witness: Record the first acquired file and line for recursable locks

and the last acquired file and line to witness object.

For recursable locks, unfortunately current implementation records only
the recurse count and the last acquired file and line, but does not
restore the previous acquired file and line on unlock. Hence it is
possible to report false acquired file and line, and that may mislead
developers and make the report by users a little harder to analyse.

Since subsequent recurse locks do not affect how witness order check,
record the first acquired file and line so that the logic is much clear.

Reported by:	bz
Reviewed by:	kib (previous version), markj
See also:	https://lists.freebsd.org/archives/freebsd-current/2025-June/007944.html
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D52496

1 files changed, 4 insertions, 6 deletions

diff --git a/sys/kern/subr_witness.c b/sys/kern/subr_witness.c
index a65c3ca128d9..c937f6a82757 100644
--- a/sys/kern/subr_witness.c
+++ b/sys/kern/subr_witness.c
@@ -1515,6 +1515,10 @@ witness_lock(struct lock_object *lock, int flags, const char *file, int line)
 	else
 		lock_list = PCPU_PTR(spinlocks);
 
+	/* Update per-witness last file and line acquire. */
+	w->w_file = file;
+	w->w_line = line;
+
 	/* Check to see if we are recursing on a lock we already own. */
 	instance = find_instance(*lock_list, lock);
 	if (instance != NULL) {
@@ -1522,15 +1526,9 @@ witness_lock(struct lock_object *lock, int flags, const char *file, int line)
 		CTR4(KTR_WITNESS, "%s: pid %d recursed on %s r=%d", __func__,
 		    td->td_proc->p_pid, lock->lo_name,
 		    instance->li_flags & LI_RECURSEMASK);
-		instance->li_file = file;
-		instance->li_line = line;
 		return;
 	}
 
-	/* Update per-witness last file and line acquire. */
-	w->w_file = file;
-	w->w_line = line;
-
 	/* Find the next open lock instance in the list and fill it. */
 	lle = *lock_list;
 	if (lle == NULL || lle->ll_count == LOCK_NCHILDREN) {