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vfs: protect vnodes with smr

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vget_prep_smr and vhold_smr can be used to ref a vnode while within vfs_smr
section, allowing consumers to get away without locking.

See vhold_smr and vdropl for comments explaining caveats.

Reviewed by:	kib
Testec by:	pho
Differential Revision:	https://reviews.freebsd.org/D23913
This commit is contained in:
Mateusz Guzik 2020-07-01 05:56:29 +00:00
parent 263a104f43
commit f8022be3e6
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=362827
2 changed files with 108 additions and 12 deletions
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103
sys/kern/vfs_subr.c
View File

@ -76,6 +76,7 @@ __FBSDID("$FreeBSD$");
#include <sys/rwlock.h>
#include <sys/sched.h>
#include <sys/sleepqueue.h>
#include <sys/smr.h>
#include <sys/smp.h>
#include <sys/stat.h>
#include <sys/sysctl.h>
@ -238,6 +239,8 @@ static uma_zone_t buf_trie_zone;
static uma_zone_t vnode_zone;
static uma_zone_t vnodepoll_zone;
__read_frequently smr_t vfs_smr;
/*
* The workitem queue.
*
@ -661,7 +664,8 @@ vntblinit(void *dummy __unused)
vnode_list_reclaim_marker = vn_alloc_marker(NULL);
TAILQ_INSERT_HEAD(&vnode_list, vnode_list_reclaim_marker, v_vnodelist);
vnode_zone = uma_zcreate("VNODE", sizeof (struct vnode), NULL, NULL,
vnode_init, vnode_fini, UMA_ALIGN_PTR, 0);
vnode_init, vnode_fini, UMA_ALIGN_PTR, UMA_ZONE_SMR);
vfs_smr = uma_zone_get_smr(vnode_zone);
vnodepoll_zone = uma_zcreate("VNODEPOLL", sizeof (struct vpollinfo),
NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
/*
@ -1603,7 +1607,7 @@ vn_alloc_hard(struct mount *mp)
if (vnlru_under(rnumvnodes, vlowat))
vnlru_kick();
mtx_unlock(&vnode_list_mtx);
return (uma_zalloc(vnode_zone, M_WAITOK));
return (uma_zalloc_smr(vnode_zone, M_WAITOK));
}
static struct vnode *
@ -1619,7 +1623,7 @@ vn_alloc(struct mount *mp)
return (vn_alloc_hard(mp));
}
return (uma_zalloc(vnode_zone, M_WAITOK));
return (uma_zalloc_smr(vnode_zone, M_WAITOK));
}
static void
@ -1627,7 +1631,7 @@ vn_free(struct vnode *vp)
{
atomic_subtract_long(&numvnodes, 1);
uma_zfree(vnode_zone, vp);
uma_zfree_smr(vnode_zone, vp);
}
/*
@ -1758,7 +1762,7 @@ freevnode(struct vnode *vp)
CTR2(KTR_VFS, "%s: destroying the vnode %p", __func__, vp);
bo = &vp->v_bufobj;
VNASSERT(vp->v_data == NULL, vp, ("cleaned vnode isn't"));
VNASSERT(vp->v_holdcnt == 0, vp, ("Non-zero hold count"));
VNPASS(vp->v_holdcnt == VHOLD_NO_SMR, vp);
VNASSERT(vp->v_usecount == 0, vp, ("Non-zero use count"));
VNASSERT(vp->v_writecount == 0, vp, ("Non-zero write count"));
VNASSERT(bo->bo_numoutput == 0, vp, ("Clean vnode has pending I/O's"));
@ -2848,7 +2852,29 @@ v_decr_devcount(struct vnode *vp)
*
* holdcnt can be manipulated using atomics without holding any locks,
* except when transitioning 1<->0, in which case the interlock is held.
*
* Consumers which don't guarantee liveness of the vnode can use SMR to
* try to get a reference. Note this operation can fail since the vnode
* may be awaiting getting freed by the time they get to it.
*/
enum vgetstate
vget_prep_smr(struct vnode *vp)
{
enum vgetstate vs;
VFS_SMR_ASSERT_ENTERED();
if (refcount_acquire_if_not_zero(&vp->v_usecount)) {
vs = VGET_USECOUNT;
} else {
if (vhold_smr(vp))
