[PATCH bpf-next 3/6] tracing: Add the comment for allowing one single recursion in process context

[Yafang Shao <laoar.shao@gmail.com>]

After TRACE_CTX_TRANSITION is applied, it will allow one single recursion
in the process context. Below is an example,

SEC("fentry/htab_map_delete_elem")
int BPF_PROG(on_delete, struct bpf_map *map)
{
    pass2++;
    bpf_map_delete_elem(&hash2, &key);
    return 0;
}

In the above test case, the recursion will be detected at the second
bpf_map_delete_elem() call in this prog. Illustrated as follows,

SEC("fentry/htab_map_delete_elem")
    pass2++;   <<<< Turn out to be 1 after this operation.
    bpf_map_delete_elem(&hash2, &key);
         SEC("fentry/htab_map_delete_elem")       <<<< no recursion
            pass2++; <<<< Turn out to be 2 after this operation.
            bpf_map_delete_elem(&hash2, &key);
                SEC("fentry/htab_map_delete_elem") <<<< RECURSION

We'd better explain this behavior explicitly.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 include/linux/trace_recursion.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
index 80de2ee..445a055 100644
--- a/include/linux/trace_recursion.h
+++ b/include/linux/trace_recursion.h
@@ -168,6 +168,8 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
 		 * will think a recursion occurred, and the event will be dropped.
 		 * Let a single instance happen via the TRANSITION_BIT to
 		 * not drop those events.
+		 * After this rule is applied, one single recursion is allowed in
+		 * the process context.
 		 */
 		bit = TRACE_CTX_TRANSITION + start;
 		if (val & (1 << bit)) {
-- 
1.8.3.1