| Message ID | 20220215135727.28521-2-hreitz@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | block: Make bdrv_refresh_limits() non-recursive | expand |
On Tue, Feb 15, 2022 at 02:57:25PM +0100, Hanna Reitz wrote: > bdrv_refresh_limits() recurses down to the node's children. That does > not seem necessary: When we refresh limits on some node, and then > recurse down and were to change one of its children's BlockLimits, then > that would mean we noticed the changed limits by pure chance. The fact > that we refresh the parent's limits has nothing to do with it, so the > reason for the change probably happened before this point in time, and > we should have refreshed the limits then. > > On the other hand, we do not have infrastructure for noticing that block > limits change after they have been initialized for the first time (this > would require propagating the change upwards to the respective node's > parents), and so evidently we consider this case impossible. > > If this case is impossible, then we will not need to recurse down in > bdrv_refresh_limits(). Every node's limits are initialized in > bdrv_open_driver(), and are refreshed whenever its children change. > We want to use the childrens' limits to get some initial default, but we > can just take them, we do not need to refresh them. > > The problem with recursing is that bdrv_refresh_limits() is not atomic. > It begins with zeroing BDS.bl, and only then sets proper, valid limits. > If we do not drain all nodes whose limits are refreshed, then concurrent > I/O requests can encounter invalid request_alignment values and crash > qemu. Therefore, a recursing bdrv_refresh_limits() requires the whole > subtree to be drained, which is currently not ensured by most callers. > > A non-recursive bdrv_refresh_limits() only requires the node in question > to not receive I/O requests, and this is done by most callers in some > way or another: > - bdrv_open_driver() deals with a new node with no parents yet > - bdrv_set_file_or_backing_noperm() acts on a drained node > - bdrv_reopen_commit() acts only on drained nodes > - bdrv_append() should in theory require the node to be drained; in > practice most callers just lock the AioContext, which should at least > be enough to prevent concurrent I/O requests from accessing invalid > limits > > So we can resolve the bug by making bdrv_refresh_limits() non-recursive. Long explanation, but very helpful. > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1879437 > Signed-off-by: Hanna Reitz <hreitz@redhat.com> > --- > block/io.c | 4 ---- > 1 file changed, 4 deletions(-) And deceptively simple fix! Reviewed-by: Eric Blake <eblake@redhat.com> > > diff --git a/block/io.c b/block/io.c > index 4e4cb556c5..c3e7301613 100644 > --- a/block/io.c > +++ b/block/io.c > @@ -189,10 +189,6 @@ void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp) > QLIST_FOREACH(c, &bs->children, next) { > if (c->role & (BDRV_CHILD_DATA | BDRV_CHILD_FILTERED | BDRV_CHILD_COW)) > { > - bdrv_refresh_limits(c->bs, tran, errp); > - if (*errp) { > - return; > - } > bdrv_merge_limits(&bs->bl, &c->bs->bl); > have_limits = true; > } > -- > 2.34.1 > >
diff --git a/block/io.c b/block/io.c index 4e4cb556c5..c3e7301613 100644 --- a/block/io.c +++ b/block/io.c @@ -189,10 +189,6 @@ void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp) QLIST_FOREACH(c, &bs->children, next) { if (c->role & (BDRV_CHILD_DATA | BDRV_CHILD_FILTERED | BDRV_CHILD_COW)) { - bdrv_refresh_limits(c->bs, tran, errp); - if (*errp) { - return; - } bdrv_merge_limits(&bs->bl, &c->bs->bl); have_limits = true; }
bdrv_refresh_limits() recurses down to the node's children. That does not seem necessary: When we refresh limits on some node, and then recurse down and were to change one of its children's BlockLimits, then that would mean we noticed the changed limits by pure chance. The fact that we refresh the parent's limits has nothing to do with it, so the reason for the change probably happened before this point in time, and we should have refreshed the limits then. On the other hand, we do not have infrastructure for noticing that block limits change after they have been initialized for the first time (this would require propagating the change upwards to the respective node's parents), and so evidently we consider this case impossible. If this case is impossible, then we will not need to recurse down in bdrv_refresh_limits(). Every node's limits are initialized in bdrv_open_driver(), and are refreshed whenever its children change. We want to use the childrens' limits to get some initial default, but we can just take them, we do not need to refresh them. The problem with recursing is that bdrv_refresh_limits() is not atomic. It begins with zeroing BDS.bl, and only then sets proper, valid limits. If we do not drain all nodes whose limits are refreshed, then concurrent I/O requests can encounter invalid request_alignment values and crash qemu. Therefore, a recursing bdrv_refresh_limits() requires the whole subtree to be drained, which is currently not ensured by most callers. A non-recursive bdrv_refresh_limits() only requires the node in question to not receive I/O requests, and this is done by most callers in some way or another: - bdrv_open_driver() deals with a new node with no parents yet - bdrv_set_file_or_backing_noperm() acts on a drained node - bdrv_reopen_commit() acts only on drained nodes - bdrv_append() should in theory require the node to be drained; in practice most callers just lock the AioContext, which should at least be enough to prevent concurrent I/O requests from accessing invalid limits So we can resolve the bug by making bdrv_refresh_limits() non-recursive. Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1879437 Signed-off-by: Hanna Reitz <hreitz@redhat.com> --- block/io.c | 4 ---- 1 file changed, 4 deletions(-)