else {
>  			dev_err(dev, "Failed to resolve %s-supply for %s\n",
> @@ -2086,6 +2087,7 @@ static int regulator_resolve_supply(struct regulator_dev *rdev)
>  			goto out;
>  		}
>  		r = dummy_regulator_rdev;
> +		BUG_ON(!r);

This doesn't actually help anything - I'd expect this to trigger probe
deferral.

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

Re: [PATCH 2/2] regulator: assert that dummy regulator has been probed before using it

[Christian Eggers]

Am Montag, 10. März 2025, 18:23:02 CET schrieb Mark Brown:
> On Mon, Mar 10, 2025 at 05:33:02PM +0100, Christian Eggers wrote:
> > Due to asynchronous driver probing there is a chance that the dummy
> > regulator hasn't already been probed when first accessing it.
> > 
> >  		if (have_full_constraints()) {
> >  		
> >  			r = dummy_regulator_rdev;
> > 
> > +			BUG_ON(!r);
> > 
> >  			get_device(&r->dev);
> >  		
> >  		} else {
> >  		
> >  			dev_err(dev, "Failed to resolve %s-supply for 
%s\n",
> > 
> > @@ -2086,6 +2087,7 @@ static int regulator_resolve_supply(struct
> > regulator_dev *rdev)> 
> >  			goto out;
> >  		
> >  		}
> >  		r = dummy_regulator_rdev;
> > 
> > +		BUG_ON(!r);
> 
> This doesn't actually help anything
My idea was to help identifying the problem (if it is reintroduced again 
later).

> I'd expect this to trigger probe deferral.
I can check for this tomorrow.  But is it worth to use deferred probing
for a shared "NOP" driver which doesn't access any hardware?  Or would this 
only introduce overhead for nothing?

regards,
Christian

Re: [PATCH 2/2] regulator: assert that dummy regulator has been probed before using it

[Mark Brown]

[-- Attachment #1: Type: text/plain, Size: 775 bytes --]

On Mon, Mar 10, 2025 at 07:22:24PM +0100, Christian Eggers wrote:
> Am Montag, 10. März 2025, 18:23:02 CET schrieb Mark Brown:

> > > +		BUG_ON(!r);

> > This doesn't actually help anything

> My idea was to help identifying the problem (if it is reintroduced again 
> later).

> > I'd expect this to trigger probe deferral.

> I can check for this tomorrow.  But is it worth to use deferred probing
> for a shared "NOP" driver which doesn't access any hardware?  Or would this 
> only introduce overhead for nothing?

My concern is that if something goes wrong in production then we've just
escallated the problem, given that there's a clear error handling
mechanism we could use we should take advantage of that rather than
doing something destructive.

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]