mirror of
https://github.com/eledio-devices/thirdparty-littlefs.git
synced 2025-10-30 16:15:40 +01:00
Removed recursion from lfs_dir_traverse
lfs_dir_traverse is a bit unpleasant in that it is inherently a recursive function, but without a strict bound of 4 calls (commit -> filter -> move -> filter), and efforts to unroll the recursion comes at a signification code cost. It turns out the best solution I've found so far is to simple create an explicit stack with an explicit bound of 4 calls (or more accurately, 3 pushed frames). --- This actually highlights one of the bigger flaws in littlefs right now, which is that this function, lfs_dir_traverse, takes O(n^2) disk reads to traverse. Note that LFS_FROM_MOVE can only occur once per commit, which is why this code is O(n^2) and not O(n^4).
This commit is contained in:
Christopher Haster
238
lfs.c
238
lfs.c
@@ -737,6 +737,7 @@ static int lfs_dir_traverse_filter(void *p,
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(LFS_MKTAG(0x7ff, 0x3ff, 0) & tag) == (
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LFS_MKTAG(LFS_TYPE_DELETE, 0, 0) |
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(LFS_MKTAG(0, 0x3ff, 0) & *filtertag))) {
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*filtertag = LFS_MKTAG(LFS_FROM_NOOP, 0, 0);
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return true;
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}
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@@ -751,98 +752,213 @@ static int lfs_dir_traverse_filter(void *p,
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#endif
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#ifndef LFS_READONLY
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// maximum recursive depth of lfs_dir_traverse, the deepest call:
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//
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// traverse with commit
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// '-> traverse with filter
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// '-> traverse with move
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// ' traverse with filter
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//
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#define LFS_DIR_TRAVERSE_DEPTH 4
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struct lfs_dir_traverse {
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const lfs_mdir_t *dir;
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lfs_off_t off;
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lfs_tag_t ptag;
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const struct lfs_mattr *attrs;
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int attrcount;
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lfs_tag_t tmask;
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lfs_tag_t ttag;
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uint16_t begin;
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uint16_t end;
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int16_t diff;
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int (*cb)(void *data, lfs_tag_t tag, const void *buffer);
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void *data;
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lfs_tag_t tag;
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const void *buffer;
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struct lfs_diskoff disk;
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};
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static int lfs_dir_traverse(lfs_t *lfs,
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const lfs_mdir_t *dir, lfs_off_t off, lfs_tag_t ptag,
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const struct lfs_mattr *attrs, int attrcount,
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lfs_tag_t tmask, lfs_tag_t ttag,
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uint16_t begin, uint16_t end, int16_t diff,
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int (*cb)(void *data, lfs_tag_t tag, const void *buffer), void *data) {
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// This function in inherently recursive, but bounded. To allow tool-based
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// analysis without unnecessary code-cost we use an explicit stack
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struct lfs_dir_traverse stack[LFS_DIR_TRAVERSE_DEPTH-1];
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unsigned sp = 0;
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int res;
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// iterate over directory and attrs
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lfs_tag_t tag;
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const void *buffer;
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struct lfs_diskoff disk;
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while (true) {
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lfs_tag_t tag;
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const void *buffer;
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struct lfs_diskoff disk;
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if (off+lfs_tag_dsize(ptag) < dir->off) {
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off += lfs_tag_dsize(ptag);
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int err = lfs_bd_read(lfs,
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NULL, &lfs->rcache, sizeof(tag),
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dir->pair[0], off, &tag, sizeof(tag));
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if (err) {
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return err;
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{
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if (off+lfs_tag_dsize(ptag) < dir->off) {
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off += lfs_tag_dsize(ptag);
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int err = lfs_bd_read(lfs,
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NULL, &lfs->rcache, sizeof(tag),
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dir->pair[0], off, &tag, sizeof(tag));
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if (err) {
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return err;
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}
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tag = (lfs_frombe32(tag) ^ ptag) | 0x80000000;
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disk.block = dir->pair[0];
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disk.off = off+sizeof(lfs_tag_t);
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buffer = &disk;
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ptag = tag;
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} else if (attrcount > 0) {
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tag = attrs[0].tag;
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buffer = attrs[0].buffer;
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attrs += 1;
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attrcount -= 1;
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} else {
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// finished traversal, pop from stack?
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res = 0;
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break;
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}
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tag = (lfs_frombe32(tag) ^ ptag) | 0x80000000;
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disk.block = dir->pair[0];
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disk.off = off+sizeof(lfs_tag_t);
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buffer = &disk;
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ptag = tag;
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} else if (attrcount > 0) {
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tag = attrs[0].tag;
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buffer = attrs[0].buffer;
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attrs += 1;
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attrcount -= 1;
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} else {
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return 0;
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// do we need to filter?
