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Adopted Brent's algorithm for cycle detection

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The previous cycle detection algorithm (a naive check against the largest
possible tail list) is simple and gets the job done, but has the potential to
take a very long time on disks with many blocks. Brent's algorithm, on
the other hand, takes at most 2x the number of blocks in the tail list.

Originally naive cycle detection was chosen over Floyd's algorithm to
avoid the extra complexity of managing two desynced traversals for every
traversal of the tail list, but Brent's algorithm is very well suited for our
use case, requiring only we keep track of an additional mdir pointer on the
stack as we traverse.
This commit is contained in:
Christopher Haster
2022-11-08 15:08:33 -06:00
parent c2147c45ee
commit 1278ec1d08
1 changed files with 54 additions and 20 deletions
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lfs.c
+54 -20
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@@ -279,14 +279,12 @@ static inline int lfs_pair_cmp(
paira[0] == pairb[1] || paira[1] == pairb[0]); paira[0] == pairb[1] || paira[1] == pairb[0]);
} }
#ifndef LFS_READONLY static inline bool lfs_pair_issync(
static inline bool lfs_pair_sync(
const lfs_block_t paira[2], const lfs_block_t paira[2],
const lfs_block_t pairb[2]) { const lfs_block_t pairb[2]) {
return (paira[0] == pairb[0] && paira[1] == pairb[1]) || return (paira[0] == pairb[0] && paira[1] == pairb[1]) ||
(paira[0] == pairb[1] && paira[1] == pairb[0]); (paira[0] == pairb[1] && paira[1] == pairb[0]);
} }
#endif
static inline void lfs_pair_fromle32(lfs_block_t pair[2]) { static inline void lfs_pair_fromle32(lfs_block_t pair[2]) {
pair[0] = lfs_fromle32(pair[0]); pair[0] = lfs_fromle32(pair[0]);
@@ -4110,14 +4108,23 @@ static int lfs_rawmount(lfs_t *lfs, const struct lfs_config *cfg) {
// scan directory blocks for superblock and any global updates // scan directory blocks for superblock and any global updates
lfs_mdir_t dir = {.tail = {0, 1}}; lfs_mdir_t dir = {.tail = {0, 1}};
lfs_block_t cycle = 0; lfs_block_t tortoise[2] = {LFS_BLOCK_NULL, LFS_BLOCK_NULL};
lfs_size_t tortoise_i = 1;
lfs_size_t tortoise_period = 1;
while (!lfs_pair_isnull(dir.tail)) { while (!lfs_pair_isnull(dir.tail)) {
if (cycle >= lfs->cfg->block_count/2) { // detect cycles with Brent's algorithm
// loop detected if (lfs_pair_issync(dir.tail, tortoise)) {
LFS_WARN("Cycle detected in tail list");
err = LFS_ERR_CORRUPT; err = LFS_ERR_CORRUPT;
goto cleanup; goto cleanup;
} }
cycle += 1; if (tortoise_i == tortoise_period) {
tortoise[0] = dir.tail[0];
tortoise[1] = dir.tail[1];
tortoise_i = 0;
tortoise_period *= 2;
}
tortoise_i += 1;
// fetch next block in tail list // fetch next block in tail list
lfs_stag_t tag = lfs_dir_fetchmatch(lfs, &dir, dir.tail, lfs_stag_t tag = lfs_dir_fetchmatch(lfs, &dir, dir.tail,
@@ -4268,13 +4275,22 @@ int lfs_fs_rawtraverse(lfs_t *lfs,
} }
#endif #endif
lfs_block_t cycle = 0; lfs_block_t tortoise[2] = {LFS_BLOCK_NULL, LFS_BLOCK_NULL};
