Date: Sat, 15 Nov 2025 05:18:57 +0100 From: Mateusz Guzik <mjguzik@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: Mark Johnston <markj@freebsd.org>, src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 99cb3dca4773 - main - vnode: Rework vput() to avoid holding the vnode lock after decrementing Message-ID: <CAGudoHHoS_ChbAivy967=Zkcxw9=4EUx7o2fq2Cck_0Hj8A1Ew@mail.gmail.com> In-Reply-To: <854dff8a-555a-4921-ae64-4c445c7a03f7@FreeBSD.org> References: <202511111459.5ABEx8Co056214@gitrepo.freebsd.org> <854dff8a-555a-4921-ae64-4c445c7a03f7@FreeBSD.org>
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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: 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);home | help
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