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

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Message-Id: <20250806173625.f83a6fc9da16099e8ae12c85@linux-foundation.org>
Date: Wed, 6 Aug 2025 17:36:25 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Marco Elver <elver@...gle.com>
Cc: Soham Bagchi <soham.bagchi@...h.edu>, andreyknvl@...il.com,
 arnd@...db.de, corbet@....net, dvyukov@...gle.com, glider@...gle.com,
 kasan-dev@...glegroups.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, sohambagchi@...look.com, tglx@...utronix.de,
 workflows@...r.kernel.org
Subject: Re: [PATCH v2] kcov: load acquire coverage count in user-space code

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

> > The load-acquire pairs with the write memory barrier
> > used in kcov_move_area()
> >
> > Signed-off-by: Soham Bagchi <soham.bagchi@...h.edu>
> 
> Reviewed-by: Marco Elver <elver@...gle.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]);
_