zfs_rlock.c revision 0a586cea3ceec7e5e50e7e54c745082a7a333ac2
/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License (the "License").
* You may not use this file except in compliance with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright 2010 Sun Microsystems, Inc. All rights reserved.
* Use is subject to license terms.
*/
/*
* This file contains the code to implement file range locking in
* ZFS, although there isn't much specific to ZFS (all that comes to mind
* support for growing the blocksize).
*
* Interface
* ---------
* Defined in zfs_rlock.h but essentially:
* rl = zfs_range_lock(zp, off, len, lock_type);
* zfs_range_unlock(rl);
* zfs_range_reduce(rl, off, len);
*
* AVL tree
* --------
* An AVL tree is used to maintain the state of the existing ranges
* that are locked for exclusive (writer) or shared (reader) use.
* The starting range offset is used for searching and sorting the tree.
*
* Common case
* -----------
* The (hopefully) usual case is of no overlaps or contention for
* locks. On entry to zfs_lock_range() a rl_t is allocated; the tree
* searched that finds no overlap, and *this* rl_t is placed in the tree.
*
* Overlaps/Reference counting/Proxy locks
* ---------------------------------------
* The avl code only allows one node at a particular offset. Also it's very
* inefficient to search through all previous entries looking for overlaps
* (because the very 1st in the ordered list might be at offset 0 but
* cover the whole file).
* So this implementation uses reference counts and proxy range locks.
* Firstly, only reader locks use reference counts and proxy locks,
* because writer locks are exclusive.
* When a reader lock overlaps with another then a proxy lock is created
* for that range and replaces the original lock. If the overlap
* is exact then the reference count of the proxy is simply incremented.
* Otherwise, the proxy lock is split into smaller lock ranges and
* new proxy locks created for non overlapping ranges.
* The reference counts are adjusted accordingly.
* Meanwhile, the orginal lock is kept around (this is the callers handle)
* and its offset and length are used when releasing the lock.
*
* Thread coordination
* -------------------
* In order to make wakeups efficient and to ensure multiple continuous
* readers on a range don't starve a writer for the same range lock,
* two condition variables are allocated in each rl_t.
* If a writer (or reader) can't get a range it initialises the writer
* (or reader) cv; sets a flag saying there's a writer (or reader) waiting;
* and waits on that cv. When a thread unlocks that range it wakes up all
* writers then all readers before destroying the lock.
*
* Append mode writes
* ------------------
* Append mode writes need to lock a range at the end of a file.
* The offset of the end of the file is determined under the
* range locking mutex, and the lock type converted from RL_APPEND to
* RL_WRITER and the range locked.
*
* Grow block handling
* -------------------
* ZFS supports multiple block sizes currently upto 128K. The smallest
* block size is used for the file which is grown as needed. During this
* growth all other writers and readers must be excluded.
* So if the block size needs to be grown then the whole file is
* exclusively locked, then later the caller will reduce the lock
* range to just the range to be written using zfs_reduce_range.
*/
#include <sys/zfs_rlock.h>
/*
* Check if a write lock can be grabbed, or wait and recheck until available.
*/
static void
zfs_range_lock_writer(znode_t *zp, rl_t *new)
{
avl_tree_t *tree = &zp->z_range_avl;
rl_t *rl;
avl_index_t where;
uint64_t end_size;
uint64_t off = new->r_off;
uint64_t len = new->r_len;
for (;;) {
/*
* Range locking is also used by zvol and uses a
* dummied up znode. However, for zvol, we don't need to
* append or grow blocksize, and besides we don't have
* a "sa" data or z_zfsvfs - so skip that processing.
*
* Yes, this is ugly, and would be solved by not handling
* grow or append in range lock code. If that was done then
* we could make the range locking code generically available
* to other non-zfs consumers.
*/
if (zp->z_vnode) { /* caller is ZPL */
/*
* If in append mode pick up the current end of file.
* This is done under z_range_lock to avoid races.
*/
if (new->r_type == RL_APPEND)
new->r_off = zp->z_size;
/*
* If we need to grow the block size then grab the whole
* file range. This is also done under z_range_lock to
* avoid races.
*/
end_size = MAX(zp->z_size, new->r_off + len);
if (end_size > zp->z_blksz && (!ISP2(zp->z_blksz) ||
zp->z_blksz < zp->z_zfsvfs->z_max_blksz)) {
new->r_off = 0;
new->r_len = UINT64_MAX;
}
}
/*
* First check for the usual case of no locks
*/
if (avl_numnodes(tree) == 0) {
new->r_type = RL_WRITER; /* convert to writer */
avl_add(tree, new);
return;
}
/*
* Look for any locks in the range.
