async zvol minor node creation interferes with receive

When we finish a zfs receive, dmu_recv_end_sync() calls
zvol_create_minors(async=TRUE).  This kicks off some other threads that
create the minor device nodes (in /dev/zvol/poolname/...).  These async
threads call zvol_prefetch_minors_impl() and zvol_create_minor(), which
both call dmu_objset_own(), which puts a "long hold" on the dataset.
Since the zvol minor node creation is asynchronous, this can happen
after the `ZFS_IOC_RECV[_NEW]` ioctl and `zfs receive` process have
completed.

After the first receive ioctl has completed, userland may attempt to do
another receive into the same dataset (e.g. the next incremental
stream).  This second receive and the asynchronous minor node creation
can interfere with one another in several different ways, because they
both require exclusive access to the dataset:

1. When the second receive is finishing up, dmu_recv_end_check() does
dsl_dataset_handoff_check(), which can fail with EBUSY if the async
minor node creation already has a "long hold" on this dataset.  This
causes the 2nd receive to fail.

2. The async udev rule can fail if zvol_id and/or systemd-udevd try to
open the device while the the second receive's async attempt at minor
node creation owns the dataset (via zvol_prefetch_minors_impl).  This
causes the minor node (/dev/zd*) to exist, but the udev-generated
/dev/zvol/... to not exist.

3. The async minor node creation can silently fail with EBUSY if the
first receive's zvol_create_minor() trys to own the dataset while the
second receive's zvol_prefetch_minors_impl already owns the dataset.

To address these problems, this change synchronously creates the minor
node.  To avoid the lock ordering problems that the asynchrony was
introduced to fix (see #3681), we create the minor nodes from open
context, with no locks held, rather than from syncing contex as was
originally done.

Implementation notes:

We generally do not need to traverse children or prefetch anything (e.g.
when running the recv, snapshot, create, or clone subcommands of zfs).
We only need recursion when importing/opening a pool and when loading
encryption keys.  The existing recursive, asynchronous, prefetching code
is preserved for use in these cases.

Channel programs may need to create zvol minor nodes, when creating a
snapshot of a zvol with the snapdev property set.  We figure out what
snapshots are created when running the LUA program in syncing context.
In this case we need to remember what snapshots were created, and then
try to create their minor nodes from open context, after the LUA code
has completed.

There are additional zvol use cases that asynchronously own the dataset,
which can cause similar problems.  E.g. changing the volmode or snapdev
properties.  These are less problematic because they are not recursive
and don't touch datasets that are not involved in the operation, there
is still potential for interference with subsequent operations.  In the
future, these cases should be similarly converted to create the zvol
minor node synchronously from open context.

The async tasks of removing and renaming minors do not own the objset,
so they do not have this problem.  However, it may make sense to also
convert these operations to happen synchronously from open context, in
the future.

Reviewed-by: Paul Dagnelie <[email protected]>
Reviewed-by: Prakash Surya <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
External-issue: DLPX-65948
Closes #7863
Closes #9885

2 files changed, 16 insertions, 8 deletions

diff --git a/include/sys/zcp.h b/include/sys/zcp.h
index 5cc520da5..3d1480050 100644
--- a/include/sys/zcp.h
+++ b/include/sys/zcp.h
@@ -132,6 +132,14 @@ typedef struct zcp_run_info {
 	nvlist_t	*zri_outnvl;
 
 	/*
+	 * The keys of this nvlist are datasets which may be zvols and may need
+	 * to have device minor nodes created.  This information is passed from
+	 * syncing context (where the zvol is created) to open context (where we
+	 * create the minor nodes).
+	 */
+	nvlist_t	*zri_new_zvols;
+
+	/*
 	 * The errno number returned to caller of zcp_eval().
 	 */
 	int		zri_result;
diff --git a/include/sys/zvol.h b/include/sys/zvol.h
index 7852838f8..8efb7f5e6 100644
--- a/include/sys/zvol.h
+++ b/include/sys/zvol.h
@@ -35,20 +35,20 @@
 #define	SPEC_MAXOFFSET_T	((1LL << ((NBBY * sizeof (daddr32_t)) + \
 				DEV_BSHIFT - 1)) - 1)
 
-extern void zvol_create_minors(spa_t *spa, const char *name, boolean_t async);
-extern void zvol_remove_minors(spa_t *spa, const char *name, boolean_t async);
-extern void zvol_rename_minors(spa_t *spa, const char *oldname,
-    const char *newname, boolean_t async);
+extern void zvol_create_minor(const char *);
+extern void zvol_create_minors_recursive(const char *);
+extern void zvol_remove_minors(spa_t *, const char *, boolean_t);
+extern void zvol_rename_minors(spa_t *, const char *, const char *, boolean_t);
 
 #ifdef _KERNEL
 struct zvol_state;
 typedef struct zvol_state zvol_state_handle_t;
 
-extern int zvol_check_volsize(uint64_t volsize, uint64_t blocksize);
-extern int zvol_check_volblocksize(const char *name, uint64_t volblocksize);
-extern int zvol_get_stats(objset_t *os, nvlist_t *nv);
+extern int zvol_check_volsize(uint64_t, uint64_t);
+extern int zvol_check_volblocksize(const char *, uint64_t);
+extern int zvol_get_stats(objset_t *, nvlist_t *);
 extern boolean_t zvol_is_zvol(const char *);
-extern void zvol_create_cb(objset_t *os, void *arg, cred_t *cr, dmu_tx_t *tx);
+extern void zvol_create_cb(objset_t *, void *, cred_t *, dmu_tx_t *);
 extern int zvol_set_volsize(const char *, uint64_t);
 extern int zvol_set_volblocksize(const char *, uint64_t);
 extern int zvol_set_snapdev(const char *, zprop_source_t, uint64_t);