vs = VGET_HOLDCNT;
else
vs = VGET_NONE;
}
return (vs);
}
enum vgetstate
vget_prep(struct vnode *vp)
{
@ -2919,6 +2945,7 @@ vget_finish(struct vnode *vp, int flags, enum vgetstate vs)
ASSERT_VI_LOCKED(vp, __func__);
else
ASSERT_VI_UNLOCKED(vp, __func__);
VNPASS(vs == VGET_HOLDCNT || vs == VGET_USECOUNT, vp);
VNPASS(vp->v_holdcnt > 0, vp);
VNPASS(vs == VGET_HOLDCNT || vp->v_usecount > 0, vp);
@ -3380,7 +3407,8 @@ vhold(struct vnode *vp)
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));
VNASSERT(old >= 0 && (old & VHOLD_ALL_FLAGS) == 0, vp,
("%s: wrong hold count %d", __func__, old));
if (old != 0)
return;
critical_enter();
@ -3405,12 +3433,40 @@ vholdnz(struct vnode *vp)
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
#ifdef INVARIANTS
int old = atomic_fetchadd_int(&vp->v_holdcnt, 1);
VNASSERT(old > 0, vp, ("%s: wrong hold count %d", __func__, old));
VNASSERT(old > 0 && (old & VHOLD_ALL_FLAGS) == 0, vp,
("%s: wrong hold count %d", __func__, old));
#else
atomic_add_int(&vp->v_holdcnt, 1);
#endif
}
/*
* Grab a hold count as long as the vnode is not getting freed.
*
* Only use this routine if vfs smr is the only protection you have against
* freeing the vnode.
*/
bool
vhold_smr(struct vnode *vp)
{
int count;
VFS_SMR_ASSERT_ENTERED();
count = atomic_load_int(&vp->v_holdcnt);
for (;;) {
if (count & VHOLD_NO_SMR) {
VNASSERT((count & ~VHOLD_NO_SMR) == 0, vp,
("non-zero hold count with flags %d\n", count));
return (false);
}
VNASSERT(count >= 0, vp, ("invalid hold count %d\n", count));
if (atomic_fcmpset_int(&vp->v_holdcnt, &count, count + 1))
return (true);
}
}
static void __noinline
vdbatch_process(struct vdbatch *vd)
{
@ -3581,11 +3637,25 @@ vdropl(struct vnode *vp)
VI_UNLOCK(vp);
return;
}
if (VN_IS_DOOMED(vp)) {
freevnode(vp);
if (!VN_IS_DOOMED(vp)) {
vdrop_deactivate(vp);
return;
}
vdrop_deactivate(vp);
/*
* We may be racing against vhold_smr.
*
* If they win we can just pretend we never got this far, they will
* vdrop later.
*/
if (!atomic_cmpset_int(&vp->v_holdcnt, 0, VHOLD_NO_SMR)) {
/*
* We lost the aforementioned race. Note that any subsequent
* access is invalid as they might have managed to vdropl on
* their own.
*/
return;
}
freevnode(vp);
}
/*
@ -4041,20 +4111,25 @@ static const char * const typename[] =
{"VNON", "VREG", "VDIR", "VBLK", "VCHR", "VLNK", "VSOCK", "VFIFO", "VBAD",
"VMARKER"};
_Static_assert((VHOLD_ALL_FLAGS & ~VHOLD_NO_SMR) == 0,
"new hold count flag not added to vn_printf");
void
vn_printf(struct vnode *vp, const char *fmt, ...)
{
va_list ap;
char buf[256], buf2[16];
u_long flags;
u_int holdcnt;
va_start(ap, fmt);
vprintf(fmt, ap);
va_end(ap);
printf("%p: ", (void *)vp);
printf("type %s\n", typename[vp->v_type]);
holdcnt = atomic_load_int(&vp->v_holdcnt);
printf(" usecount %d, writecount %d, refcount %d",
vp->v_usecount, vp->v_writecount, vp->v_holdcnt);
vp->v_usecount, vp->v_writecount, holdcnt & ~VHOLD_ALL_FLAGS);
switch (vp->v_type) {
case VDIR:
printf(" mountedhere %p\n", vp->v_mountedhere);
@ -4072,6 +4147,12 @@ vn_printf(struct vnode *vp, const char *fmt, ...)
printf("\n");
break;
}
buf[0] = '\0';
buf[1] = '\0';
if (holdcnt & VHOLD_NO_SMR)
strlcat(buf, "|VHOLD_NO_SMR", sizeof(buf));
printf(" hold count flags (%s)\n", buf + 1);
buf[0] = '\0';
buf[1] = '\0';
if (vp->v_irflag & VIRF_DOOMED)

