Re: [PATCH 1/5] kcov: add kcov_pause()/kcov_resume() helpers

[Bradley Morgan]

On 7 August 2026 21:50:23 BST, Karl Mehltretter <kmehltretter@xxxxxxxxx>
wrote:
>Interrupt-return work can run after HARDIRQ_OFFSET is dropped, when
>in_task() is true. KCOV then attributes instrumented callees to the
>interrupted task.
>
>Add a KCOV_PAUSED bit next to KCOV_IN_CTXSW and mask both in
>kcov_mode_enabled(). The context switch suppression keeps its own bit:
>kcov_prepare_switch() runs on the previous task and kcov_finish_switch()
>on the one switched in, so its lifetime is not a pause section.
>
>Sections nest by passing the state returned by kcov_pause() to
>kcov_resume(). Both operate on current. When task KCOV is active, remote
>softirq sections save and restore the complete mode, preserving the
>pause state.
>
>The helpers are __always_inline, and the caller must be uninstrumented:
>inlining does not remove the caller's own coverage callbacks.
>

Hmm. Okay!

Reviewed-by: Bradley Morgan <include@xxxxxxxxx>
 

I don't see anything wrong with it, LGTM, thanks for the patch 

>Assisted-by: Claude:claude-opus-4-8
>Assisted-by: Claude:claude-fable-5
>Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
>---
> include/linux/kcov.h | 25 ++++++++++++++++++++++++-
> kernel/kcov.c        |  2 +-
> 2 files changed, 25 insertions(+), 2 deletions(-)
>
>diff --git a/include/linux/kcov.h b/include/linux/kcov.h
>index 895b761b2db1..5a0a1a9bb7ef 100644
>--- a/include/linux/kcov.h
>+++ b/include/linux/kcov.h
>@@ -2,6 +2,7 @@
> #ifndef _LINUX_KCOV_H
> #define _LINUX_KCOV_H
> 
>+#include <linux/bits.h>
> #include <linux/sched.h>
> #include <uapi/linux/kcov.h>
> 
>@@ -23,7 +24,8 @@ enum kcov_mode {
> 	KCOV_MODE_TRACE_CMP = 3,
> };
> 
>-#define KCOV_IN_CTXSW	(1 << 30)
>+#define KCOV_IN_CTXSW	BIT(30)
>+#define KCOV_PAUSED	BIT(29)
> 
> void kcov_task_init(struct task_struct *t);
> void kcov_task_exit(struct task_struct *t);
>@@ -38,6 +40,25 @@ do {						\
> 	(t)->kcov_mode &= ~KCOV_IN_CTXSW;	\
> } while (0)
> 
>+/*
>+ * Pause coverage for current. Pass the returned state to kcov_resume().
>+ * Callers must be uninstrumented.
>+ */
>+static __always_inline unsigned int kcov_pause(struct task_struct *t)
>+{
>+	unsigned int paused;
>+
>+	paused = t->kcov_mode & KCOV_PAUSED;
>+	t->kcov_mode |= KCOV_PAUSED;
>+	return paused;
>+}
>+
>+static __always_inline void kcov_resume(struct task_struct *t, unsigned int paused)
>+{
>+	if (!paused)
>+		t->kcov_mode &= ~KCOV_PAUSED;
>+}
>+
> /* See Documentation/dev-tools/kcov.rst for usage details. */
> void kcov_remote_start(u64 handle);
> void kcov_remote_stop(void);
>@@ -93,6 +114,8 @@ void __sanitizer_cov_trace_switch(kcov_u64 val, void *cases);
> 
> static inline void kcov_task_init(struct task_struct *t) {}
> static inline void kcov_task_exit(struct task_struct *t) {}
>+static inline unsigned int kcov_pause(struct task_struct *t) { return 0; }
>+static inline void kcov_resume(struct task_struct *t, unsigned int paused) {}
> static inline void kcov_prepare_switch(struct task_struct *t) {}
> static inline void kcov_finish_switch(struct task_struct *t) {}
> static inline void kcov_remote_start(u64 handle) {}
>diff --git a/kernel/kcov.c b/kernel/kcov.c
>index 1df373fb562b..83d53e383822 100644
>--- a/kernel/kcov.c
>+++ b/kernel/kcov.c
>@@ -830,7 +830,7 @@ static const struct file_operations kcov_fops = {
> 
> static inline bool kcov_mode_enabled(unsigned int mode)
> {
>-	return (mode & ~KCOV_IN_CTXSW) != KCOV_MODE_DISABLED;
>+	return (mode & ~(KCOV_IN_CTXSW | KCOV_PAUSED)) != KCOV_MODE_DISABLED;
> }
> 
> static void kcov_remote_softirq_start(struct task_struct *t)
>

Thanks!