Fixes for procfs files backed by linked lists

There are some issues with the way the seq_file interface is implemented
for kstats backed by linked lists (zfs_dbgmsgs and certain per-pool
debugging info):

* We don't account for the fact that seq_file sometimes visits a node
  multiple times, which results in missing messages when read through
  procfs.
* We don't keep separate state for each reader of a file, so concurrent
  readers will receive incorrect results.
* We don't account for the fact that entries may have been removed from
  the list between read syscalls, so reading from these files in procfs
  can cause the system to crash.

This change fixes these issues and adds procfs_list, a wrapper around a
linked list which abstracts away the details of implementing the
seq_file interface for a list and exposing the contents of the list
through procfs.

Reviewed by: Don Brady <[email protected]>
Reviewed-by: Serapheim Dimitropoulos <[email protected]>
Reviewed by: Brad Lewis <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: John Gallagher <[email protected]>
External-issue: LX-1211
Closes #7819 

1 files changed, 14 insertions, 9 deletions

diff --git a/include/sys/spa.h b/include/sys/spa.h
index b86c65557..443d835a1 100644
--- a/include/sys/spa.h
+++ b/include/sys/spa.h
@@ -863,22 +863,27 @@ extern boolean_t spa_refcount_zero(spa_t *spa);
 #define	SCL_STATE_ALL	(SCL_STATE | SCL_L2ARC | SCL_ZIO)
 
 /* Historical pool statistics */
-typedef struct spa_stats_history {
+typedef struct spa_history_kstat {
 	kmutex_t		lock;
 	uint64_t		count;
 	uint64_t		size;
 	kstat_t			*kstat;
 	void			*private;
 	list_t			list;
-} spa_stats_history_t;
+} spa_history_kstat_t;
+
+typedef struct spa_history_list {
+	uint64_t		size;
+	procfs_list_t		procfs_list;
+} spa_history_list_t;
 
 typedef struct spa_stats {
-	spa_stats_history_t	read_history;
-	spa_stats_history_t	txg_history;
-	spa_stats_history_t	tx_assign_histogram;
-	spa_stats_history_t	io_history;
-	spa_stats_history_t	mmp_history;
-	spa_stats_history_t	state;		/* pool state */
+	spa_history_list_t	read_history;
+	spa_history_list_t	txg_history;
+	spa_history_kstat_t	tx_assign_histogram;
+	spa_history_kstat_t	io_history;
+	spa_history_list_t	mmp_history;
+	spa_history_kstat_t	state;		/* pool state */
 } spa_stats_t;
 
 typedef enum txg_state {
@@ -911,7 +916,7 @@ extern void spa_tx_assign_add_nsecs(spa_t *spa, uint64_t nsecs);
 extern int spa_mmp_history_set_skip(spa_t *spa, uint64_t mmp_kstat_id);
 extern int spa_mmp_history_set(spa_t *spa, uint64_t mmp_kstat_id, int io_error,
     hrtime_t duration);
-extern void *spa_mmp_history_add(spa_t *spa, uint64_t txg, uint64_t timestamp,
+extern void spa_mmp_history_add(spa_t *spa, uint64_t txg, uint64_t timestamp,
     uint64_t mmp_delay, vdev_t *vd, int label, uint64_t mmp_kstat_id,
     int error);