OpenZFS 9425 - channel programs can be interrupted

Problem Statement
=================
ZFS Channel program scripts currently require a timeout, so that hung or
long-running scripts return a timeout error instead of causing ZFS to get
wedged. This limit can currently be set up to 100 million Lua instructions.
Even with a limit in place, it would be desirable to have a sys admin
(support engineer) be able to cancel a script that is taking a long time.

Proposed Solution
=================
Make it possible to abort a channel program by sending an interrupt signal.In
the underlying txg_wait_sync function, switch the cv_wait to a cv_wait_sig to
catch the signal. Once a signal is encountered, the dsl_sync_task function can
install a Lua hook that will get called before the Lua interpreter executes a
new line of code. The dsl_sync_task can resume with a standard txg_wait_sync
call and wait for the txg to complete.  Meanwhile, the hook will abort the
script and indicate that the channel program was canceled. The kernel returns
a EINTR to indicate that the channel program run was canceled.

Porting notes: Added missing return value from cv_wait_sig()

Authored by: Don Brady <[email protected]>
Reviewed by: Sebastien Roy <[email protected]>
Reviewed by: Serapheim Dimitropoulos <[email protected]>
Reviewed by: Matt Ahrens <[email protected]>
Reviewed by: Sara Hartse <[email protected]>
Reviewed by: Brian Behlendorf <[email protected]>
Approved by: Robert Mustacchi <[email protected]>
Ported-by: Don Brady <[email protected]>
Signed-off-by: Don Brady <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/9425
OpenZFS-commit: https://github.com/illumos/illumos-gate/commit/d0cb1fb926
Closes #8904 

1 files changed, 21 insertions, 3 deletions

diff --git a/module/zfs/dsl_synctask.c b/module/zfs/dsl_synctask.c
index b63ce5cad..b225eed37 100644
--- a/module/zfs/dsl_synctask.c
+++ b/module/zfs/dsl_synctask.c
@@ -41,7 +41,7 @@ dsl_null_checkfunc(void *arg, dmu_tx_t *tx)
 
 static int
 dsl_sync_task_common(const char *pool, dsl_checkfunc_t *checkfunc,
-    dsl_syncfunc_t *syncfunc, void *arg,
+    dsl_syncfunc_t *syncfunc, dsl_sigfunc_t *sigfunc, void *arg,
     int blocks_modified, zfs_space_check_t space_check, boolean_t early)
 {
 	spa_t *spa;
@@ -85,6 +85,11 @@ top:
 
 	dmu_tx_commit(tx);
 
+	if (sigfunc != NULL && txg_wait_synced_sig(dp, dst.dst_txg)) {
+		/* current contract is to call func once */
+		sigfunc(arg, tx);
+		sigfunc = NULL;	/* in case we're performing an EAGAIN retry */
+	}
 	txg_wait_synced(dp, dst.dst_txg);
 
 	if (dst.dst_error == EAGAIN) {
@@ -124,7 +129,7 @@ dsl_sync_task(const char *pool, dsl_checkfunc_t *checkfunc,
     dsl_syncfunc_t *syncfunc, void *arg,
     int blocks_modified, zfs_space_check_t space_check)
 {
-	return (dsl_sync_task_common(pool, checkfunc, syncfunc, arg,
+	return (dsl_sync_task_common(pool, checkfunc, syncfunc, NULL, arg,
 	    blocks_modified, space_check, B_FALSE));
 }
 
@@ -146,10 +151,23 @@ dsl_early_sync_task(const char *pool, dsl_checkfunc_t *checkfunc,
     dsl_syncfunc_t *syncfunc, void *arg,
     int blocks_modified, zfs_space_check_t space_check)
 {
-	return (dsl_sync_task_common(pool, checkfunc, syncfunc, arg,
+	return (dsl_sync_task_common(pool, checkfunc, syncfunc, NULL, arg,
 	    blocks_modified, space_check, B_TRUE));
 }
 
+/*
+ * A standard synctask that can be interrupted from a signal. The sigfunc
+ * is called once if a signal occurred while waiting for the task to sync.
+ */
+int
+dsl_sync_task_sig(const char *pool, dsl_checkfunc_t *checkfunc,
+    dsl_syncfunc_t *syncfunc, dsl_sigfunc_t *sigfunc, void *arg,
+    int blocks_modified, zfs_space_check_t space_check)
+{
+	return (dsl_sync_task_common(pool, checkfunc, syncfunc, sigfunc, arg,
+	    blocks_modified, space_check, B_FALSE));
+}
+
 static void
 dsl_sync_task_nowait_common(dsl_pool_t *dp, dsl_syncfunc_t *syncfunc, void *arg,
     int blocks_modified, zfs_space_check_t space_check, dmu_tx_t *tx,