*/
rl = avl_find(tree, new, &where);
if (rl)
goto wait; /* already locked at same offset */
rl = (rl_t *)avl_nearest(tree, where, AVL_AFTER);
if (rl && (rl->r_off < new->r_off + new->r_len))
goto wait;
rl = (rl_t *)avl_nearest(tree, where, AVL_BEFORE);
if (rl && rl->r_off + rl->r_len > new->r_off)
goto wait;
new->r_type = RL_WRITER; /* convert possible RL_APPEND */
avl_insert(tree, new, where);
return;
wait:
if (!rl->r_write_wanted) {
cv_init(&rl->r_wr_cv, NULL, CV_DEFAULT, NULL);
rl->r_write_wanted = B_TRUE;
}
cv_wait(&rl->r_wr_cv, &zp->z_range_lock);
/* reset to original */
new->r_off = off;
new->r_len = len;
}
}
/*
* If this is an original (non-proxy) lock then replace it by
* a proxy and return the proxy.
*/
static rl_t *
zfs_range_proxify(avl_tree_t *tree, rl_t *rl)
{
rl_t *proxy;
if (rl->r_proxy)
return (rl); /* already a proxy */
ASSERT3U(rl->r_cnt, ==, 1);
ASSERT(rl->r_write_wanted == B_FALSE);
ASSERT(rl->r_read_wanted == B_FALSE);
avl_remove(tree, rl);
rl->r_cnt = 0;
/* create a proxy range lock */
proxy = kmem_alloc(sizeof (rl_t), KM_SLEEP);
proxy->r_off = rl->r_off;
proxy->r_len = rl->r_len;
proxy->r_cnt = 1;
proxy->r_type = RL_READER;
proxy->r_proxy = B_TRUE;
proxy->r_write_wanted = B_FALSE;
proxy->r_read_wanted = B_FALSE;
avl_add(tree, proxy);
return (proxy);
}
/*
* Split the range lock at the supplied offset
* returning the *front* proxy.
*/
static rl_t *
zfs_range_split(avl_tree_t *tree, rl_t *rl, uint64_t off)
{
rl_t *front, *rear;
ASSERT3U(rl->r_len, >, 1);
ASSERT3U(off, >, rl->r_off);
ASSERT3U(off, <, rl->r_off + rl->r_len);
ASSERT(rl->r_write_wanted == B_FALSE);
ASSERT(rl->r_read_wanted == B_FALSE);
/* create the rear proxy range lock */
rear = kmem_alloc(sizeof (rl_t), KM_SLEEP);
rear->r_off = off;
rear->r_len = rl->r_off + rl->r_len - off;
rear->r_cnt = rl->r_cnt;
rear->r_type = RL_READER;
rear->r_proxy = B_TRUE;
rear->r_write_wanted = B_FALSE;
rear->r_read_wanted = B_FALSE;
front = zfs_range_proxify(tree, rl);
front->r_len = off - rl->r_off;
avl_insert_here(tree, rear, front, AVL_AFTER);
return (front);
}
/*
* Create and add a new proxy range lock for the supplied range.
*/
static void
zfs_range_new_proxy(avl_tree_t *tree, uint64_t off, uint64_t len)
{
rl_t *rl;
ASSERT(len);
rl = kmem_alloc(sizeof (rl_t), KM_SLEEP);
rl->r_off = off;
rl->r_len = len;
rl->r_cnt = 1;
rl->r_type = RL_READER;
rl->r_proxy = B_TRUE;
rl->r_write_wanted = B_FALSE;
rl->r_read_wanted = B_FALSE;
avl_add(tree, rl);
}
static void
zfs_range_add_reader(avl_tree_t *tree, rl_t *new, rl_t *prev, avl_index_t where)
{
rl_t *next;
uint64_t off = new->r_off;
uint64_t len = new->r_len;
/*
* prev arrives either:
* - pointing to an entry at the same offset
* - pointing to the entry with the closest previous offset whose
* range may overlap with the new range
* - null, if there were no ranges starting before the new one
*/
if (prev) {
if (prev->r_off + prev->r_len <= off) {
prev = NULL;
} else if (prev->r_off != off) {
/*
* convert to proxy if needed then
* split this entry and bump ref count
*/
prev = zfs_range_split(tree, prev, off);
prev = AVL_NEXT(tree, prev); /* move to rear range */
}
}
ASSERT((prev == NULL) || (prev->r_off == off));
if (prev)
next = prev;
else
next = (rl_t *)avl_nearest(tree, where, AVL_AFTER);
if (next == NULL || off + len <= next->r_off) {
/* no overlaps, use the original new rl_t in the tree */
avl_insert(tree, new, where);
return;
}
if (off < next->r_off) {
/* Add a proxy for initial range before the overlap */
zfs_range_new_proxy(tree, off, next->r_off - off);
}
new->r_cnt = 0; /* will use proxies in tree */
/*
* We now search forward through the ranges, until we go past the end
* of the new range. For each entry we make it a proxy if it
* isn't already, then bump its reference count. If there's any
* gaps between the ranges then we create a new proxy range.
