Re: mkfs.btrfs on 24 disks in parallel crashes kernel

[Ian Kent <raven@themaw.net>]

On Mon, 2010-11-08 at 09:22 +0200, Albert Strasheim wrote:
> Hello all
> 
> I did some experiments on Fedora 14 with 2.6.35.6, running mkfs.btrfs
> followed by a mount in parallel on 24 disks.
> 
> This seems to crash reliably.
> 
> I reported the bug to Fedora here:
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=650261
> 
> The bug report has some stacktraces and stuff attached.
> 
> I saw stuff like this:
> 
> [  203.861368] Btrfs loaded
> [  203.871418] device label myvol-12b0f8ba-e1ef-4d00-aae4-578d59d955e4
> devid 1 transid 7 /dev/sdv
> [  203.981371] device label myvol-fe7fdbcd-8f2f-4b34-aa6b-0d3bb7582ebc
> devid 1 transid 7 /dev/sdl
> [  203.990234] BUG: unable to handle kernel NULL pointer dereference
> at 0000000000000128
> [  204.001153] IP: [<ffffffffa019a010>] btrfs_test_super+0x10/0x26 [btrfs]
> [  204.016840] PGD 0
> [  204.018696] Oops: 0000 [#1] SMP
> [  204.021264] last sysfs file:
> /sys/devices/pci0000:00/0000:00:05.0/0000:0d:00.0/host7/port-7:0/expander-7:0/port-7:0:9/end_device-7:0:9/target7:0:9/7:0:9:0/uevent
> [  204.045966] CPU 0
> [  204.055933] Modules linked in: btrfs zlib_deflate libcrc32c ipv6
> mlx4_ib ib_mad ib_core mlx4_en igb mlx4_core ses iTCO_wdt ioatdma dca
> iTCO_vendor_support i7core_edac edac_core enclosure i2c_i801 i2c_core
> microcode joydev serio_raw mptsas mptscsih mptbase scsi_transport_sas
> [last unloaded: scsi_wait_scan]
> [  204.100672]
> [  204.103025] Pid: 2166, comm: mount Not tainted
> 2.6.35.6-48.fc14.x86_64 #1 ASSY,BLADE,X6270      /SUN BLADE X6270
> SERVER MODULE
> [  204.121895] RIP: 0010:[<ffffffffa019a010>]  [<ffffffffa019a010>]
> btrfs_test_super+0x10/0x26 [btrfs]
> [  204.139279] RSP: 0018:ffff8803768ddcd8  EFLAGS: 00010287
> [  204.144675] RAX: 0000000000000000 RBX: ffff8803772df800 RCX: ffff8801f7d34480
> [  204.160885] RDX: 0000000000000120 RSI: ffff8801f7d34480 RDI: ffff8803772df800
> [  204.179473] RBP: ffff8803768ddcd8 R08: ffff8801f7d344f8 R09: ffff880375c78760
> [  204.185673] R10: ffff8803768ddb68 R11: ffff8801f7d34480 R12: ffffffffa01f13d0
> [  204.200174] R13: ffffffffa019a000 R14: ffff8801f7d34480 R15: ffffffffa01f1400
> [  204.221347] FS:  00007f766050d800(0000) GS:ffff880022200000(0000)
> knlGS:0000000000000000
> [  204.226597] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [  204.238144] CR2: 0000000000000128 CR3: 00000001f7504000 CR4: 00000000000006f0
> [  204.256446] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [  204.263975] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
> [  204.280097] Process mount (pid: 2166, threadinfo ffff8803768dc000,
> task ffff880376ab8000)
> [  204.296529] Stack:
> [  204.298975]  ffff8803768ddd38 ffffffff81118df6 ffffffffa01f13d0
> 0000000000000000
> [  204.305955] <0> ffffffff811184a7 0000000000000000 ffff8803768ddd38
> 0000000000000003
> [  204.320999] <0> ffffffffa01f13d0 0000000000000000 ffff880375c78680
> ffff8801f4f59d00
> [  204.338658] Call Trace:
> [  204.341138]  [<ffffffff81118df6>] sget+0x54/0x367
> [  204.355474]  [<ffffffff811184a7>] ? set_anon_super+0x0/0xe7
> [  204.360487]  [<ffffffffa019a821>] btrfs_get_sb+0x108/0x3eb [btrfs]
> [  204.375799]  [<ffffffff81118b99>] vfs_kern_mount+0xad/0x1ac
> [  204.379952]  [<ffffffff81118d00>] do_kern_mount+0x4d/0xef
> [  204.395757]  [<ffffffff8112e45a>] do_mount+0x700/0x75d
> [  204.400676]  [<ffffffff810e5aec>] ? strndup_user+0x54/0x85
> [  204.417219]  [<ffffffff8112e6e7>] sys_mount+0x88/0xc2
> [  204.420399]  [<ffffffff81009cf2>] system_call_fastpath+0x16/0x1b
> [  204.435716] Code: <48> 8b 80 28 01 00 00 48 39 b0 28 22 00 00 c9 0f
> 94 c0 0f b6 c0 c3
> [  204.447369] RIP  [<ffffffffa019a010>] btrfs_test_super+0x10/0x26 [btrfs]
> [  204.458280]  RSP <ffff8803768ddcd8>
> [  204.462330] CR2: 0000000000000128
> [  204.466479] ---[ end trace c8bb842fa664d021 ]---
> [  204.498273] device label myvol-11d50321-7482-4ee2-8da5-5ee5f623ae17
> devid 1 transid 7 /dev/sdk

