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Rearranged lfsr_mkdir to avoid tracking an imaginary mid

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lfsr_mkdir creates two mid entries atomically, one for the bookmark and
one for the actual dir entry. It looks up where to insert both, which is
necessary for some other checks, and then inserts one mid while tracking
the position of the other mid using our opened-mdir subsystem.

But it's a bit of a challenge to track an mid that hasn't been created
yet.

Consider what happends if we create a bookmark immediately adjacent to
our dir entry:

  0.0 bookmark parent  ->  0.0 bookmark parent  ->  0.0 bookmark parent
  (tracking 0.1)           0.1 bookmark child       0.1 bookmark child
                           (tracking 0.2)           0.2 dir child

That's not right.

To fix this we could add some special handling to our opened-mdir
subsystem to track opened-mdirs specially if they don't actually exist.

Or, as it turns out, just create the dir/bookmark entries in the
opposite order. Which happens to avoid this problem completely:

  0.0 bookmark parent  ->  0.0 bookmark parent  ->  0.0 bookmark parent
  (tracking 0.1)           0.1 dir child            0.1 dir child
                           (tracking 0.2)           0.2 bookmark child

The reason this works is that, thanks to our bookmarks, we can never
actually have a bookmark immediately preceding our dir entry. The
imaginary dir entry will always be preceded by either our parent or
other entry thanks to ordering by did first:

  (tracking 0.0)       ->  (tracking 0.0)       ->  0.0 bookmark child
  0.0 bookmark parent      0.0 bookmark parent      0.1 bookmark parent
  (tracking 0.1)           0.1 dir child            0.2 dir child

