illumos/illumos-gate@11ceac77ea11ceac77eahttps://www.illumos.org/issues/6844
dnode_next_offset is used in a variety of places to iterate over the holes or
allocated blocks in a dnode. It operates under the premise that it can iterate
over the blockpointers of a dnode in open context while holding only the
dn_struct_rwlock as reader. Unfortunately, this premise does not hold.
When we create the zio for a dbuf, we pass in the actual block pointer in the
indirect block above that dbuf. When we later zero the bp in
zio_write_compress, we are directly modifying the bp. The state of the bp is
now inconsistent from the perspective of dnode_next_offset: the bp will appear
to be a hole until zio_dva_allocate finally finishes filling it in. In the
meantime, dnode_next_offset can detect a hole in the dnode when none exists.
I was able to experimentally demonstrate this behavior with the following
setup:
1. Create a file with 1 million dbufs.
2. Create a thread that randomly dirties L2 blocks by writing to the first L0
block under them.
3. Observe dnode_next_offset, waiting for it to skip over a hole in the middle
of a file.
4. Do dnode_next_offset in a loop until we skip over such a non-existent hole.
The fix is to ensure that it is valid to iterate over the indirect blocks in a
dnode while holding the dn_struct_rwlock by passing the zio a copy of the BP
and updating the actual BP in dbuf_write_ready while holding the lock.
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Boris Protopopov <bprotopopov@hotmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Alex Reece <alex@delphix.com>
MFC after: 3 weeks
e7ed92bbbc