[Ocfs2-devel] [PATCH] ocfs2/dlm: remove lockres from purge when a lock is added

[Srinivas Eeda]

Wengang, thanks for the patch. My comments are inline :)

On 6/8/2010 7:38 AM, Wengang Wang wrote:
> dlm_thread(when purges a lockres) races with another thread that is running on
> dlmlock_remote(). dlmlock_remote() can add a lock to the blocked list of the
> lockres without taking dlm->spinlock. This can lead a BUG() in dlm_thread because
> of the added lock.
>
> Fix
> We take dlm->spinlock when adding a lock to the blocked list and make sure the
> lockres is not in purge list. If we removed the lockres from purge list, try to
> add it back.
>
> Signed-off-by: Wengang Wang <wen.gang.wang at oracle.com>
> ---
>  fs/ocfs2/dlm/dlmcommon.h |   15 +++++++++++++++
>  fs/ocfs2/dlm/dlmlock.c   |   18 +++++++++++++++---
>  fs/ocfs2/dlm/dlmthread.c |   28 ++++++++++++++++++++++++++++
>  3 files changed, 58 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ocfs2/dlm/dlmcommon.h b/fs/ocfs2/dlm/dlmcommon.h
> index 4b6ae2c..134973a 100644
> --- a/fs/ocfs2/dlm/dlmcommon.h
> +++ b/fs/ocfs2/dlm/dlmcommon.h
> @@ -1002,6 +1002,9 @@ void dlm_move_lockres_to_recovery_list(struct dlm_ctxt *dlm,
>  
>  /* will exit holding res->spinlock, but may drop in function */
>  void __dlm_wait_on_lockres_flags(struct dlm_lock_resource *res, int flags);
> +void __dlm_wait_on_lockres_flags_lock_dlm(struct dlm_ctxt *dlm,
> +					  struct dlm_lock_resource *res,
> +					  int flags);
>  void __dlm_wait_on_lockres_flags_set(struct dlm_lock_resource *res, int flags);
>  
>  /* will exit holding res->spinlock, but may drop in function */
> @@ -1012,6 +1015,18 @@ static inline void __dlm_wait_on_lockres(struct dlm_lock_resource *res)
>  					  DLM_LOCK_RES_MIGRATING));
>  }
>  
> +/*
> + * will exit holding dlm->spinlock and res->spinlock, but may drop in
> + * function
> + */
> +static inline void __dlm_wait_on_lockres_lock_dlm(struct dlm_ctxt *dlm,
> +						  struct dlm_lock_resource *res)
> +{
> +	__dlm_wait_on_lockres_flags_lock_dlm(dlm, res,
> +					     (DLM_LOCK_RES_IN_PROGRESS|
> +					      DLM_LOCK_RES_RECOVERING|
> +					      DLM_LOCK_RES_MIGRATING));
> +}
>  void __dlm_unlink_mle(struct dlm_ctxt *dlm, struct dlm_master_list_entry *mle);
>  void __dlm_insert_mle(struct dlm_ctxt *dlm, struct dlm_master_list_entry *mle);
>  
> diff --git a/fs/ocfs2/dlm/dlmlock.c b/fs/ocfs2/dlm/dlmlock.c
> index 69cf369..fb3c178 100644
> --- a/fs/ocfs2/dlm/dlmlock.c
> +++ b/fs/ocfs2/dlm/dlmlock.c
> @@ -222,23 +222,35 @@ static enum dlm_status dlmlock_remote(struct dlm_ctxt *dlm,
>  				      struct dlm_lock *lock, int flags)
>  {
>  	enum dlm_status status = DLM_DENIED;
> -	int lockres_changed = 1;
> +	int lockres_changed = 1, recalc;
>  
>  	mlog_entry("type=%d\n", lock->ml.type);
>  	mlog(0, "lockres %.*s, flags = 0x%x\n", res->lockname.len,
>  	     res->lockname.name, flags);
>  
> +	spin_lock(&dlm->spinlock);
>  	spin_lock(&res->spinlock);
>  
>  	/* will exit this call with spinlock held */
> -	__dlm_wait_on_lockres(res);
> +	__dlm_wait_on_lockres_lock_dlm(dlm, res);
>  	res->state |= DLM_LOCK_RES_IN_PROGRESS;
>  
>  	/* add lock to local (secondary) queue */
>  	dlm_lock_get(lock);
>  	list_add_tail(&lock->list, &res->blocked);
>  	lock->lock_pending = 1;
> +	/*
> +	 * make sure this lockres is not in purge list if we remove the
> +	 * lockres from purge list, we try to add it back if locking failed.
> +	 *
> +	 * there is a race with dlm_thread purging this lockres.
> +	 * in this case, it can trigger a BUG() because we added a lock to the
> +	 * blocked list.
> +	 */
> +	recalc = !list_empty(&res->purge);
> +	__dlm_lockres_calc_usage(dlm, res);
>   
Thanks for this patch, looks good. A few comments on another approach, 
let me know your thoughts.

