Add INVARIANTS-only fences around lockless vnode refcount updates.
Some internal KASSERTs access the v_iflag field without the vnode interlock held after such a refcount update. The fences are needed for the assertions to be correct in the face of store reordering. Reported and tested by: jhibbits Reviewed by: kib, mjg MFC after: 2 weeks Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D16756
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svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=337974
@ -117,6 +117,22 @@ static void vfs_knl_assert_unlocked(void *arg);
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static void vnlru_return_batches(struct vfsops *mnt_op);
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static void destroy_vpollinfo(struct vpollinfo *vi);
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/*
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* These fences are intended for cases where some synchronization is
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* needed between access of v_iflags and lockless vnode refcount (v_holdcnt
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* and v_usecount) updates. Access to v_iflags is generally synchronized
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* by the interlock, but we have some internal assertions that check vnode
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* flags * without acquiring the lock. Thus, these fences are INVARIANTS-only
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* for now.
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*/
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#ifdef INVARIANTS
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#define VNODE_REFCOUNT_FENCE_ACQ() atomic_thread_fence_acq()
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#define VNODE_REFCOUNT_FENCE_REL() atomic_thread_fence_rel()
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#else
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#define VNODE_REFCOUNT_FENCE_ACQ()
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#define VNODE_REFCOUNT_FENCE_REL()
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#endif
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/*
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* Number of vnodes in existence. Increased whenever getnewvnode()
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* allocates a new vnode, decreased in vdropl() for VI_DOOMED vnode.
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@ -1018,6 +1034,7 @@ vnlru_free_locked(int count, struct vfsops *mnt_op)
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*/
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freevnodes--;
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vp->v_iflag &= ~VI_FREE;
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VNODE_REFCOUNT_FENCE_REL();
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refcount_acquire(&vp->v_holdcnt);
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mtx_unlock(&vnode_free_list_mtx);
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@ -2495,6 +2512,7 @@ v_incr_usecount(struct vnode *vp)
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if (vp->v_type != VCHR &&
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refcount_acquire_if_not_zero(&vp->v_usecount)) {
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VNODE_REFCOUNT_FENCE_ACQ();
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VNASSERT((vp->v_iflag & VI_OWEINACT) == 0, vp,
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("vnode with usecount and VI_OWEINACT set"));
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} else {
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@ -2593,6 +2611,7 @@ vget(struct vnode *vp, int flags, struct thread *td)
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} else {
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oweinact = 1;
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vp->v_iflag &= ~VI_OWEINACT;
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VNODE_REFCOUNT_FENCE_REL();
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}
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refcount_acquire(&vp->v_usecount);
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v_incr_devcount(vp);
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@ -2807,6 +2826,7 @@ _vhold(struct vnode *vp, bool locked)
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CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
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if (!locked) {
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if (refcount_acquire_if_not_zero(&vp->v_holdcnt)) {
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VNODE_REFCOUNT_FENCE_ACQ();
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VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
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("_vhold: vnode with holdcnt is free"));
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return;
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