md/raid5: Use conf->device_lock protect changing of multi-thread resources.

When we change group_thread_cnt from sysfs entry, it can OOPS.

The kernel messages are:
[  135.299021] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  135.299073] IP: [<ffffffff815188ab>] handle_active_stripes+0x32b/0x440
[  135.299107] PGD 0
[  135.299122] Oops: 0000 [#1] SMP
[  135.299144] Modules linked in: netconsole e1000e ptp pps_core
[  135.299188] CPU: 3 PID: 2225 Comm: md0_raid5 Not tainted 3.12.0+ #24
[  135.299214] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./To be filled by O.E.M., BIOS 080015  11/09/2011
[  135.299255] task: ffff8800b9638f80 ti: ffff8800b77a4000 task.ti: ffff8800b77a4000
[  135.299283] RIP: 0010:[<ffffffff815188ab>]  [<ffffffff815188ab>] handle_active_stripes+0x32b/0x440
[  135.299323] RSP: 0018:ffff8800b77a5c48  EFLAGS: 00010002
[  135.299344] RAX: ffff880037bb5c70 RBX: 0000000000000000 RCX: 0000000000000008
[  135.299371] RDX: ffff880037bb5cb8 RSI: 0000000000000001 RDI: ffff880037bb5c00
[  135.299398] RBP: ffff8800b77a5d08 R08: 0000000000000001 R09: 0000000000000000
[  135.299425] R10: ffff8800b77a5c98 R11: 00000000ffffffff R12: ffff880037bb5c00
[  135.299452] R13: 0000000000000000 R14: 0000000000000000 R15: ffff880037bb5c70
[  135.299479] FS:  0000000000000000(0000) GS:ffff88013fd80000(0000) knlGS:0000000000000000
[  135.299510] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  135.299532] CR2: 0000000000000000 CR3: 0000000001c0b000 CR4: 00000000000407e0
[  135.299559] Stack:
[  135.299570]  ffff8800b77a5c88 ffffffff8107383e ffff8800b77a5c88 ffff880037a64300
[  135.299611]  000000000000ec08 ffff880037bb5cb8 ffff8800b77a5c98 ffffffffffffffd8
[  135.299654]  000000000000ec08 ffff880037bb5c60 ffff8800b77a5c98 ffff8800b77a5c98
[  135.299696] Call Trace:
[  135.299711]  [<ffffffff8107383e>] ? __wake_up+0x4e/0x70
[  135.299733]  [<ffffffff81518f88>] raid5d+0x4c8/0x680
[  135.299756]  [<ffffffff817174ed>] ? schedule_timeout+0x15d/0x1f0
[  135.299781]  [<ffffffff81524c9f>] md_thread+0x11f/0x170
[  135.299804]  [<ffffffff81069cd0>] ? wake_up_bit+0x40/0x40
[  135.299826]  [<ffffffff81524b80>] ? md_rdev_init+0x110/0x110
[  135.299850]  [<ffffffff81069656>] kthread+0xc6/0xd0
[  135.299871]  [<ffffffff81069590>] ? kthread_freezable_should_stop+0x70/0x70
[  135.299899]  [<ffffffff81722ffc>] ret_from_fork+0x7c/0xb0
[  135.299923]  [<ffffffff81069590>] ? kthread_freezable_should_stop+0x70/0x70
[  135.299951] Code: ff ff ff 0f 84 d7 fe ff ff e9 5c fe ff ff 66 90 41 8b b4 24 d8 01 00 00 45 31 ed 85 f6 0f 8e 7b fd ff ff 49 8b 9c 24 d0 01 00 00 <48> 3b 1b 49 89 dd 0f 85 67 fd ff ff 48 8d 43 28 31 d2 eb 17 90
[  135.300005] RIP  [<ffffffff815188ab>] handle_active_stripes+0x32b/0x440
[  135.300005]  RSP <ffff8800b77a5c48>
[  135.300005] CR2: 0000000000000000
[  135.300005] ---[ end trace 504854e5bb7562ed ]---
[  135.300005] Kernel panic - not syncing: Fatal exception

This is because raid5d() can be running when the multi-thread
resources are changed via system. We see need to provide locking.

mddev->device_lock is suitable, but we cannot simple call
alloc_thread_groups under this lock as we cannot allocate memory
while holding a spinlock.
So change alloc_thread_groups() to allocate and return the data
structures, then raid5_store_group_thread_cnt() can take the lock
while updating the pointers to the data structures.

This fixes a bug introduced in 3.12 and so is suitable for the 3.12.x
stable series.