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lfs_tag_t mask = LFS_MKTAG(0x7ff, 0, 0);
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if ((mask & tmask & tag) != (mask & tmask & ttag)) {
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continue;
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}
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if (lfs_tag_id(tmask) != 0) {
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LFS_ASSERT(sp < LFS_DIR_TRAVERSE_DEPTH);
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// recurse, scan for duplicates, and update tag based on
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// creates/deletes
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stack[sp] = (struct lfs_dir_traverse){
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.dir = dir,
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.off = off,
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.ptag = ptag,
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.attrs = attrs,
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.attrcount = attrcount,
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.tmask = tmask,
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.ttag = ttag,
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.begin = begin,
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.end = end,
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.diff = diff,
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.cb = cb,
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.data = data,
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.tag = tag,
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.buffer = buffer,
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.disk = disk,
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};
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sp += 1;
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dir = dir;
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off = off;
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ptag = ptag;
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attrs = attrs;
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attrcount = attrcount;
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tmask = 0;
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ttag = 0;
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begin = 0;
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end = 0;
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diff = 0;
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cb = lfs_dir_traverse_filter;
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data = &stack[sp-1].tag;
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continue;
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}
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}
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lfs_tag_t mask = LFS_MKTAG(0x7ff, 0, 0);
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if ((mask & tmask & tag) != (mask & tmask & ttag)) {
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popped:
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// in filter range?
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if (lfs_tag_id(tmask) != 0 &&
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!(lfs_tag_id(tag) >= begin && lfs_tag_id(tag) < end)) {
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continue;
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}
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// do we need to filter? inlining the filtering logic here allows
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// for some minor optimizations
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if (lfs_tag_id(tmask) != 0) {
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// scan for duplicates and update tag based on creates/deletes
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int filter = lfs_dir_traverse(lfs,
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dir, off, ptag, attrs, attrcount,
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0, 0, 0, 0, 0,
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lfs_dir_traverse_filter, &tag);
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if (filter < 0) {
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return filter;
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}
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if (filter) {
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continue;
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}
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// in filter range?
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if (!(lfs_tag_id(tag) >= begin && lfs_tag_id(tag) < end)) {
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continue;
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}
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}
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// handle special cases for mcu-side operations
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if (lfs_tag_type3(tag) == LFS_FROM_NOOP) {
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// do nothing
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} else if (lfs_tag_type3(tag) == LFS_FROM_MOVE) {
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// recurse into move
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stack[sp] = (struct lfs_dir_traverse){
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.dir = dir,
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.off = off,
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.ptag = ptag,
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.attrs = attrs,
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.attrcount = attrcount,
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.tmask = tmask,
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.ttag = ttag,
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.begin = begin,
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.end = end,
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.diff = diff,
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.cb = cb,
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.data = data,
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.tag = LFS_MKTAG(LFS_FROM_NOOP, 0, 0),
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};
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sp += 1;
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uint16_t fromid = lfs_tag_size(tag);
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uint16_t toid = lfs_tag_id(tag);
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int err = lfs_dir_traverse(lfs,
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buffer, 0, 0xffffffff, NULL, 0,
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LFS_MKTAG(0x600, 0x3ff, 0),
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LFS_MKTAG(LFS_TYPE_STRUCT, 0, 0),
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fromid, fromid+1, toid-fromid+diff,
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cb, data);
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if (err) {
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return err;
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}
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dir = buffer;
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off = 0;
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ptag = 0xffffffff;
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attrs = NULL;
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attrcount = 0;
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tmask = LFS_MKTAG(0x600, 0x3ff, 0);
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ttag = LFS_MKTAG(LFS_TYPE_STRUCT, 0, 0);
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begin = fromid;
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end = fromid+1;
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diff = toid-fromid+diff;
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} else if (lfs_tag_type3(tag) == LFS_FROM_USERATTRS) {
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for (unsigned i = 0; i < lfs_tag_size(tag); i++) {
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const struct lfs_attr *a = buffer;
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int err = cb(data, LFS_MKTAG(LFS_TYPE_USERATTR + a[i].type,
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res = cb(data, LFS_MKTAG(LFS_TYPE_USERATTR + a[i].type,
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lfs_tag_id(tag) + diff, a[i].size), a[i].buffer);
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if (err) {
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return err;
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if (res < 0) {
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return res;
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}
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if (res) {
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break;
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}
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}
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} else {
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int err = cb(data, tag + LFS_MKTAG(0, diff, 0), buffer);
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if (err) {
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return err;
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res = cb(data, tag + LFS_MKTAG(0, diff, 0), buffer);
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if (res < 0) {
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return res;
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}
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if (res) {
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break;
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}
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}
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}
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if (sp > 0) {
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// pop from the stack and return, fortunately all pops share
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// a destination
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dir = stack[sp-1].dir;
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off = stack[sp-1].off;
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ptag = stack[sp-1].ptag;
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attrs = stack[sp-1].attrs;
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attrcount = stack[sp-1].attrcount;
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tmask = stack[sp-1].tmask;
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ttag = stack[sp-1].ttag;
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begin = stack[sp-1].begin;
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end = stack[sp-1].end;
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diff = stack[sp-1].diff;
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cb = stack[sp-1].cb;
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data = stack[sp-1].data;
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tag = stack[sp-1].tag;
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buffer = stack[sp-1].buffer;
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disk = stack[sp-1].disk;
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sp -= 1;
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goto popped;
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} else {
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return res;
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}
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}
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#endif
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