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Fix read errors race after block cloning

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Investigating read errors triggering panic fixed in #16042 I've
found that we have a race in a sync process between the moment
dirty record for cloned block is removed and the moment dbuf is
destroyed.  If dmu_buf_hold_array_by_dnode() take a hold on a
cloned dbuf before it is synced/destroyed, then dbuf_read_impl()
may see it still in DB_NOFILL state, but without the dirty record.
Such case is not an error, but equivalent to DB_UNCACHED, since
the dbuf block pointer is already updated by dbuf_write_ready().
Unfortunately it is impossible to safely change the dbuf state
to DB_UNCACHED there, since there may already be another cloning
in progress, that dropped dbuf lock before creating a new dirty
record, protected only by the range lock.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Robert Evans <evansr@google.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #16052
This commit is contained in:
Alexander Motin 2024-04-08 15:03:18 -04:00 committed by Brian Behlendorf
parent d5fb6abd36
commit 602b5dca7b
1 changed files with 20 additions and 21 deletions
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41
module/zfs/dbuf.c
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@ -1548,7 +1548,7 @@ dbuf_read_impl(dmu_buf_impl_t *db, dnode_t *dn, zio_t *zio, uint32_t flags,
zbookmark_phys_t zb;
uint32_t aflags = ARC_FLAG_NOWAIT;
int err, zio_flags;
blkptr_t bp, *bpp;
blkptr_t bp, *bpp = NULL;
ASSERT(!zfs_refcount_is_zero(&db->db_holds));
ASSERT(MUTEX_HELD(&db->db_mtx));
@ -1562,29 +1562,28 @@ dbuf_read_impl(dmu_buf_impl_t *db, dnode_t *dn, zio_t *zio, uint32_t flags,
goto early_unlock;
}
if (db->db_state == DB_UNCACHED) {
if (db->db_blkptr == NULL) {
bpp = NULL;
} else {
bp = *db->db_blkptr;
/*
* If we have a pending block clone, we don't want to read the
* underlying block, but the content of the block being cloned,
* pointed by the dirty record, so we have the most recent data.
* If there is no dirty record, then we hit a race in a sync
* process when the dirty record is already removed, while the
* dbuf is not yet destroyed. Such case is equivalent to uncached.
*/
if (db->db_state == DB_NOFILL) {
dbuf_dirty_record_t *dr = list_head(&db->db_dirty_records);
if (dr != NULL) {
if (!dr->dt.dl.dr_brtwrite) {
err = EIO;
goto early_unlock;
}
bp = dr->dt.dl.dr_overridden_by;
bpp = &bp;
}
} else {
dbuf_dirty_record_t *dr;
}
ASSERT3S(db->db_state, ==, DB_NOFILL);
/*
* Block cloning: If we have a pending block clone,
* we don't want to read the underlying block, but the content
* of the block being cloned, so we have the most recent data.
*/
dr = list_head(&db->db_dirty_records);
if (dr == NULL || !dr->dt.dl.dr_brtwrite) {
err = EIO;
goto early_unlock;
}
bp = dr->dt.dl.dr_overridden_by;
if (bpp == NULL && db->db_blkptr != NULL) {
bp = *db->db_blkptr;
bpp = &bp;
}