Re: [PATCH v2] cleanup: adjust scoped_guard() to avoid potential warning

[David Lechner <dlechner@baylibre.com>]

On 10/9/24 6:44 AM, Przemek Kitszel wrote:
> Change scoped_guard() to make reasoning about it easier for static
> analysis tools (smatch, compiler diagnostics), especially to enable them
> to tell if the given scoped_guard() is conditional (interruptible-locks,
> try-locks) or not (like simple mutex_lock()).
> 
> Add compile-time error if scoped_cond_guard() is used for non-conditional
> lock class.
> 
> Beyond easier tooling and a little shrink reported by bloat-o-meter:
> add/remove: 3/2 grow/shrink: 45/55 up/down: 1573/-2069 (-496)
> this patch enables developer to write code like:
> 
> int foo(struct my_drv *adapter)
> {
> 	scoped_guard(spinlock, &adapter->some_spinlock)
> 		return adapter->spinlock_protected_var;
> }
> > 
> Current scoped_guard() implementation does not support that,
> due to compiler complaining:
> error: control reaches end of non-void function [-Werror=return-type]

I was hoping that this would allow us to do the same with
scoped_cond_guard() so that we could remove a bunch of
unreachable(); that we had to add in the IIO subsystem. But
with this patch we still get compile errors if we remove them.

Is it possible to apply the same if/goto magic to scoped_cond_guard()
to make it better too?

---

Here is a test case if that helps...

For example, I made this change:

diff --git a/drivers/iio/adc/ad7380.c b/drivers/iio/adc/ad7380.c
index e8bddfb0d07d..f1c85690af1e 100644
--- a/drivers/iio/adc/ad7380.c
+++ b/drivers/iio/adc/ad7380.c
@@ -577,7 +577,6 @@ static int ad7380_debugfs_reg_access(struct iio_dev *indio_dev, u32 reg,
 		else
 			return regmap_write(st->regmap, reg, writeval);
 	}
-	unreachable();
 }
 
 /*
@@ -824,7 +823,6 @@ static int ad7380_read_raw(struct iio_dev *indio_dev,
 			return ad7380_read_direct(st, chan->scan_index,
 						  scan_type, val);
 		}
-		unreachable();
 	case IIO_CHAN_INFO_SCALE:
 		/*
 		 * According to the datasheet, the LSB size is:
@@ -933,7 +931,6 @@ static int ad7380_write_raw(struct iio_dev *indio_dev,
 					FIELD_PREP(AD7380_CONFIG2_RESET,
 						   AD7380_CONFIG2_RESET_SOFT));
 		}
-		unreachable();
 	default:
 		return -EINVAL;
 	}

And then I get the following compiler errors/warnings:

/home/david/work/linux/drivers/iio/adc/ad7380.c: In function ‘ad7380_debugfs_reg_access’:
/home/david/work/linux/drivers/iio/adc/ad7380.c:580:1: error: control reaches end of non-void function [-Werror=return-type]
  580 | }
      | ^
In file included from /home/david/work/linux/include/linux/irqflags.h:17,
                 from /home/david/work/linux/arch/arm/include/asm/bitops.h:28,
                 from /home/david/work/linux/include/linux/bitops.h:68,
                 from /home/david/work/linux/drivers/iio/adc/ad7380.c:20:
/home/david/work/linux/drivers/iio/adc/ad7380.c: In function ‘ad7380_write_raw’:
/home/david/work/linux/include/linux/cleanup.h:337:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  337 |         for (CLASS(_name, scope)(args), \
      |         ^~~
/home/david/work/linux/include/linux/iio/iio.h:689:9: note: in expansion of macro ‘scoped_cond_guard’
  689 |         scoped_cond_guard(iio_claim_direct_try, fail, iio_dev)
      |         ^~~~~~~~~~~~~~~~~
/home/david/work/linux/drivers/iio/adc/ad7380.c:910:17: note: in expansion of macro ‘iio_device_claim_direct_scoped’
  910 |                 iio_device_claim_direct_scoped(return -EBUSY, indio_dev) {
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/david/work/linux/drivers/iio/adc/ad7380.c:934:9: note: here
  934 |         default:
      |         ^~~~~~~
/home/david/work/linux/drivers/iio/adc/ad7380.c: In function ‘ad7380_read_raw’:
/home/david/work/linux/include/linux/cleanup.h:337:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
  337 |         for (CLASS(_name, scope)(args), \
      |         ^~~
/home/david/work/linux/include/linux/iio/iio.h:689:9: note: in expansion of macro ‘scoped_cond_guard’
  689 |         scoped_cond_guard(iio_claim_direct_try, fail, iio_dev)
      |         ^~~~~~~~~~~~~~~~~
/home/david/work/linux/drivers/iio/adc/ad7380.c:822:17: note: in expansion of macro ‘iio_device_claim_direct_scoped’
  822 |                 iio_device_claim_direct_scoped(return -EBUSY, indio_dev) {
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/david/work/linux/drivers/iio/adc/ad7380.c:826:9: note: here
  826 |         case IIO_CHAN_INFO_SCALE:
      |         ^~~~

Using compiler:

$ arm-linux-gnueabi-gcc --version
arm-linux-gnueabi-gcc (Ubuntu 13.2.0-23ubuntu4) 13.2.0