Re: git: 99cb3dca4773 - main - vnode: Rework vput() to avoid holding the vnode lock after decrementing
Date: Sat, 15 Nov 2025 18:08:31 UTC
On Sat, Nov 15, 2025 at 05:18:57AM +0100, Mateusz Guzik wrote:
> On Sat, Nov 15, 2025 at 3:12 AM John Baldwin <jhb@freebsd.org> wrote:
> >
> > On 11/11/25 09:59, Mark Johnston wrote:
> > > The branch main has been updated by markj:
> > >
> > > URL: https://cgit.FreeBSD.org/src/commit/?id=99cb3dca4773fe4a16c500f9cb55fcd62cd8d7f3
> > >
> > > commit 99cb3dca4773fe4a16c500f9cb55fcd62cd8d7f3
> > > Author: Mark Johnston <markj@FreeBSD.org>
> > > AuthorDate: 2025-11-11 14:47:06 +0000
> > > Commit: Mark Johnston <markj@FreeBSD.org>
> > > CommitDate: 2025-11-11 14:58:59 +0000
> > >
> > > vnode: Rework vput() to avoid holding the vnode lock after decrementing
> > >
> > > It is not safe to modify the vnode structure after releasing one's
> > > reference. Modify vput() to avoid this. Use refcount_release_if_last()
> > > to opportunistically call vput_final() with the vnode lock held since we
> > > need the vnode lock in order to deactivate the vnode, and it's silly to
> > > drop the vnode lock and immediately reacquire it in this common case.
> > >
> > > Note that vunref() has a similar flaw. D52628 aims to fix the problem
> > > more holistically, but this change fixes observable panics in the
> > > meantime.
> > >
> > > Reported by: syzbot+6676b3ff282d590b0fb3@syzkaller.appspotmail.com
> > > Reported by: syzbot+38e26cf6f959e886f110@syzkaller.appspotmail.com
> > > Reviewed by: kib
> > > MFC after: 3 days
> > > Differential Revision: https://reviews.freebsd.org/D52608
> > > ---
> > > sys/kern/vfs_subr.c | 7 ++++---
> > > 1 file changed, 4 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
> > > index 58975f7ac932..9cf983f6f89d 100644
> > > --- a/sys/kern/vfs_subr.c
> > > +++ b/sys/kern/vfs_subr.c
> > > @@ -3713,11 +3713,12 @@ vput(struct vnode *vp)
> > >
> > > ASSERT_VOP_LOCKED(vp, __func__);
> > > ASSERT_VI_UNLOCKED(vp, __func__);
> > > - if (!refcount_release(&vp->v_usecount)) {
> > > - VOP_UNLOCK(vp);
> > > + if (refcount_release_if_last(&vp->v_usecount)) {
> > > + vput_final(vp, VPUT);
> > > return;
> > > }
> > > - vput_final(vp, VPUT);
> > > + VOP_UNLOCK(vp);
> > > + vrele(vp);
> > > }
> >
> > This commit results in reliable hangs for me when I do a 'make -j 16 buildkernel'
> > in a VM where the source directory is mounted over NFS. Even if there is nothing
> > to build so the make is just traversing all the directories. The hung process is
> > stuck on a vnode lock.
> >
> > Some of the stacks from procstat:
> >
> > PID TID COMM TDNAME KSTACK
> > 1858 100343 make - mi_switch+0x172 sleepq_switch+0x109 sleeplk+0x110 lockmgr_slock_hard+0x3c3 VOP_LOCK1_APV+0x32 nfs_lock+0x2c vop_sigdefer+0x30 VOP_LOCK1_APV+0x32 _vn_lock+0x53 vget_finish+0xaf cache_lookup+0x2be nfs_lookup+0x4e1 vop_sigdefer+0x30 VOP_LOOKUP_APV+0x57 vfs_lookup+0x5aa namei+0x35d vn_open_cred+0x592 openatfp+0x2b7
> > root@head:~ # procstat -kk 1860
> > 1860 100360 make - mi_switch+0x172 sleepq_switch+0x109 sleeplk+0x110 lockmgr_slock_hard+0x3c3 VOP_LOCK1_APV+0x32 nfs_lock+0x2c vop_sigdefer+0x30 VOP_LOCK1_APV+0x32 _vn_lock+0x53 vget_finish+0xaf cache_lookup+0x2be nfs_lookup+0x4e1 vop_sigdefer+0x30 VOP_LOOKUP_APV+0x57 vfs_lookup+0x5aa namei+0x35d vn_open_cred+0x592 openatfp+0x2b7
> > root@head:~ # procstat -kk 1855
> > 1855 100314 make - mi_switch+0x172 sleepq_switch+0x109 sleeplk+0x110 lockmgr_xlock_hard+0x3d0 VOP_LOCK1_APV+0x32 nfs_lock+0x2c vop_sigdefer+0x30 VOP_LOCK1_APV+0x32 _vn_lock+0x53 vput_final+0x269 vput+0xab vfs_lookup+0xa7a namei+0x35d vn_open_cred+0x592 openatfp+0x2b7 sys_open+0x28 amd64_syscall+0x169 fast_syscall_common+0xf8
> >
> > This last one is kind of interesting as it calls vput_final?
