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 <don.brady@delphix.com>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: John Gallagher <john.gallagher@delphix.com>
External-issue: LX-1211
Closes #7819

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);
 

include/sys/zfs_context.h

@@ -62,6 +62,7 @@
 #include <sys/ctype.h>
 #include <sys/disp.h>
 #include <sys/trace.h>
+#include <sys/procfs_list.h>
 #include <linux/dcache_compat.h>
 #include <linux/utsname_compat.h>
 
@@ -351,6 +352,37 @@ extern void kstat_set_raw_ops(kstat_t *ksp,
     int (*data)(char *buf, size_t size, void *data),
     void *(*addr)(kstat_t *ksp, loff_t index));
 
+/*
+ * procfs list manipulation
+ */
+
+struct seq_file { };
+void seq_printf(struct seq_file *m, const char *fmt, ...);
+
+typedef struct procfs_list {
+	void		*pl_private;
+	kmutex_t	pl_lock;
+	list_t		pl_list;
+	uint64_t	pl_next_id;
+	size_t		pl_node_offset;
+} procfs_list_t;
+
+typedef struct procfs_list_node {
+	list_node_t	pln_link;
+	uint64_t	pln_id;
+} procfs_list_node_t;
+
+void procfs_list_install(const char *module,
+    const char *name,
+    procfs_list_t *procfs_list,
+    int (*show)(struct seq_file *f, void *p),
+    int (*show_header)(struct seq_file *f),
+    int (*clear)(procfs_list_t *procfs_list),
+    size_t procfs_list_node_off);
+void procfs_list_uninstall(procfs_list_t *procfs_list);
+void procfs_list_destroy(procfs_list_t *procfs_list);
+void procfs_list_add(procfs_list_t *procfs_list, void *p);
+
 /*
  * Kernel memory
  */

include/sys/zfs_debug.h

@@ -76,13 +76,6 @@ extern void __dprintf(const char *file, const char *func,
 
 extern void zfs_panic_recover(const char *fmt, ...);
 
-typedef struct zfs_dbgmsg {
-	list_node_t zdm_node;
-	time_t zdm_timestamp;
-	int zdm_size;
-	char zdm_msg[1]; /* variable length allocation */
-} zfs_dbgmsg_t;
-
 extern void zfs_dbgmsg_init(void);
 extern void zfs_dbgmsg_fini(void);