Use _Noreturn (C11; GNU89) properly

A function that returns with no value is a different thing from a
function that doesn't return at all.  Those are two orthogonal
concepts, commonly confused.

pthread_create(3) expects a pointer to a start routine that has a
very precise prototype:

    void *(*start_routine)(void *);

However, other thread functions, such as kernel ones, expect:

    void (*start_routine)(void *);

Providing a different one is incorrect, and has only been working
because the ABIs happen to produce a compatible function.

We should use '_Noreturn void', since it's the natural type, and
then provide a '_Noreturn void *' wrapper for pthread functions.

For consistency, replace most cases of __NORETURN or
__attribute__((noreturn)) by _Noreturn.  _Noreturn is understood
by -std=gnu89, so it should be safe to use everywhere.

Ref: https://github.com/openzfs/zfs/pull/13110#discussion_r808450136
Ref: https://software.codidact.com/posts/285972
Reviewed-by: Brian Behlendorf <[email protected]>
Co-authored-by: Ahelenia Ziemiańska <[email protected]>
Signed-off-by: Alejandro Colomar <[email protected]>
Closes #13120 

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/libzpool/taskq.c b/lib/libzpool/taskq.c
index 8a6113091..146044bb8 100644
--- a/lib/libzpool/taskq.c
+++ b/lib/libzpool/taskq.c
@@ -211,7 +211,7 @@ taskq_wait_outstanding(taskq_t *tq, taskqid_t id)
 	taskq_wait(tq);
 }
 
-static void
+static _Noreturn void
 taskq_thread(void *arg)
 {
 	taskq_t *tq = arg;