> >
> > (kgdb) p vp
> > $2 = (struct vnode *) 0xfffff8016bb6c898
> > (kgdb) p vp->v_lock
> > $3 = {lock_object = {lo_name = 0xffffffff81467118 <nfs_vnode_tag> "nfs",
> > lo_flags = 117112832, lo_data = 0, lo_witness = 0xfffff8043fd83900},
> > lk_lock = 39, lk_exslpfail = 0, lk_pri = 44, lk_timo = 6}
> > (kgdb) p/x vp->v_lock->lk_lock
> > $4 = 0x27
> >
> > So there is one shared lock, and both shared and exclusive waiters. Seems like
> > a vnode lock has been leaked? Or rather, the vput_final() in this case is
> > called from vrele(), so the vput() for 1855 has hit the new path. I wonder if
> > when it dropped its lock via VOP_UNLOCK() it is no longer eligible to re-lock
> > it as it needs some other sharer to also unlock before it can re-acquire the
> > lock in vput_final? LK_UPGRADE attempts don't block if there are shared waiters
> > but only one 1 shared lock held, but if you do LK_UNLOCK and then LK_XLOCK that
> > will indeed honor the other waiters and not acquire the lock, so a vput that
> > held the last shared lock would have "worked" to ugprade before even with
> > other waiters, but now it can drop the lock and then get stuck. I guess the
> > question though in this case is that some other thread must have snuck in
> > and acquired a shared lock between the VOP_UNLOCK and the vrele() and is
> > blocked while holding it.
> >
> > Aha, so most of the other makes are stuck in VOP_LOOKUP on this same vnode
> > (which I believe is "/usr/src/sys/dev" as in 1855 the vput() is inside of
> > namei() and this is the state of ndp:
> >
> > (kgdb) p *ndp
> > $9 = {
> > ni_dirp = 0x481afdaa2d7d <error: Cannot access memory at address 0x481afdaa2d7d>, ni_segflg = UIO_USERSPACE, ni_rightsneeded = 0xfffffe00d83d2d90,
> > ni_startdir = 0x0, ni_rootdir = 0xfffff80005e461b8, ni_topdir = 0x0,
> > ni_dirfd = -100, ni_lcf = 0, ni_filecaps = {fc_rights = {cr_rights = {0,
> > 0}}, fc_ioctls = 0x0, fc_nioctls = -1, fc_fcntls = 0},
> > ni_vp = 0xfffff801bc007000, ni_dvp = 0xfffff8016bb6c898, ni_resflags = 1,
> > ni_debugflags = 3, ni_loopcnt = 0, ni_pathlen = 1,
> > ni_next = 0xfffff8000a0b0817 "", ni_cnd = {cn_flags = 8936259908,
> > cn_cred = 0xfffff80005e53c00, cn_nameiop = LOOKUP, cn_lkflags = 524288,
> > cn_pnbuf = 0xfffff8000a0b0800 "/usr/src/sys/dev/acpica",
> > cn_nameptr = 0xfffff8000a0b0811 "acpica", cn_namelen = 6},
> > ni_cap_tracker = {tqh_first = 0x0, tqh_last = 0xfffffe00d83d2d48},
> > ni_rbeneath_dpp = 0x0, ni_nctrack_mnt = 0x0, ni_dvp_seqc = 0, ni_vp_seqc = 0}
> >
> > But pid 1844 is a make blocked on another vnode:
> >
> > (kgdb) p vp
> > $24 = (struct vnode *) 0xfffff801bc007000
> >
> > And that one appears to be the "acpica" child directory:
> >
> > (kgdb) p *vp->v_cache_dst.tqh_first
> > $27 = {nc_src = {le_next = 0xfffff801bc01b9b8, le_prev = 0xfffff801bc01b898},
> > nc_dst = {tqe_next = 0x0, tqe_prev = 0xfffff801bc007058}, nc_hash = {
> > csle_next = 0x0}, nc_dvp = 0xfffff8016bb6c898, n_un = {
> > nu_vp = 0xfffff801bc007000, nu_neg = {neg_flag = 0 '\000',
> > neg_hit = 112 'p'}}, nc_flag = 12 '\f', nc_nlen = 6 '\006',
> > nc_name = 0xfffff801bc01b962 "acpica"}
> >
> > And note it's nc_dvp is the vnode 1855 is trying to vput and all the other
> > makes are trying to VOP_LOCK in nfs_lookup().
