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, 3 insertions, 0 deletions

diff --git a/include/sys/dsl_synctask.h b/include/sys/dsl_synctask.h
index da6c7a40d..957963ffe 100644
--- a/include/sys/dsl_synctask.h
+++ b/include/sys/dsl_synctask.h
@@ -37,6 +37,7 @@ struct dsl_pool;
 
 typedef int (dsl_checkfunc_t)(void *, dmu_tx_t *);
 typedef void (dsl_syncfunc_t)(void *, dmu_tx_t *);
+typedef void (dsl_sigfunc_t)(void *, dmu_tx_t *);
 
 typedef enum zfs_space_check {
 	/*
@@ -116,6 +117,8 @@ int dsl_early_sync_task(const char *, dsl_checkfunc_t *,
     dsl_syncfunc_t *, void *, int, zfs_space_check_t);
 void dsl_early_sync_task_nowait(struct dsl_pool *, dsl_syncfunc_t *,
     void *, int, zfs_space_check_t, dmu_tx_t *);
+int dsl_sync_task_sig(const char *, dsl_checkfunc_t *, dsl_syncfunc_t *,
+    dsl_sigfunc_t *, void *, int, zfs_space_check_t);
 
 #ifdef	__cplusplus
 }