[RFC PATCH v2 03/14] kcov: add barriers to recursion guard in kcov_df_write

[Yunseong Kim <yunseong.kim@est.tech>]

The recursion guard (bit-31 of kcov_df_seq) prevents reentry when
copy_from_kernel_nofault() or other called functions are instrumented
with INSTRUMENT_ALL. Without compiler barriers, the guard set/clear
can be reordered relative to the function body, making the protection
ineffective under optimization.

Add barrier() after setting the guard and before clearing it, ensuring
the compiler does not move instrumented operations outside the guarded
region.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Yunseong Kim <yunseong.kim@est.tech>
---
 kernel/kcov_dataflow.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/kcov_dataflow.c b/kernel/kcov_dataflow.c
index df7e8bf70bfa..5248293280d5 100644
--- a/kernel/kcov_dataflow.c
+++ b/kernel/kcov_dataflow.c
@@ -86,6 +86,7 @@ kcov_df_write(u64 type_marker, u64 pc, u64 meta, void *ptr,
 	if (t->kcov_df_seq & (1U << 31))
 		return;
 	t->kcov_df_seq |= (1U << 31);
+	barrier();
 
 	area = (u64 *)t->kcov_df_area;
 	if (!area)
@@ -147,6 +148,7 @@ kcov_df_write(u64 type_marker, u64 pc, u64 meta, void *ptr,
 		}
 	}
 out:
+	barrier();
 	t->kcov_df_seq &= ~(1U << 31);
 }
 

-- 
2.43.0