*/
for (prev = NULL; next; prev = next, next = AVL_NEXT(tree, next)) {
if (off + len <= next->r_off)
break;
if (prev && prev->r_off + prev->r_len < next->r_off) {
/* there's a gap */
ASSERT3U(next->r_off, >, prev->r_off + prev->r_len);
zfs_range_new_proxy(tree, prev->r_off + prev->r_len,
next->r_off - (prev->r_off + prev->r_len));
}
if (off + len == next->r_off + next->r_len) {
/* exact overlap with end */
next = zfs_range_proxify(tree, next);
next->r_cnt++;
return;
}
if (off + len < next->r_off + next->r_len) {
/* new range ends in the middle of this block */
next = zfs_range_split(tree, next, off + len);
next->r_cnt++;
return;
}
ASSERT3U(off + len, >, next->r_off + next->r_len);
next = zfs_range_proxify(tree, next);
next->r_cnt++;
}
/* Add the remaining end range. */
zfs_range_new_proxy(tree, prev->r_off + prev->r_len,
(off + len) - (prev->r_off + prev->r_len));
}
/*
* Check if a reader lock can be grabbed, or wait and recheck until available.
*/
static void
zfs_range_lock_reader(znode_t *zp, rl_t *new)
{
avl_tree_t *tree = &zp->z_range_avl;
rl_t *prev, *next;
avl_index_t where;
uint64_t off = new->r_off;
uint64_t len = new->r_len;
/*
* Look for any writer locks in the range.
*/
retry:
prev = avl_find(tree, new, &where);
if (prev == NULL)
prev = (rl_t *)avl_nearest(tree, where, AVL_BEFORE);
/*
* Check the previous range for a writer lock overlap.
*/
if (prev && (off < prev->r_off + prev->r_len)) {
if ((prev->r_type == RL_WRITER) || (prev->r_write_wanted)) {
if (!prev->r_read_wanted) {
cv_init(&prev->r_rd_cv, NULL, CV_DEFAULT, NULL);
prev->r_read_wanted = B_TRUE;
}
cv_wait(&prev->r_rd_cv, &zp->z_range_lock);
goto retry;
}
if (off + len < prev->r_off + prev->r_len)
goto got_lock;
}
/*
* Search through the following ranges to see if there's
* write lock any overlap.
*/
if (prev)
next = AVL_NEXT(tree, prev);
else
next = (rl_t *)avl_nearest(tree, where, AVL_AFTER);
for (; next; next = AVL_NEXT(tree, next)) {
if (off + len <= next->r_off)
goto got_lock;
if ((next->r_type == RL_WRITER) || (next->r_write_wanted)) {
if (!next->r_read_wanted) {
cv_init(&next->r_rd_cv, NULL, CV_DEFAULT, NULL);
next->r_read_wanted = B_TRUE;
}
cv_wait(&next->r_rd_cv, &zp->z_range_lock);
goto retry;
}
if (off + len <= next->r_off + next->r_len)
goto got_lock;
}
got_lock:
/*
* Add the read lock, which may involve splitting existing
* locks and bumping ref counts (r_cnt).
*/
zfs_range_add_reader(tree, new, prev, where);
}
/*
* Lock a range (offset, length) as either shared (RL_READER)
* or exclusive (RL_WRITER). Returns the range lock structure
* for later unlocking or reduce range (if entire file
* previously locked as RL_WRITER).
*/
rl_t *
zfs_range_lock(znode_t *zp, uint64_t off, uint64_t len, rl_type_t type)
{
rl_t *new;
ASSERT(type == RL_READER || type == RL_WRITER || type == RL_APPEND);
new = kmem_alloc(sizeof (rl_t), KM_SLEEP);
new->r_zp = zp;
new->r_off = off;
if (len + off < off) /* overflow */
len = UINT64_MAX - off;
new->r_len = len;
new->r_cnt = 1; /* assume it's going to be in the tree */
new->r_type = type;
new->r_proxy = B_FALSE;
new->r_write_wanted = B_FALSE;
new->r_read_wanted = B_FALSE;
mutex_enter(&zp->z_range_lock);
if (type == RL_READER) {
/*
* First check for the usual case of no locks
*/
if (avl_numnodes(&zp->z_range_avl) == 0)
avl_add(&zp->z_range_avl, new);
else
zfs_range_lock_reader(zp, new);
} else
zfs_range_lock_writer(zp, new); /* RL_WRITER or RL_APPEND */
mutex_exit(&zp->z_range_lock);
return (new);
}
/*
* Unlock a reader lock
*/
static void
zfs_range_unlock_reader(znode_t *zp, rl_t *remove)
{
avl_tree_t *tree = &zp->z_range_avl;
rl_t *rl, *next;
uint64_t len;
/*
* The common case is when the remove entry is in the tree
* (cnt == 1) meaning there's been no other reader locks overlapping
* with this one. Otherwise the remove entry will have been
* removed from the tree and replaced by proxies (one or
* more ranges mapping to the entire range).
