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Use more atomics in refcounts.
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Use atomic_load_64() for zfs_refcount_count() to prevent torn reads
on 32-bit platforms.  On 64-bit ones it should not change anything.

When built with ZFS_DEBUG but running without tracking enabled use
atomics instead of mutexes same as for builds without ZFS_DEBUG.
Since rc_tracked can't change live we can check it without lock.

Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
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@amotin
amotin committed Jul 26, 2021
1 parent ab8a8f0 commit d85cdf2
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Showing 2 changed files with 26 additions and 33 deletions.
8 changes: 4 additions & 4 deletions include/sys/zfs_refcount.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -96,22 +96,22 @@ typedef struct refcount {
#define zfs_refcount_create_tracked(rc) ((rc)->rc_count = 0)
#define zfs_refcount_destroy(rc) ((rc)->rc_count = 0)
#define zfs_refcount_destroy_many(rc, number) ((rc)->rc_count = 0)
#define zfs_refcount_is_zero(rc) ((rc)->rc_count == 0)
#define zfs_refcount_count(rc) ((rc)->rc_count)
#define zfs_refcount_is_zero(rc) (zfs_refcount_count(rc) == 0)
#define zfs_refcount_count(rc) atomic_load_64(&(rc)->rc_count)
#define zfs_refcount_add(rc, holder) atomic_inc_64_nv(&(rc)->rc_count)
#define zfs_refcount_remove(rc, holder) atomic_dec_64_nv(&(rc)->rc_count)
#define zfs_refcount_add_many(rc, number, holder) \
atomic_add_64_nv(&(rc)->rc_count, number)
#define zfs_refcount_remove_many(rc, number, holder) \
atomic_add_64_nv(&(rc)->rc_count, -number)
#define zfs_refcount_transfer(dst, src) { \
uint64_t __tmp = (src)->rc_count; \
uint64_t __tmp = zfs_refcount_count(src); \
atomic_add_64(&(src)->rc_count, -__tmp); \
atomic_add_64(&(dst)->rc_count, __tmp); \
}
#define zfs_refcount_transfer_ownership(rc, ch, nh) ((void)0)
#define zfs_refcount_transfer_ownership_many(rc, nr, ch, nh) ((void)0)
#define zfs_refcount_held(rc, holder) ((rc)->rc_count > 0)
#define zfs_refcount_held(rc, holder) (zfs_refcount_count(rc) > 0)
#define zfs_refcount_not_held(rc, holder) (B_TRUE)

#define zfs_refcount_init()
Expand Down
51 changes: 22 additions & 29 deletions module/zfs/refcount.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -112,13 +112,13 @@ zfs_refcount_destroy(zfs_refcount_t *rc)
int
zfs_refcount_is_zero(zfs_refcount_t *rc)
{
return (rc->rc_count == 0);
return (zfs_refcount_count(rc) == 0);
}

int64_t
zfs_refcount_count(zfs_refcount_t *rc)
{
return (rc->rc_count);
return (atomic_load_64(&rc->rc_count));
}

int64_t
Expand All @@ -127,15 +127,18 @@ zfs_refcount_add_many(zfs_refcount_t *rc, uint64_t number, const void *holder)
reference_t *ref = NULL;
int64_t count;

if (rc->rc_tracked) {
ref = kmem_cache_alloc(reference_cache, KM_SLEEP);
ref->ref_holder = holder;
ref->ref_number = number;
if (!rc->rc_tracked) {
count = atomic_add_64_nv(&(rc)->rc_count, number);
ASSERT3U(count, >=, number);
return (count);
}

ref = kmem_cache_alloc(reference_cache, KM_SLEEP);
ref->ref_holder = holder;
ref->ref_number = number;
mutex_enter(&rc->rc_mtx);
ASSERT3U(rc->rc_count, >=, 0);
if (rc->rc_tracked)
list_insert_head(&rc->rc_list, ref);
list_insert_head(&rc->rc_list, ref);
rc->rc_count += number;
count = rc->rc_count;
mutex_exit(&rc->rc_mtx);
Expand All @@ -156,16 +159,14 @@ zfs_refcount_remove_many(zfs_refcount_t *rc, uint64_t number,
reference_t *ref;
int64_t count;

mutex_enter(&rc->rc_mtx);
ASSERT3U(rc->rc_count, >=, number);

if (!rc->rc_tracked) {
rc->rc_count -= number;
count = rc->rc_count;
mutex_exit(&rc->rc_mtx);
count = atomic_add_64_nv(&(rc)->rc_count, -number);
ASSERT3S(count, >=, 0);
return (count);
}

mutex_enter(&rc->rc_mtx);
ASSERT3U(rc->rc_count, >=, number);
for (ref = list_head(&rc->rc_list); ref;
ref = list_next(&rc->rc_list, ref)) {
if (ref->ref_holder == holder && ref->ref_number == number) {
Expand Down Expand Up @@ -242,12 +243,10 @@ zfs_refcount_transfer_ownership_many(zfs_refcount_t *rc, uint64_t number,
reference_t *ref;
boolean_t found = B_FALSE;

mutex_enter(&rc->rc_mtx);
if (!rc->rc_tracked) {
mutex_exit(&rc->rc_mtx);
if (!rc->rc_tracked)
return;
}

mutex_enter(&rc->rc_mtx);
for (ref = list_head(&rc->rc_list); ref;
ref = list_next(&rc->rc_list, ref)) {
if (ref->ref_holder == current_holder &&
Expand Down Expand Up @@ -279,13 +278,10 @@ zfs_refcount_held(zfs_refcount_t *rc, const void *holder)
{
reference_t *ref;

mutex_enter(&rc->rc_mtx);

if (!rc->rc_tracked) {
mutex_exit(&rc->rc_mtx);
return (rc->rc_count > 0);
}
if (!rc->rc_tracked)
return (zfs_refcount_count(rc) > 0);

mutex_enter(&rc->rc_mtx);
for (ref = list_head(&rc->rc_list); ref;
ref = list_next(&rc->rc_list, ref)) {
if (ref->ref_holder == holder) {
Expand All @@ -307,13 +303,10 @@ zfs_refcount_not_held(zfs_refcount_t *rc, const void *holder)
{
reference_t *ref;

mutex_enter(&rc->rc_mtx);

if (!rc->rc_tracked) {
mutex_exit(&rc->rc_mtx);
if (!rc->rc_tracked)
return (B_TRUE);
}

mutex_enter(&rc->rc_mtx);
for (ref = list_head(&rc->rc_list); ref;
ref = list_next(&rc->rc_list, ref)) {
if (ref->ref_holder == holder) {
Expand Down

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