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Subject[PATCH] dm thin: Initialize refcount and completion earlier
Date
The commit 80e96c5484be (dm thin: do not allow thin device activation
while pool is suspended) delayed the initialization of the completion
and setting the initial refcount to 1 until after the new thin is
added to the pool's active thins list and the pool lock is released.
This opens a race with a worker thread that walks the list and calls
thin_get/put, noticing that the refcount goes to 0 and calling
complete, freezing up the system and giving the oops below:

kernel: BUG: unable to handle kernel NULL pointer dereference at (null)
kernel: IP: [<ffffffff810d360b>] __wake_up_common+0x2b/0x90

kernel: Call Trace:
kernel: [<ffffffff810d3683>] __wake_up_locked+0x13/0x20
kernel: [<ffffffff810d3dc7>] complete+0x37/0x50
kernel: [<ffffffffa0595c50>] thin_put+0x20/0x30 [dm_thin_pool]
kernel: [<ffffffffa059aab7>] do_worker+0x667/0x870 [dm_thin_pool]
kernel: [<ffffffff816a8a4c>] ? __schedule+0x3ac/0x9a0
kernel: [<ffffffff810b1aef>] process_one_work+0x14f/0x400
kernel: [<ffffffff810b206b>] worker_thread+0x6b/0x490
kernel: [<ffffffff810b2000>] ? rescuer_thread+0x260/0x260
kernel: [<ffffffff810b6a7b>] kthread+0xdb/0x100
kernel: [<ffffffff810b69a0>] ? kthread_create_on_node+0x170/0x170
kernel: [<ffffffff816ad7ec>] ret_from_fork+0x7c/0xb0
kernel: [<ffffffff810b69a0>] ? kthread_create_on_node+0x170/0x170

Set the initial refcount and initialize the completion
before dropping the pool lock.

Signed-off-by: Marc Dionne <marc.dionne@your-file-system.com>
---
drivers/md/dm-thin.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index 8735543..4133223 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -3814,6 +3814,8 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)
r = -EINVAL;
goto bad;
}
+ atomic_set(&tc->refcount, 1);
+ init_completion(&tc->can_destroy);
list_add_tail_rcu(&tc->list, &tc->pool->active_thins);
spin_unlock_irqrestore(&tc->pool->lock, flags);
/*
@@ -3826,9 +3828,6 @@ static int thin_ctr(struct dm_target *ti, unsigned argc, char **argv)

dm_put(pool_md);

- atomic_set(&tc->refcount, 1);
- init_completion(&tc->can_destroy);
-
return 0;

bad:
--
2.1.0


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