17
sys/sys/vnode.h
View File

@ -58,7 +58,7 @@
enum vtype { VNON, VREG, VDIR, VBLK, VCHR, VLNK, VSOCK, VFIFO, VBAD,
VMARKER };
enum vgetstate { VGET_HOLDCNT, VGET_USECOUNT };
enum vgetstate { VGET_NONE, VGET_HOLDCNT, VGET_USECOUNT };
/*
* Each underlying filesystem allocates its own private area and hangs
* it from v_data. If non-null, this area is freed in getnewvnode().
@ -236,6 +236,9 @@ struct xvnode {
* VIRF_DOOMED is doubly protected by the interlock and vnode lock. Both
* are required for writing but the status may be checked with either.
*/
#define VHOLD_NO_SMR (1<<29) /* Disable vhold_smr */
#define VHOLD_ALL_FLAGS (VHOLD_NO_SMR)
#define VIRF_DOOMED 0x0001 /* This vnode is being recycled */
#define VI_TEXT_REF 0x0001 /* Text ref grabbed use ref */
@ -657,12 +660,14 @@ void vdrop(struct vnode *);
void vdropl(struct vnode *);
int vflush(struct mount *mp, int rootrefs, int flags, struct thread *td);
int vget(struct vnode *vp, int flags, struct thread *td);
enum vgetstate vget_prep_smr(struct vnode *vp);
enum vgetstate vget_prep(struct vnode *vp);
int vget_finish(struct vnode *vp, int flags, enum vgetstate vs);
void vgone(struct vnode *vp);
void vhold(struct vnode *);
void vholdl(struct vnode *);
void vholdnz(struct vnode *);
bool vhold_smr(struct vnode *);
void vinactive(struct vnode *vp);
int vinvalbuf(struct vnode *vp, int save, int slpflag, int slptimeo);
int vtruncbuf(struct vnode *vp, off_t length, int blksize);
@ -974,6 +979,16 @@ int vn_dir_check_exec(struct vnode *vp, struct componentname *cnp);
SYSINIT(vfs_vector_##vnodeops##_f, SI_SUB_VFS, SI_ORDER_ANY, \
vfs_vector_op_register, &vnodeops)
#define VFS_SMR_DECLARE \
extern smr_t vfs_smr
#define VFS_SMR() vfs_smr
#define vfs_smr_enter() smr_enter(VFS_SMR())
#define vfs_smr_exit() smr_exit(VFS_SMR())
#define VFS_SMR_ASSERT_ENTERED() SMR_ASSERT_ENTERED(VFS_SMR())
#define VFS_SMR_ASSERT_NOT_ENTERED() SMR_ASSERT_NOT_ENTERED(VFS_SMR())
#define VFS_SMR_ZONE_SET(zone) uma_zone_set_smr((zone), VFS_SMR())
#endif /* _KERNEL */
#endif /* !_SYS_VNODE_H_ */