kcov: load acquire coverage count in user-space code

[PATCH v2] kcov: load acquire coverage count in user-space code

Posted by Soham Bagchi 6 months, 1 week ago

Updating the KCOV documentation to use a load-acquire
operation for the first element of the shared memory
buffer between kernel-space and user-space.

The load-acquire pairs with the write memory barrier
used in kcov_move_area()

Signed-off-by: Soham Bagchi <soham.bagchi@utah.edu>
---

Changes in v2:
- note for load-acquire shifted to block comment
  in code rather than in the preceding paragraphs
---
 Documentation/dev-tools/kcov.rst | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
index 6611434e2dd..40a4b500073 100644
--- a/Documentation/dev-tools/kcov.rst
+++ b/Documentation/dev-tools/kcov.rst
@@ -361,7 +361,12 @@ local tasks spawned by the process and the global task that handles USB bus #1:
 	 */
 	sleep(2);
 
-	n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
+        /*
+         * The load to the coverage count should be an acquire to pair with 
+         * pair with the corresponding write memory barrier (smp_wmb()) on 
+         * the kernel-side in kcov_move_area().
+         */
+	n = __atomic_load_n(&cover[0], __ATOMIC_ACQUIRE);
 	for (i = 0; i < n; i++)
 		printf("0x%lx\n", cover[i + 1]);
 	if (ioctl(fd, KCOV_DISABLE, 0))
-- 
2.34.1

Re: [PATCH v2] kcov: load acquire coverage count in user-space code

Posted by Marco Elver 6 months, 1 week ago

On Sun, 3 Aug 2025 at 20:06, Soham Bagchi <soham.bagchi@utah.edu> wrote:
>
> Updating the KCOV documentation to use a load-acquire
> operation for the first element of the shared memory
> buffer between kernel-space and user-space.
>
> The load-acquire pairs with the write memory barrier
> used in kcov_move_area()
>
> Signed-off-by: Soham Bagchi <soham.bagchi@utah.edu>

Reviewed-by: Marco Elver <elver@google.com>

> ---
>
> Changes in v2:

Btw, it is customary to send out the whole patch series on a version
bump, even if only one of the patches changed.
https://www.kernel.org/doc/html/latest/process/submitting-patches.html#explicit-in-reply-to-headers

> - note for load-acquire shifted to block comment
>   in code rather than in the preceding paragraphs
> ---
>  Documentation/dev-tools/kcov.rst | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> index 6611434e2dd..40a4b500073 100644
> --- a/Documentation/dev-tools/kcov.rst
> +++ b/Documentation/dev-tools/kcov.rst
> @@ -361,7 +361,12 @@ local tasks spawned by the process and the global task that handles USB bus #1:
>          */
>         sleep(2);
>
> -       n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED);
> +        /*
> +         * The load to the coverage count should be an acquire to pair with
> +         * pair with the corresponding write memory barrier (smp_wmb()) on
> +         * the kernel-side in kcov_move_area().
> +         */
> +       n = __atomic_load_n(&cover[0], __ATOMIC_ACQUIRE);
>         for (i = 0; i < n; i++)
>                 printf("0x%lx\n", cover[i + 1]);
>         if (ioctl(fd, KCOV_DISABLE, 0))
> --
> 2.34.1
>

Re: [PATCH v2] kcov: load acquire coverage count in user-space code

Posted by Andrew Morton 6 months ago

On Mon, 4 Aug 2025 08:00:00 +0200 Marco Elver <elver@google.com> wrote:

> > The load-acquire pairs with the write memory barrier
> > used in kcov_move_area()
> >
> > Signed-off-by: Soham Bagchi <soham.bagchi@utah.edu>
> 
> Reviewed-by: Marco Elver <elver@google.com>
> 
> > ---
> >
> > Changes in v2:
> 
> Btw, it is customary to send out the whole patch series on a version
> bump, even if only one of the patches changed.
> https://www.kernel.org/doc/html/latest/process/submitting-patches.html#explicit-in-reply-to-headers

Yes please, try to keep everything together.  We look at a lot of
patches!

I queued this as a -fix against the original
https://lkml.kernel.org/r/20250728184318.1839137-2-soham.bagchi@utah.edu

--- a/Documentation/dev-tools/kcov.rst~kcov-load-acquire-coverage-count-in-user-space-code-v2
+++ a/Documentation/dev-tools/kcov.rst
@@ -287,11 +287,6 @@ handle instance id.
 The following program demonstrates using KCOV to collect coverage from both
 local tasks spawned by the process and the global task that handles USB bus #1:
 
-The user-space code for KCOV should also use an acquire to fetch the count
-of coverage entries in the shared buffer. This acquire pairs with the
-corresponding write memory barrier (smp_wmb()) on the kernel-side in
-kcov_move_area().
-
 .. code-block:: c
 
     /* Same includes and defines as above. */
@@ -366,6 +361,11 @@ kcov_move_area().
 	 */
 	sleep(2);
 
+        /*
+         * The load to the coverage count should be an acquire to pair with
+         * pair with the corresponding write memory barrier (smp_wmb()) on
+         * the kernel-side in kcov_move_area().
+         */
 	n = __atomic_load_n(&cover[0], __ATOMIC_ACQUIRE);
 	for (i = 0; i < n; i++)
 		printf("0x%lx\n", cover[i + 1]);
_