This looks like a fairly obvious race.
How about some comments on the suitability of this totally untested
patch?

I thought about trying to merge the tests following sget() (those
surrounded by the mutex) into btrfs_fill_super() but that seemed a bit
too hard. Maybe someone can advise on how that should be done if this
approach is not OK.

btrfs - fix race in btrfs_get_sb()

From: Ian Kent <raven@themaw.net>

When mounting a btrfs file system btrfs_test_super() may attempt to
use sb->s_fs_info before it is set. This is because it isn't set until
btrfs_fill_super() is called but the super block creation locks are
dropped earlier during sget().

Also, for the same reason, it looks like there is a possible race
with the s->s_root check which should also be dealt with by this
patch.
---

 fs/btrfs/super.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)


diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 718b10d..9b463b9 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -565,6 +565,8 @@ static int btrfs_test_super(struct super_block *s, void *data)
 	struct btrfs_fs_devices *test_fs_devices = data;
 	struct btrfs_root *root = btrfs_sb(s);
 
+	if (!root)
+		return 0;
 	return root->fs_info->fs_devices == test_fs_devices;
 }
 
@@ -585,6 +587,7 @@ static int btrfs_get_sb(struct file_system_type *fs_type, int flags,
 	char *subvol_name = NULL;
 	u64 subvol_objectid = 0;
 	int error = 0;
+	DEFINE_MUTEX(super_mutex);
 
 	if (!(flags & MS_RDONLY))
 		mode |= FMODE_WRITE;
@@ -613,8 +616,10 @@ static int btrfs_get_sb(struct file_system_type *fs_type, int flags,
 	if (IS_ERR(s))
 		goto error_s;
 
+	mutex_lock(&super_mutex);
 	if (s->s_root) {
 		if ((flags ^ s->s_flags) & MS_RDONLY) {
+			mutex_unlock(&super_mutex);
 			deactivate_locked_super(s);
 			error = -EBUSY;
 			goto error_close_devices;
@@ -629,6 +634,7 @@ static int btrfs_get_sb(struct file_system_type *fs_type, int flags,
 		error = btrfs_fill_super(s, fs_devices, data,
 					 flags & MS_SILENT ? 1 : 0);
 		if (error) {
+			mutex_unlock(&super_mutex);
 			deactivate_locked_super(s);
 			goto error_free_subvol_name;
 		}
@@ -636,6 +642,7 @@ static int btrfs_get_sb(struct file_system_type *fs_type, int flags,
 		btrfs_sb(s)->fs_info->bdev_holder = fs_type;
 		s->s_flags |= MS_ACTIVE;
 	}
+	mutex_unlock(&super_mutex);
 
 	root = get_default_root(s, subvol_objectid);
 	if (IS_ERR(root)) {