git: 4dbe6628179d - main - devfs: make destroy_dev() a release barrier for cdevpriv destructors runs

The branch main has been updated by kib:

URL: https://cgit.FreeBSD.org/src/commit/?id=4dbe6628179d8e6bf400bfdb4bfa869bdc102a56

commit 4dbe6628179d8e6bf400bfdb4bfa869bdc102a56
Author:     Konstantin Belousov <kib@FreeBSD.org>
AuthorDate: 2025-10-23 18:37:00 +0000
Commit:     Konstantin Belousov <kib@FreeBSD.org>
CommitDate: 2025-10-24 16:09:25 +0000

    devfs: make destroy_dev() a release barrier for cdevpriv destructors runs
    
    Ensure that all destructors for cdevpriv finished running before
    destroy_dev() returns to the caller.  Otherwise, since
    devfs_destroy_cdevpriv() removes the cdevpriv data from the list, drops
    the cdevpriv_mtx, and then starts the destructor, it is possible for
    destroy_dev() to return before destructor finished in other thread.
    
    This should allow drivers to safely remove cdev instance data that might
    be referenced by cdevpriv data.
    
    Diagnosed by:   kevans
    Reviewed by:    kevans, markj
    Sponsored by:   The FreeBSD Foundation
    MFC after:      1 week
    Differential revision:  https://reviews.freebsd.org/D53303
---
 sys/fs/devfs/devfs_int.h   |  1 +
 sys/fs/devfs/devfs_vnops.c | 17 ++++++++++++++---
 sys/kern/kern_conf.c       |  3 +++
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/sys/fs/devfs/devfs_int.h b/sys/fs/devfs/devfs_int.h
index 916297425b53..9fa75c0e90ad 100644
--- a/sys/fs/devfs/devfs_int.h
+++ b/sys/fs/devfs/devfs_int.h
@@ -67,6 +67,7 @@ struct cdev_priv {
 	void			*cdp_dtr_cb_arg;
 
 	LIST_HEAD(, cdev_privdata) cdp_fdpriv;
+	u_int			cdp_fdpriv_dtrc;
 
 	struct mtx		cdp_threadlock;
 };
diff --git a/sys/fs/devfs/devfs_vnops.c b/sys/fs/devfs/devfs_vnops.c
index caadf257b8ad..323f1e0fa961 100644
--- a/sys/fs/devfs/devfs_vnops.c
+++ b/sys/fs/devfs/devfs_vnops.c
@@ -200,14 +200,25 @@ devfs_foreach_cdevpriv(struct cdev *dev, int (*cb)(void *data, void *arg),
 void
 devfs_destroy_cdevpriv(struct cdev_privdata *p)
 {
+	struct file *fp;
+	struct cdev_priv *cdp;
 
 	mtx_assert(&cdevpriv_mtx, MA_OWNED);
-	KASSERT(p->cdpd_fp->f_cdevpriv == p,
-	    ("devfs_destoy_cdevpriv %p != %p", p->cdpd_fp->f_cdevpriv, p));
-	p->cdpd_fp->f_cdevpriv = NULL;
+	fp = p->cdpd_fp;
+	KASSERT(fp->f_cdevpriv == p,
+	    ("devfs_destoy_cdevpriv %p != %p", fp->f_cdevpriv, p));
+	cdp = cdev2priv((struct cdev *)fp->f_data);
+	cdp->cdp_fdpriv_dtrc++;
+	fp->f_cdevpriv = NULL;
 	LIST_REMOVE(p, cdpd_list);
 	mtx_unlock(&cdevpriv_mtx);
 	(p->cdpd_dtr)(p->cdpd_data);
+	mtx_lock(&cdevpriv_mtx);
+	MPASS(cdp->cdp_fdpriv_dtrc >= 1);
+	cdp->cdp_fdpriv_dtrc--;
+	if (cdp->cdp_fdpriv_dtrc == 0)
+		wakeup(&cdp->cdp_fdpriv_dtrc);
+	mtx_unlock(&cdevpriv_mtx);
 	free(p, M_CDEVPDATA);
 }
 
diff --git a/sys/kern/kern_conf.c b/sys/kern/kern_conf.c
index b891ed84957a..dcf3309898ab 100644
--- a/sys/kern/kern_conf.c
+++ b/sys/kern/kern_conf.c
@@ -1163,6 +1163,9 @@ destroy_devl(struct cdev *dev)
 		devfs_destroy_cdevpriv(p);
 		mtx_lock(&cdevpriv_mtx);
 	}
+	while (cdp->cdp_fdpriv_dtrc != 0) {
+		msleep(&cdp->cdp_fdpriv_dtrc, &cdevpriv_mtx, 0, "cdfdpc", 0);
+	}
 	mtx_unlock(&cdevpriv_mtx);
 	dev_lock();