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Fixed sync issue where data writes could appear before metadata writes

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Long story short we aren't calling sync correctly in littlefs. This
fixes that.

Some forms of storage, mainly anything with an FTL, eMMC, SD, etc, do
not guarantee a strict write order for writes to different blocks. In
theory this is what bd sync is for, to tell the bd when it is important
for the writes to be ordered.

Currently, littlefs calls bd sync after committing metadata. This is
useful as it ensures that user code can rely on lfs_file_sync for
ordering external side-effects.

But this is insufficient for handling storage with out-of-order writes.

Consider the simple case of a file with one data block:

1. lfs_file_write(blablabla) => writes data into a new data block

2. lfs_file_sync() => commits metadata to point to the new data block

But with out-of-order writes, the bd is free to reorder things such that
the metadata is updated _before_ the data is written. If we lose power,
that would be bad.

The solution to this is to call bd sync twice: Once before we commit
the metadata to tell the bd that these writes must be ordered, and once
after we commit the metadata to allow ordering with user code.

As a small optimization, we only call bd sync if the current file is not
inlined and has actually been modified (LFS_F_DIRTY). It's possible for
inlined files to be interleaved with writes to other files.

Found by MFaehling and alex31
This commit is contained in:
Christopher Haster 2024-02-27 14:00:10 -06:00
parent f2a6f45eef
commit 6352185949
1 changed files with 9 additions and 0 deletions
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9
lfs.c
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@ -3401,6 +3401,15 @@ static int lfs_file_sync_(lfs_t *lfs, lfs_file_t *file) {
if ((file->flags & LFS_F_DIRTY) &&
!lfs_pair_isnull(file->m.pair)) {
// before we commit metadata, we need sync the disk to make sure
// data writes don't complete after metadata writes
if (!(file->flags & LFS_F_INLINE)) {
err = lfs_bd_sync(lfs, &lfs->pcache, &lfs->rcache, false);
if (err) {
return err;
}
}
// update dir entry
uint16_t type;
const void *buffer;