The patch I sent for review marks the lockres to be in use (before 
dlmlock_remote is called) in dlm_get_lockresource. I have to do that to 
protect the lockres from unhashing once it's being used. But since 
lockres is still in purgelist the BUG you mentioned can still trigger in 
dlm_thread.

once a lockres is on purge list there are quite a few cases(case this 
patch is addressing, case the patch I just sent for review is addresing, 
and the case of the recovery) that can trigger the lockres to be reused, 
before or while it is getting purged. dlm_thread(dlm_run_purge_list) 
already expects that a lockres on purge list could just getting started 
to be reused. However it doesn't handle the above cases. I think it's 
better to enhance dlm_run_purge_list/dlm_purge_lockres code to handle 
these cases. I'am working on this change.

There also seems to be a case thats not handled. Assume dlm_thread sent 
deref message and waiting for response, now dlmlock_remote removed it 
from purge list. dlm_thread got response from master and continues and 
unhashes the lockres.
>  	spin_unlock(&res->spinlock);
> +	spin_unlock(&dlm->spinlock);
>  
>  	/* spec seems to say that you will get DLM_NORMAL when the lock
>  	 * has been queued, meaning we need to wait for a reply here. */
> @@ -282,7 +294,7 @@ static enum dlm_status dlmlock_remote(struct dlm_ctxt *dlm,
>  	}
>  	spin_unlock(&res->spinlock);
>  
> -	if (lockres_changed)
> +	if (recalc || lockres_changed)
>  		dlm_lockres_calc_usage(dlm, res);
>  
>  	wake_up(&res->wq);
> diff --git a/fs/ocfs2/dlm/dlmthread.c b/fs/ocfs2/dlm/dlmthread.c
> index d4f73ca..8961767 100644
> --- a/fs/ocfs2/dlm/dlmthread.c
> +++ b/fs/ocfs2/dlm/dlmthread.c
> @@ -77,6 +77,34 @@ repeat:
>  	__set_current_state(TASK_RUNNING);
>  }
>  
> +/*
> + * same as __dlm_wait_on_lockres_flags, but also hold dlm->spinlock on exit and
> + * may drop it in function
> + */
> +void __dlm_wait_on_lockres_flags_lock_dlm(struct dlm_ctxt *dlm,
> +					  struct dlm_lock_resource *res,
> +					  int flags)
> +{
> +	DECLARE_WAITQUEUE(wait, current);
> +
> +	assert_spin_locked(&dlm->spinlock);
> +	assert_spin_locked(&res->spinlock);
> +
> +	add_wait_queue(&res->wq, &wait);
> +repeat:
> +	set_current_state(TASK_UNINTERRUPTIBLE);
> +	if (res->state & flags) {
> +		spin_unlock(&res->spinlock);
> +		spin_unlock(&dlm->spinlock);
> +		schedule();
> +		spin_lock(&dlm->spinlock);
> +		spin_lock(&res->spinlock);
> +		goto repeat;
> +	}
> +	remove_wait_queue(&res->wq, &wait);
> +	__set_current_state(TASK_RUNNING);
> +}
> +
>  int __dlm_lockres_has_locks(struct dlm_lock_resource *res)
>  {
>  	if (list_empty(&res->granted) &&
>