> >
> > So my guess is that 1844 was able to VOP_LOCK() "/usr/src/sys/dev" in the
> > window between VOP_UNLOCK() and the vput_final() invoked from vrele().
> >
> > Humm, except that the vnode in question shouldn't be in vput_final at all??
> >
> > Back to 1855 (proc doing vput_final):
> >
> > (kgdb)
> > #12 0xffffffff80c9bc89 in vput_final (vp=0xfffff8016bb6c898,
> > func=func@entry=VRELE) at /usr/src/sys/kern/vfs_subr.c:3628
> > 3628 error = vn_lock(vp, LK_EXCLUSIVE | LK_INTERLOCK);
> > (kgdb) p vp
> > $6 = (struct vnode *) 0xfffff8016bb6c898
> > (kgdb) p vp->v_usecount
> > $7 = 14
> > (kgdb) p vp->v_holdcnt
> > $8 = 7
> >
> > Or did the other make's manage to acquire new use counts while 1855 was blocked
> > on the lock in vput_final?
> >
> > Anyway, it seems 14 make's are blocked on this vnode (/usr/src/sys/dev). 1844
> > is blocked on the "acpica" vnode, and another make process (1856) is blocked on
> > "acpica", but via getdirentries() instead of namei():
> >
> > #4 0xffffffff80b62ec0 in sleeplk (lk=lk@entry=0xfffff801bc007070,
> > flags=flags@entry=2098176, ilk=ilk@entry=0xfffff801bc007098,
> > wmesg=wmesg@entry=0xffffffff81467118 <nfs_vnode_tag> "nfs",
> > pri=<optimized out>, pri@entry=44, timo=timo@entry=6, queue=1)
> > at /usr/src/sys/kern/kern_lock.c:302
> > #5 0xffffffff80b60fc3 in lockmgr_slock_hard (lk=0xfffff801bc007070,
> > flags=2098176, ilk=0xfffff801bc007098, file=<optimized out>, line=4333,
> > lwa=0x0) at /usr/src/sys/kern/kern_lock.c:693
> > #6 0xffffffff811b97a2 in VOP_LOCK1_APV (
> > vop=0xffffffff81aeeb38 <default_vnodeops>, a=a@entry=0xfffffe00d8445c88)
> > at vnode_if.c:2103
> > #7 0xffffffff80a6050c in nfs_lock (ap=ap@entry=0xfffffe00d8445c88)
> > at /usr/src/sys/fs/nfsclient/nfs_clvnops.c:344
> > #8 0xffffffff80c83d90 in vop_sigdefer (vop=<optimized out>,
> > a=0xfffffe00d8445c88) at /usr/src/sys/kern/vfs_default.c:1502
> > #9 0xffffffff811b97a2 in VOP_LOCK1_APV (
> > vop=0xffffffff81aada28 <newnfs_vnodeops>, a=a@entry=0xfffffe00d8445c88)
> > at vnode_if.c:2103
> > #10 0xffffffff80cb6b43 in VOP_LOCK1 (vp=0xfffff801bc007000, flags=2098176,
> > file=0xffffffff81359deb "/usr/src/sys/kern/vfs_syscalls.c", line=4333)
> > at ./vnode_if.h:1316
> > #11 _vn_lock (vp=vp@entry=0xfffff801bc007000, flags=flags@entry=2098176,
> > file=<unavailable>, line=<unavailable>, line@entry=4333)
> > at /usr/src/sys/kern/vfs_vnops.c:1970
> > #12 0xffffffff80cb2d94 in kern_getdirentries (td=0xfffff80006cb5000,
> > fd=<optimized out>,
> > buf=0x2088dba27000 <error: Cannot access memory at address 0x2088dba27000>, count=4096, basep=basep@entry=0xfffffe00d8445df0, residp=residp@entry=0x0,
> > bufseg=UIO_USERSPACE) at /usr/src/sys/kern/vfs_syscalls.c:4333
> > #13 0xffffffff80cb32a9 in sys_getdirentries (td=<unavailable>,
> > uap=0xfffff80006cb5428) at /usr/src/sys/kern/vfs_syscalls.c:4287
> >
> > The "acpica" lock is held by a writer at least:
> >
> > (kgdb) p vp
> > $9 = (struct vnode *) 0xfffff801bc007000
> > (kgdb) p vp->v_lock
> > $10 = {lock_object = {lo_name = 0xffffffff81467118 <nfs_vnode_tag> "nfs",
> > lo_flags = 117112832, lo_data = 0, lo_witness = 0xfffff8043fd83900},
> > lk_lock = 18446735277917198210, lk_exslpfail = 0, lk_pri = 44, lk_timo = 6}
> > (kgdb) p/x vp->v_lock.lk_lock
> > $14 = 0xfffff80011ebd782
> > (kgdb) set $td = (struct thread *)0xfffff80011ebd780
> > (kgdb) p $td->td_tid
> > $15 = 100314
> > (kgdb) p $td->td_proc->p_comm
> > $16 = "make", '\000' <repeats 15 times>
> > (kgdb) p $td->td_proc->p_pid
> > $17 = 1855
> >
> > Oh dear, so that's the deadlock.
> >
> > 1855 needs to get an exclusive lock on "/usr/src/sys/dev/" to finish its vput()
> > _while_ it holds an exclusive lock on "/usr/src/sys/dev/acpica". And the other
> > shared lock on "/usr/src/sys/dev" is held by 1844 who is blocked trying to share
> > lock "/usr/src/sys/dev/acpica" during lookup().
> >
> > So by doing the VOP_UNLOCK() and then reacquiring the lock via vrele(), you've
> > introduced a vnode LOR as the vput() of the parent directory can now try to
> > re-lock the vnode of the parent directory while we hold a vnode on the
> > child vnode.
> >
>
> This should be easy to fix though.
>
> While the routine must not vrele, the VPUT should be patched to
> trylock (aka LK_NOWAIT) and EAGAIN if that fails, which in turn will
> punt the vnode to a workqueue.
>
> However, one has to wonder if failing to lock here will be frequent
> enough to constitute a problem. There is a counter for these
> (deferred_inactive).
>
> The following was only boot-tested:
This patch also causes vput() to unlock and immediately re-lock the
vnode when deactivating, which is a common case.
For 15.0 at least, I'd rather work around the bug more directly:
https://reviews.freebsd.org/D53774
> diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
> index 58975f7ac932..5cde78162e6a 100644
> --- a/sys/kern/vfs_subr.c
> +++ b/sys/kern/vfs_subr.c
> @@ -3640,11 +3640,8 @@ vput_final(struct vnode *vp, enum vput_op func)
> break;
> case VPUT:
> want_unlock = true;
> - if (VOP_ISLOCKED(vp) != LK_EXCLUSIVE) {
> - error = VOP_LOCK(vp, LK_UPGRADE | LK_INTERLOCK |
> - LK_NOWAIT);
> - VI_LOCK(vp);
> - }
> + error = VOP_LOCK(vp, LK_EXCLUSIVE | LK_INTERLOCK | LK_NOWAIT);
> + VI_LOCK(vp);
> break;
> case VUNREF:
> if (VOP_ISLOCKED(vp) != LK_EXCLUSIVE) {
> @@ -3672,12 +3669,9 @@ vput_final(struct vnode *vp, enum vput_op func)
> }
> if (func == VUNREF)
> vp->v_vflag &= ~VV_UNREF;
> - vdropl(vp);
> - return;
> out:
> - if (func == VPUT)
> - VOP_UNLOCK(vp);
> vdropl(vp);
> + return;
> }
>
> /*
> @@ -3713,8 +3707,8 @@ vput(struct vnode *vp)
>
> ASSERT_VOP_LOCKED(vp, __func__);
> ASSERT_VI_UNLOCKED(vp, __func__);
> + VOP_UNLOCK(vp);
> if (!refcount_release(&vp->v_usecount)) {
> - VOP_UNLOCK(vp);
> return;
> }
> vput_final(vp, VPUT);