Fixes: b721420e
Cc: stable@vger.kernel.org (3.12)
Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Shaohua Li <shli@kernel.org>

drivers/md/raid5.c

@@ -5387,15 +5387,18 @@ raid5_show_group_thread_cnt(struct mddev *mddev, char *page)
 		return 0;
 }
 
-static int alloc_thread_groups(struct r5conf *conf, int cnt);
+static int alloc_thread_groups(struct r5conf *conf, int cnt,
+			       int *group_cnt,
+			       int *worker_cnt_per_group,
+			       struct r5worker_group **worker_groups);
 static ssize_t
 raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 {
 	struct r5conf *conf = mddev->private;
 	unsigned long new;
 	int err;
-	struct r5worker_group *old_groups;
-	int old_group_cnt;
+	struct r5worker_group *new_groups, *old_groups;
+	int group_cnt, worker_cnt_per_group;
 
 	if (len >= PAGE_SIZE)
 		return -EINVAL;
@@ -5411,17 +5414,19 @@ raid5_store_group_thread_cnt(struct mddev *mddev, const char *page, size_t len)
 	mddev_suspend(mddev);
 
 	old_groups = conf->worker_groups;
-	old_group_cnt = conf->worker_cnt_per_group;
-
 	if (old_groups)
 		flush_workqueue(raid5_wq);
 
-	conf->worker_groups = NULL;
-	err = alloc_thread_groups(conf, new);
-	if (err) {
-		conf->worker_groups = old_groups;
-		conf->worker_cnt_per_group = old_group_cnt;
-	} else {
+	err = alloc_thread_groups(conf, new,
+				  &group_cnt, &worker_cnt_per_group,
+				  &new_groups);
+	if (!err) {
+		spin_lock_irq(&conf->device_lock);
+		conf->group_cnt = group_cnt;
+		conf->worker_cnt_per_group = worker_cnt_per_group;
+		conf->worker_groups = new_groups;
+		spin_unlock_irq(&conf->device_lock);
+
 		if (old_groups)
 			kfree(old_groups[0].workers);
 		kfree(old_groups);
@@ -5451,33 +5456,36 @@ static struct attribute_group raid5_attrs_group = {
 	.attrs = raid5_attrs,
 };
 
-static int alloc_thread_groups(struct r5conf *conf, int cnt)
+static int alloc_thread_groups(struct r5conf *conf, int cnt,
+			       int *group_cnt,
+			       int *worker_cnt_per_group,
+			       struct r5worker_group **worker_groups)
 {
 	int i, j, k;
 	ssize_t size;
 	struct r5worker *workers;
 
-	conf->worker_cnt_per_group = cnt;
+	*worker_cnt_per_group = cnt;
 	if (cnt == 0) {
-		conf->worker_groups = NULL;
+		*group_cnt = 0;
+		*worker_groups = NULL;
 		return 0;
 	}
-	conf->group_cnt = num_possible_nodes();
+	*group_cnt = num_possible_nodes();
 	size = sizeof(struct r5worker) * cnt;
-	workers = kzalloc(size * conf->group_cnt, GFP_NOIO);
-	conf->worker_groups = kzalloc(sizeof(struct r5worker_group) *
-				conf->group_cnt, GFP_NOIO);
-	if (!conf->worker_groups || !workers) {
+	workers = kzalloc(size * *group_cnt, GFP_NOIO);
+	*worker_groups = kzalloc(sizeof(struct r5worker_group) *
+				*group_cnt, GFP_NOIO);
+	if (!*worker_groups || !workers) {
 		kfree(workers);
-		kfree(conf->worker_groups);
-		conf->worker_groups = NULL;
+		kfree(*worker_groups);
 		return -ENOMEM;
 	}
 
-	for (i = 0; i < conf->group_cnt; i++) {
+	for (i = 0; i < *group_cnt; i++) {
 		struct r5worker_group *group;
 
-		group = &conf->worker_groups[i];
+		group = worker_groups[i];
 		INIT_LIST_HEAD(&group->handle_list);
 		group->conf = conf;
 		group->workers = workers + i * cnt;
@@ -5640,6 +5648,8 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 	struct disk_info *disk;
 	char pers_name[6];
 	int i;
+	int group_cnt, worker_cnt_per_group;
+	struct r5worker_group *new_group;
 
 	if (mddev->new_level != 5
 	    && mddev->new_level != 4
@@ -5674,7 +5684,12 @@ static struct r5conf *setup_conf(struct mddev *mddev)
 	if (conf == NULL)
 		goto abort;
 	/* Don't enable multi-threading by default*/
-	if (alloc_thread_groups(conf, 0))
+	if (!alloc_thread_groups(conf, 0, &group_cnt, &worker_cnt_per_group,
+				 &new_group)) {
+		conf->group_cnt = group_cnt;
+		conf->worker_cnt_per_group = worker_cnt_per_group;
+		conf->worker_groups = new_group;
+	} else
 		goto abort;
 	spin_lock_init(&conf->device_lock);
 	seqcount_init(&conf->gen_lock);