*/
if (remove->r_cnt == 1) {
avl_remove(tree, remove);
if (remove->r_write_wanted) {
cv_broadcast(&remove->r_wr_cv);
cv_destroy(&remove->r_wr_cv);
}
if (remove->r_read_wanted) {
cv_broadcast(&remove->r_rd_cv);
cv_destroy(&remove->r_rd_cv);
}
} else {
ASSERT3U(remove->r_cnt, ==, 0);
ASSERT3U(remove->r_write_wanted, ==, 0);
ASSERT3U(remove->r_read_wanted, ==, 0);
/*
* Find start proxy representing this reader lock,
* then decrement ref count on all proxies
* that make up this range, freeing them as needed.
*/
rl = avl_find(tree, remove, NULL);
ASSERT(rl);
ASSERT(rl->r_cnt);
ASSERT(rl->r_type == RL_READER);
for (len = remove->r_len; len != 0; rl = next) {
len -= rl->r_len;
if (len) {
next = AVL_NEXT(tree, rl);
ASSERT(next);
ASSERT(rl->r_off + rl->r_len == next->r_off);
ASSERT(next->r_cnt);
ASSERT(next->r_type == RL_READER);
}
rl->r_cnt--;
if (rl->r_cnt == 0) {
avl_remove(tree, rl);
if (rl->r_write_wanted) {
cv_broadcast(&rl->r_wr_cv);
cv_destroy(&rl->r_wr_cv);
}
if (rl->r_read_wanted) {
cv_broadcast(&rl->r_rd_cv);
cv_destroy(&rl->r_rd_cv);
}
kmem_free(rl, sizeof (rl_t));
}
}
}
kmem_free(remove, sizeof (rl_t));
}
/*
* Unlock range and destroy range lock structure.
*/
void
zfs_range_unlock(rl_t *rl)
{
znode_t *zp = rl->r_zp;
ASSERT(rl->r_type == RL_WRITER || rl->r_type == RL_READER);
ASSERT(rl->r_cnt == 1 || rl->r_cnt == 0);
ASSERT(!rl->r_proxy);
mutex_enter(&zp->z_range_lock);
if (rl->r_type == RL_WRITER) {
/* writer locks can't be shared or split */
avl_remove(&zp->z_range_avl, rl);
mutex_exit(&zp->z_range_lock);
if (rl->r_write_wanted) {
cv_broadcast(&rl->r_wr_cv);
cv_destroy(&rl->r_wr_cv);
}
if (rl->r_read_wanted) {
cv_broadcast(&rl->r_rd_cv);
cv_destroy(&rl->r_rd_cv);
}
kmem_free(rl, sizeof (rl_t));
} else {
/*
* lock may be shared, let zfs_range_unlock_reader()
* release the lock and free the rl_t
*/
zfs_range_unlock_reader(zp, rl);
mutex_exit(&zp->z_range_lock);
}
}
/*
* Reduce range locked as RL_WRITER from whole file to specified range.
* Asserts the whole file is exclusivly locked and so there's only one
* entry in the tree.
*/
void
zfs_range_reduce(rl_t *rl, uint64_t off, uint64_t len)
{
znode_t *zp = rl->r_zp;
/* Ensure there are no other locks */
ASSERT(avl_numnodes(&zp->z_range_avl) == 1);
ASSERT(rl->r_off == 0);
ASSERT(rl->r_type == RL_WRITER);
ASSERT(!rl->r_proxy);
ASSERT3U(rl->r_len, ==, UINT64_MAX);
ASSERT3U(rl->r_cnt, ==, 1);
mutex_enter(&zp->z_range_lock);
rl->r_off = off;
rl->r_len = len;
mutex_exit(&zp->z_range_lock);
if (rl->r_write_wanted)
cv_broadcast(&rl->r_wr_cv);
if (rl->r_read_wanted)
cv_broadcast(&rl->r_rd_cv);
}
/*
* AVL comparison function used to order range locks
* Locks are ordered on the start offset of the range.
*/
int
zfs_range_compare(const void *arg1, const void *arg2)
{
const rl_t *rl1 = arg1;
const rl_t *rl2 = arg2;
if (rl1->r_off > rl2->r_off)
return (1);
if (rl1->r_off < rl2->r_off)
return (-1);
return (0);
}