git: 4869c1571f34 - stable/13 - vfs: lockless writecount adjustment in set/unset text

[Mateusz Guzik]

The branch stable/13 has been updated by mjg:

URL: https://cgit.FreeBSD.org/src/commit/?id=4869c1571f34b603279a9addd18181aefc4f17e9

commit 4869c1571f34b603279a9addd18181aefc4f17e9
Author:     Mateusz Guzik <mjg at FreeBSD.org>
AuthorDate: 2021-05-07 14:04:27 +0000
Commit:     Mateusz Guzik <mjg at FreeBSD.org>
CommitDate: 2021-05-22 18:22:03 +0000

    vfs: lockless writecount adjustment in set/unset text
    
    ... for cases where this is not the first/last exec.
    
    (cherry picked from commit b5fb9ae6872c499f1a02bec41f48b163a73a2aaa)
---
 sys/kern/vfs_default.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/sys/kern/vfs_default.c b/sys/kern/vfs_default.c
index 4b9b1b43f1ce..eb584feb6c41 100644
--- a/sys/kern/vfs_default.c
+++ b/sys/kern/vfs_default.c
@@ -1177,9 +1177,23 @@ vop_stdset_text(struct vop_set_text_args *ap)
 {
 	struct vnode *vp;
 	struct mount *mp;
-	int error;
+	int error, n;
 
 	vp = ap->a_vp;
+
+	/*
+	 * Avoid the interlock if execs are already present.
+	 */
+	n = atomic_load_int(&vp->v_writecount);
+	for (;;) {
+		if (n > -1) {
+			break;
+		}
+		if (atomic_fcmpset_int(&vp->v_writecount, &n, n - 1)) {
+			return (0);
+		}
+	}
+
 	VI_LOCK(vp);
 	if (vp->v_writecount > 0) {
 		error = ETXTBSY;
@@ -1207,10 +1221,24 @@ static int
 vop_stdunset_text(struct vop_unset_text_args *ap)
 {
 	struct vnode *vp;
-	int error;
+	int error, n;
 	bool last;
 
 	vp = ap->a_vp;
+
+	/*
+	 * Avoid the interlock if this is not the last exec.
+	 */
+	n = atomic_load_int(&vp->v_writecount);
+	for (;;) {
+		if (n >= -1) {
+			break;
+		}
+		if (atomic_fcmpset_int(&vp->v_writecount, &n, n + 1)) {
+			return (0);
+		}
+	}
+
 	last = false;
 	VI_LOCK(vp);
 	if (vp->v_writecount < 0) {