Previously we relied on rid=-1 to handle this as a special case, but
this workaround would stop working with compressed mids, where rid=-1
becomes unrepresentable.
This commit is contained in:
Christopher Haster
2023-08-25 01:11:42 -05:00
parent 302bfa17de
commit af5f4bff21
1 changed files with 56 additions and 52 deletions
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lfs.c
+56 -52
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@@ -6774,14 +6774,14 @@ int lfsr_mkdir(lfs_t *lfs, const char *path) {
}
// lookup our parent
lfsr_openedmdir_t parent;
lfs_size_t parent_did;
lfsr_mdir_t mdir;
lfs_size_t did;
const char *name;
lfs_size_t name_size;
err = lfsr_mtree_pathlookup(lfs, path,
&parent.mdir, NULL,
&parent_did, &name, &name_size);
if (err && (err != LFS_ERR_NOENT || lfsr_mdir_isroot(&parent.mdir))) {
&mdir, NULL,
&did, &name, &name_size);
if (err && (err != LFS_ERR_NOENT || lfsr_mdir_isroot(&mdir))) {
return err;
}
@@ -6825,14 +6825,14 @@ int lfsr_mkdir(lfs_t *lfs, const char *path) {
lfs_nlog2(lfsr_mtree_weight(lfs))
+ lfs_nlog2(lfs->cfg->block_size/32),
32)) - 1;
lfs_size_t did = lfs_crc32c(0, path, strlen(path)) & dmask;
lfs_size_t did_ = lfs_crc32c(0, path, strlen(path)) & dmask;
// Check if we have a collision. If we do, search for the next
// available did
lfsr_mdir_t mdir;
lfsr_openedmdir_t bookmark;
while (true) {
int err = lfsr_mtree_namelookup(lfs, did, NULL, 0,
&mdir, NULL, NULL);
int err = lfsr_mtree_namelookup(lfs, did_, NULL, 0,
&bookmark.mdir, NULL, NULL);
if (err && err != LFS_ERR_NOENT) {
return err;
}
@@ -6841,42 +6841,46 @@ int lfsr_mkdir(lfs_t *lfs, const char *path) {
}
// try the next did
did = (did + 1) & dmask;
did_ = (did_ + 1) & dmask;
}
// found a good did, now to commit to the mtree
// Note when we write to the mtree, it's possible it changes our
// parent's mdir/rid. We can catch this by tracking our parent
// as "opened" temporarily
// TODO is this the best workaround for rid update issues?
parent.mdir.mid.rid -= 1;
lfsr_mdir_addopened(lfs, LFS_TYPE_REG, &parent);
// A problem: we need to create both 1. the metadata entry and 2. the
// bookmark entry.
//
// To do this atomically, we first create the metadata entry with a grm
// to delete-self in case of powerloss, then create the bookmark while
// atomically cancelling the grm.
//
// These commits can change the relative mids of each other, so we track
// the bookmark mdir as an "open file" temporarily.
//
// Note! The metadata/bookmark order is important! Attempting to create
// the bookmark first risks inserting the bookmark before the metadata
// entry, which breaks things.
//
lfsr_mdir_addopened(lfs, LFS_TYPE_REG, &bookmark);
// Conveniently, we just found where our bookmark should go. The bookmark
// tag is an empty entry that marks our directory as being allocated.
//
// We include a GRM here so the bookmark is automatically removed if we
// lose power before writing the entry in our parent
//
// commit our new directory into our parent, creating a grm to self-remove
// in case of powerloss
err = lfsr_mdir_commit(lfs, &mdir, LFSR_ATTRS(
LFSR_ATTR(mdir.mid.rid, BOOKMARK, +1, NAME(did, NULL, 0)),
LFSR_ATTR(mdir.mid.rid, DIR, +1,
NAME(did, name, name_size)),
LFSR_ATTR(mdir.mid.rid, DID, 0, LEB128(did_)),
LFSR_ATTR(-1, GRM, 0, GRM(&((lfsr_grm_t){{
lfsr_mdir_mid(&mdir),
LFSR_MID(-1, -1)}})))));
if (err) {
goto failed_with_parent;
goto failed_with_bookmark;
}
lfsr_mdir_removeopened(lfs, LFS_TYPE_REG, &parent);
parent.mdir.mid.rid += 1;
lfsr_mdir_removeopened(lfs, LFS_TYPE_REG, &bookmark);
// commit our new directory into our parent, zeroing out our grm
// in the process
err = lfsr_mdir_commit(lfs, &parent.mdir, LFSR_ATTRS(
LFSR_ATTR(parent.mdir.mid.rid, DIR, +1,
NAME(parent_did, name, name_size)),
LFSR_ATTR(parent.mdir.mid.rid, DID, 0, LEB128(did)),
// commit our bookmark and zero the grm, the bookmark tag is an empty
// entry that marks our did as allocated
err = lfsr_mdir_commit(lfs, &bookmark.mdir, LFSR_ATTRS(
LFSR_ATTR(bookmark.mdir.mid.rid, BOOKMARK, +1, NAME(did_, NULL, 0)),
LFSR_ATTR(-1, GRM, 0, GRM(&((lfsr_grm_t){{
LFSR_MID(-1, -1),
LFSR_MID(-1, -1)}})))));
@@ -6886,8 +6890,8 @@ int lfsr_mkdir(lfs_t *lfs, const char *path) {
return 0;
failed_with_parent:
lfsr_mdir_removeopened(lfs, LFS_TYPE_REG, &parent);
failed_with_bookmark:
lfsr_mdir_removeopened(lfs, LFS_TYPE_REG, &bookmark);
return err;
}
@@ -6927,33 +6931,33 @@ int lfsr_remove(lfs_t *lfs, const char *path) {
}
// then lookup the bookmark entry
lfsr_mdir_t mdir_;
lfsr_mdir_t bookmark_mdir;
err = lfsr_mtree_namelookup(lfs, did, NULL, 0,
&mdir_, NULL, NULL);
&bookmark_mdir, NULL, NULL);
if (err) {
LFS_ASSERT(err != LFS_ERR_NOENT);
return err;
}
// create a grm to remove the bookmark entry
lfsr_grm_pushrm(&grm, lfsr_mdir_mid(&mdir_));
lfsr_grm_pushrm(&grm, lfsr_mdir_mid(&bookmark_mdir));
// check that the directory is empty
err = lfsr_mtree_seek(lfs, &mdir_, 1);
err = lfsr_mtree_seek(lfs, &bookmark_mdir, 1);
if (err && err != LFS_ERR_NOENT) {
return err;
}
if (err != LFS_ERR_NOENT) {
lfsr_tag_t tag_;
err = lfsr_mdir_lookup(lfs, &mdir_,
mdir_.mid.rid, LFSR_TAG_WIDE(NAME),
&tag_, NULL);
lfsr_tag_t bookmark_tag;
err = lfsr_mdir_lookup(lfs, &bookmark_mdir,
bookmark_mdir.mid.rid, LFSR_TAG_WIDE(NAME),
&bookmark_tag, NULL);
if (err) {
return err;
}
if (tag_ != LFSR_TAG_BOOKMARK) {
if (bookmark_tag != LFSR_TAG_BOOKMARK) {
return LFS_ERR_NOTEMPTY;
}
}
@@ -7059,33 +7063,33 @@ int lfsr_rename(lfs_t *lfs, const char *old_path, const char *new_path) {
}
// then lookup the bookmark entry
lfsr_mdir_t mdir_;
lfsr_mdir_t bookmark_mdir;
err = lfsr_mtree_namelookup(lfs, did, NULL, 0,
&mdir_, NULL, NULL);
&bookmark_mdir, NULL, NULL);
if (err) {
LFS_ASSERT(err != LFS_ERR_NOENT);
return err;
}
// create a grm to remove the bookmark entry
lfsr_grm_pushrm(&grm, lfsr_mdir_mid(&mdir_));
lfsr_grm_pushrm(&grm, lfsr_mdir_mid(&bookmark_mdir));
// check that the directory is empty
err = lfsr_mtree_seek(lfs, &mdir_, 1);
err = lfsr_mtree_seek(lfs, &bookmark_mdir, 1);
if (err && err != LFS_ERR_NOENT) {
return err;
}
if (err != LFS_ERR_NOENT) {
lfsr_tag_t tag_;
err = lfsr_mdir_lookup(lfs, &mdir_,
mdir_.mid.rid, LFSR_TAG_WIDE(NAME),
&tag_, NULL);
lfsr_tag_t bookmark_tag;
err = lfsr_mdir_lookup(lfs, &bookmark_mdir,
bookmark_mdir.mid.rid, LFSR_TAG_WIDE(NAME),
&bookmark_tag, NULL);
if (err) {
return err;
}
if (tag_ != LFSR_TAG_BOOKMARK) {
if (bookmark_tag != LFSR_TAG_BOOKMARK) {
return LFS_ERR_NOTEMPTY;
}
}