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vfs: allow v_holdcnt to transition 0->1 without the interlock

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Since r356672 ("vfs: rework vnode list management") there is nothing to do
apart from altering freevnodes count, but this much can be safely done based
on the result of atomic_fetchadd.

Reviewed by:	kib
Tested by:	pho
Differential Revision:	https://reviews.freebsd.org/D23186
This commit is contained in:
mjg 2020-01-19 17:47:04 +00:00
parent 1ba7593a69
commit 44fb757863
1 changed files with 30 additions and 55 deletions
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85
sys/kern/vfs_subr.c
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@ -2826,38 +2826,26 @@ v_decr_devcount(struct vnode *vp)
* see doomed vnodes. If inactive processing was delayed in
* vput try to do it here.
*
* Both holdcnt and usecount can be manipulated using atomics without holding
* any locks except in these cases which require the vnode interlock:
* holdcnt: 1->0 and 0->1
* usecount: 0->1
*
* usecount is permitted to transition 1->0 without the interlock because
* vnode is kept live by holdcnt.
* usecount is manipulated using atomics without holding any locks,
* except when transitioning 0->1 in which case the interlock is held.
* holdcnt is manipulated using atomics without holding any locks,
* except when transitioning 1->0 in which case the interlock is held.
*/
static enum vgetstate __always_inline
_vget_prep(struct vnode *vp, bool interlock)
enum vgetstate
vget_prep(struct vnode *vp)
{
enum vgetstate vs;
if (refcount_acquire_if_not_zero(&vp->v_usecount)) {
vs = VGET_USECOUNT;
} else {
if (interlock)
vholdl(vp);
else
vhold(vp);
vhold(vp);
vs = VGET_HOLDCNT;
}
return (vs);
}
enum vgetstate
vget_prep(struct vnode *vp)
{
return (_vget_prep(vp, false));
}
int
vget(struct vnode *vp, int flags, struct thread *td)
{
@ -2865,7 +2853,7 @@ vget(struct vnode *vp, int flags, struct thread *td)
MPASS(td == curthread);
vs = _vget_prep(vp, (flags & LK_INTERLOCK) != 0);
vs = vget_prep(vp);
return (vget_finish(vp, flags, vs));
}
@ -3234,37 +3222,21 @@ vunref(struct vnode *vp)
vputx(vp, VPUTX_VUNREF);
}
/*
* Increase the hold count and activate if this is the first reference.
*/
static void
vhold_activate(struct vnode *vp)
void
vhold(struct vnode *vp)
{
struct vdbatch *vd;
int old;
ASSERT_VI_LOCKED(vp, __func__);
VNASSERT(vp->v_holdcnt == 0, vp,
("%s: wrong hold count", __func__));
VNASSERT(vp->v_op != NULL, vp,
("%s: vnode already reclaimed.", __func__));
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
old = atomic_fetchadd_int(&vp->v_holdcnt, 1);
VNASSERT(old >= 0, vp, ("%s: wrong hold count %d", __func__, old));
if (old != 0)
return;
critical_enter();
vd = DPCPU_PTR(vd);
vd->freevnodes--;
critical_exit();
refcount_acquire(&vp->v_holdcnt);
}
void
vhold(struct vnode *vp)
{
ASSERT_VI_UNLOCKED(vp, __func__);
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
if (refcount_acquire_if_not_zero(&vp->v_holdcnt))
return;
VI_LOCK(vp);
vholdl(vp);
VI_UNLOCK(vp);
}
void
@ -3273,11 +3245,7 @@ vholdl(struct vnode *vp)
ASSERT_VI_LOCKED(vp, __func__);
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
if (vp->v_holdcnt > 0) {
refcount_acquire(&vp->v_holdcnt);
return;
}
vhold_activate(vp);
vhold(vp);
}
void
@ -3417,8 +3385,6 @@ vdrop_deactivate(struct vnode *vp)
("vdrop: returning doomed vnode"));
VNASSERT(vp->v_op != NULL, vp,
("vdrop: vnode already reclaimed."));
VNASSERT(vp->v_holdcnt == 0, vp,
("vdrop: freeing when we shouldn't"));
VNASSERT((vp->v_iflag & VI_OWEINACT) == 0, vp,
("vnode with VI_OWEINACT set"));
VNASSERT((vp->v_iflag & VI_DEFINACT) == 0, vp,
@ -3426,9 +3392,18 @@ vdrop_deactivate(struct vnode *vp)
if (vp->v_mflag & VMP_LAZYLIST) {
mp = vp->v_mount;
mtx_lock(&mp->mnt_listmtx);
vp->v_mflag &= ~VMP_LAZYLIST;
TAILQ_REMOVE(&mp->mnt_lazyvnodelist, vp, v_lazylist);
mp->mnt_lazyvnodelistsize--;
VNASSERT(vp->v_mflag & VMP_LAZYLIST, vp, ("lost VMP_LAZYLIST"));
/*
* Don't remove the vnode from the lazy list if another thread
* has increased the hold count. It may have re-enqueued the
* vnode to the lazy list and is now responsible for its
* removal.
*/
if (vp->v_holdcnt == 0) {
vp->v_mflag &= ~VMP_LAZYLIST;
TAILQ_REMOVE(&mp->mnt_lazyvnodelist, vp, v_lazylist);
mp->mnt_lazyvnodelistsize--;
}
mtx_unlock(&mp->mnt_listmtx);
}
vdbatch_enqueue(vp);