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Add INVARIANTS-only fences around lockless vnode refcount updates.

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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
This commit is contained in:
Mark Johnston 2018-08-17 15:41:01 +00:00
parent c85b199444
commit 3ccbdc8254
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=337974
1 changed files with 20 additions and 0 deletions
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20
sys/kern/vfs_subr.c
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@ -117,6 +117,22 @@ static void vfs_knl_assert_unlocked(void *arg);
static void vnlru_return_batches(struct vfsops *mnt_op);
static void destroy_vpollinfo(struct vpollinfo *vi);
/*
* These fences are intended for cases where some synchronization is
* needed between access of v_iflags and lockless vnode refcount (v_holdcnt
* and v_usecount) updates. Access to v_iflags is generally synchronized
* by the interlock, but we have some internal assertions that check vnode
* flags * without acquiring the lock. Thus, these fences are INVARIANTS-only
* for now.
*/
#ifdef INVARIANTS
#define VNODE_REFCOUNT_FENCE_ACQ() atomic_thread_fence_acq()
#define VNODE_REFCOUNT_FENCE_REL() atomic_thread_fence_rel()
#else
#define VNODE_REFCOUNT_FENCE_ACQ()
#define VNODE_REFCOUNT_FENCE_REL()
#endif
/*
* Number of vnodes in existence. Increased whenever getnewvnode()
* allocates a new vnode, decreased in vdropl() for VI_DOOMED vnode.
@ -1018,6 +1034,7 @@ vnlru_free_locked(int count, struct vfsops *mnt_op)
*/
freevnodes--;
vp->v_iflag &= ~VI_FREE;
VNODE_REFCOUNT_FENCE_REL();
refcount_acquire(&vp->v_holdcnt);
mtx_unlock(&vnode_free_list_mtx);
@ -2495,6 +2512,7 @@ v_incr_usecount(struct vnode *vp)
if (vp->v_type != VCHR &&
refcount_acquire_if_not_zero(&vp->v_usecount)) {
VNODE_REFCOUNT_FENCE_ACQ();
VNASSERT((vp->v_iflag & VI_OWEINACT) == 0, vp,
("vnode with usecount and VI_OWEINACT set"));
} else {
@ -2593,6 +2611,7 @@ vget(struct vnode *vp, int flags, struct thread *td)
} else {
oweinact = 1;
vp->v_iflag &= ~VI_OWEINACT;
VNODE_REFCOUNT_FENCE_REL();
}
refcount_acquire(&vp->v_usecount);
v_incr_devcount(vp);
@ -2807,6 +2826,7 @@ _vhold(struct vnode *vp, bool locked)
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
if (!locked) {
if (refcount_acquire_if_not_zero(&vp->v_holdcnt)) {
VNODE_REFCOUNT_FENCE_ACQ();
VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
("_vhold: vnode with holdcnt is free"));
return;