* [PATCH] Btrfs: fix deadlock when throttling transactions
@ 2011-07-14 17:26 Josef Bacik
2011-07-15 2:56 ` liubo
0 siblings, 1 reply; 3+ messages in thread
From: Josef Bacik @ 2011-07-14 17:26 UTC (permalink / raw)
To: linux-btrfs
Hit this nice little deadlock. What happens is this
__btrfs_end_transaction with throttle set, --use_count so it equals 0
btrfs_commit_transaction
<somebody else actually manages to start the commit>
btrfs_end_transaction --use_count so now its -1 <== BAD
we just return and wait on the transaction
This is bad because we just return after our use_count is -1 and don't let go
of our num_writer count on the transaction, so the guy committing the
transaction just sits there forever. Fix this by inc'ing our use_count if we're
going to call commit_transaction so that if we call btrfs_end_transaction it's
valid. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
---
fs/btrfs/transaction.c | 13 ++++++++++---
1 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 654755b..00b81fb5 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -497,10 +497,17 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
}
if (lock && cur_trans->blocked && !cur_trans->in_commit) {
- if (throttle)
+ if (throttle) {
+ /*
+ * We may race with somebody else here so end up having
+ * to call end_transaction on ourselves again, so inc
+ * our use_count.
+ */
+ trans->use_count++;
return btrfs_commit_transaction(trans, root);
- else
+ } else {
wake_up_process(info->transaction_kthread);
+ }
}
WARN_ON(cur_trans != info->running_transaction);
@@ -1225,7 +1232,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
if (cur_trans->in_commit) {
spin_unlock(&cur_trans->commit_lock);
atomic_inc(&cur_trans->use_count);
- btrfs_end_transaction(trans, root);
+ __btrfs_end_transaction(trans, root, 0, 1);
ret = wait_for_commit(root, cur_trans);
BUG_ON(ret);
--
1.7.5.2
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH] Btrfs: fix deadlock when throttling transactions
2011-07-14 17:26 [PATCH] Btrfs: fix deadlock when throttling transactions Josef Bacik
@ 2011-07-15 2:56 ` liubo
2011-07-15 13:54 ` Josef Bacik
0 siblings, 1 reply; 3+ messages in thread
From: liubo @ 2011-07-15 2:56 UTC (permalink / raw)
To: Josef Bacik; +Cc: linux-btrfs
On 07/15/2011 01:26 AM, Josef Bacik wrote:
> Hit this nice little deadlock. What happens is this
>
> __btrfs_end_transaction with throttle set, --use_count so it equals 0
> btrfs_commit_transaction
> <somebody else actually manages to start the commit>
> btrfs_end_transaction --use_count so now its -1 <== BAD
> we just return and wait on the transaction
>
> This is bad because we just return after our use_count is -1 and don't let go
> of our num_writer count on the transaction, so the guy committing the
> transaction just sits there forever. Fix this by inc'ing our use_count if we're
> going to call commit_transaction so that if we call btrfs_end_transaction it's
> valid. Thanks,
>
> Signed-off-by: Josef Bacik <josef@redhat.com>
> ---
> fs/btrfs/transaction.c | 13 ++++++++++---
> 1 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index 654755b..00b81fb5 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -497,10 +497,17 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
> }
>
> if (lock && cur_trans->blocked && !cur_trans->in_commit) {
> - if (throttle)
> + if (throttle) {
> + /*
> + * We may race with somebody else here so end up having
> + * to call end_transaction on ourselves again, so inc
> + * our use_count.
> + */
> + trans->use_count++;
> return btrfs_commit_transaction(trans, root);
> - else
> + } else {
> wake_up_process(info->transaction_kthread);
> + }
> }
>
> WARN_ON(cur_trans != info->running_transaction);
> @@ -1225,7 +1232,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
> if (cur_trans->in_commit) {
> spin_unlock(&cur_trans->commit_lock);
> atomic_inc(&cur_trans->use_count);
> - btrfs_end_transaction(trans, root);
> + __btrfs_end_transaction(trans, root, 0, 1);
>
Looks good.
BTW, btrfs_end_transaction(trans, root) is just __btrfs_end_transaction(trans, root, 0, 1).
thanks,
liubo
> ret = wait_for_commit(root, cur_trans);
> BUG_ON(ret);
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH] Btrfs: fix deadlock when throttling transactions
2011-07-15 2:56 ` liubo
@ 2011-07-15 13:54 ` Josef Bacik
0 siblings, 0 replies; 3+ messages in thread
From: Josef Bacik @ 2011-07-15 13:54 UTC (permalink / raw)
To: liubo; +Cc: linux-btrfs
On 07/14/2011 10:56 PM, liubo wrote:
> On 07/15/2011 01:26 AM, Josef Bacik wrote:
>> Hit this nice little deadlock. What happens is this
>>
>> __btrfs_end_transaction with throttle set, --use_count so it equals 0
>> btrfs_commit_transaction
>> <somebody else actually manages to start the commit>
>> btrfs_end_transaction --use_count so now its -1 <== BAD
>> we just return and wait on the transaction
>>
>> This is bad because we just return after our use_count is -1 and don't let go
>> of our num_writer count on the transaction, so the guy committing the
>> transaction just sits there forever. Fix this by inc'ing our use_count if we're
>> going to call commit_transaction so that if we call btrfs_end_transaction it's
>> valid. Thanks,
>>
>> Signed-off-by: Josef Bacik <josef@redhat.com>
>> ---
>> fs/btrfs/transaction.c | 13 ++++++++++---
>> 1 files changed, 10 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
>> index 654755b..00b81fb5 100644
>> --- a/fs/btrfs/transaction.c
>> +++ b/fs/btrfs/transaction.c
>> @@ -497,10 +497,17 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
>> }
>>
>> if (lock && cur_trans->blocked && !cur_trans->in_commit) {
>> - if (throttle)
>> + if (throttle) {
>> + /*
>> + * We may race with somebody else here so end up having
>> + * to call end_transaction on ourselves again, so inc
>> + * our use_count.
>> + */
>> + trans->use_count++;
>> return btrfs_commit_transaction(trans, root);
>> - else
>> + } else {
>> wake_up_process(info->transaction_kthread);
>> + }
>> }
>>
>> WARN_ON(cur_trans != info->running_transaction);
>> @@ -1225,7 +1232,7 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
>> if (cur_trans->in_commit) {
>> spin_unlock(&cur_trans->commit_lock);
>> atomic_inc(&cur_trans->use_count);
>> - btrfs_end_transaction(trans, root);
>> + __btrfs_end_transaction(trans, root, 0, 1);
>>
>
> Looks good.
>
> BTW, btrfs_end_transaction(trans, root) is just __btrfs_end_transaction(trans, root, 0, 1).
>
Oops you're right, I saw the 1 for lock and thought it was for throttle.
Thanks,
Josef
^ permalink raw reply [flat|nested] 3+ messages in thread
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2011-07-14 17:26 [PATCH] Btrfs: fix deadlock when throttling transactions Josef Bacik
2011-07-15 2:56 ` liubo
2011-07-15 13:54 ` Josef Bacik
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