lfs_size_t tortoise_i = 1;
lfs_size_t tortoise_period = 1;
while (!lfs_pair_isnull(dir.tail)) { while (!lfs_pair_isnull(dir.tail)) {
if (cycle >= lfs->cfg->block_count/2) { // detect cycles with Brent's algorithm
// loop detected if (lfs_pair_issync(dir.tail, tortoise)) {
LFS_WARN("Cycle detected in tail list");
return LFS_ERR_CORRUPT; return LFS_ERR_CORRUPT;
} }
cycle += 1; if (tortoise_i == tortoise_period) {
tortoise[0] = dir.tail[0];
tortoise[1] = dir.tail[1];
tortoise_i = 0;
tortoise_period *= 2;
}
tortoise_i += 1;
for (int i = 0; i < 2; i++) { for (int i = 0; i < 2; i++) {
int err = cb(data, dir.tail[i]); int err = cb(data, dir.tail[i]);
@@ -4353,13 +4369,22 @@ static int lfs_fs_pred(lfs_t *lfs,
// iterate over all directory directory entries // iterate over all directory directory entries
pdir->tail[0] = 0; pdir->tail[0] = 0;
pdir->tail[1] = 1; pdir->tail[1] = 1;
lfs_block_t cycle = 0; lfs_block_t tortoise[2] = {LFS_BLOCK_NULL, LFS_BLOCK_NULL};
lfs_size_t tortoise_i = 1;
lfs_size_t tortoise_period = 1;
while (!lfs_pair_isnull(pdir->tail)) { while (!lfs_pair_isnull(pdir->tail)) {
if (cycle >= lfs->cfg->block_count/2) { // detect cycles with Brent's algorithm
// loop detected if (lfs_pair_issync(pdir->tail, tortoise)) {
LFS_WARN("Cycle detected in tail list");
return LFS_ERR_CORRUPT; return LFS_ERR_CORRUPT;
} }
cycle += 1; if (tortoise_i == tortoise_period) {
tortoise[0] = pdir->tail[0];
tortoise[1] = pdir->tail[1];
tortoise_i = 0;
tortoise_period *= 2;
}
tortoise_i += 1;
if (lfs_pair_cmp(pdir->tail, pair) == 0) { if (lfs_pair_cmp(pdir->tail, pair) == 0) {
return 0; return 0;
@@ -4409,13 +4434,22 @@ static lfs_stag_t lfs_fs_parent(lfs_t *lfs, const lfs_block_t pair[2],
// use fetchmatch with callback to find pairs // use fetchmatch with callback to find pairs
parent->tail[0] = 0; parent->tail[0] = 0;
parent->tail[1] = 1; parent->tail[1] = 1;
lfs_block_t cycle = 0; lfs_block_t tortoise[2] = {LFS_BLOCK_NULL, LFS_BLOCK_NULL};
lfs_size_t tortoise_i = 1;
lfs_size_t tortoise_period = 1;
while (!lfs_pair_isnull(parent->tail)) { while (!lfs_pair_isnull(parent->tail)) {
if (cycle >= lfs->cfg->block_count/2) { // detect cycles with Brent's algorithm
// loop detected if (lfs_pair_issync(parent->tail, tortoise)) {
LFS_WARN("Cycle detected in tail list");
return LFS_ERR_CORRUPT; return LFS_ERR_CORRUPT;
} }
cycle += 1; if (tortoise_i == tortoise_period) {
tortoise[0] = parent->tail[0];
tortoise[1] = parent->tail[1];
tortoise_i = 0;
tortoise_period *= 2;
}
tortoise_i += 1;
lfs_stag_t tag = lfs_dir_fetchmatch(lfs, parent, parent->tail, lfs_stag_t tag = lfs_dir_fetchmatch(lfs, parent, parent->tail,
LFS_MKTAG(0x7ff, 0, 0x3ff), LFS_MKTAG(0x7ff, 0, 0x3ff),
@@ -4532,7 +4566,7 @@ restart:
} }
lfs_pair_fromle32(pair); lfs_pair_fromle32(pair);
if (!lfs_pair_sync(pair, pdir.tail)) { if (!lfs_pair_issync(pair, pdir.tail)) {
// we have desynced // we have desynced
LFS_DEBUG("Fixing half-orphan " LFS_DEBUG("Fixing half-orphan "
"{0x%"PRIx32", 0x%"PRIx32"} " "{0x%"PRIx32", 0x%"PRIx32"} "