[Ocfs2-devel] [PATCH] ocfs2/dlm: remove potential deadlock -V2

[Wengang Wang]

Hi,

Any comment on this?

regards,
wengang.
On 10-06-21 21:43, Wengang Wang wrote:
> When we need to take both dlm_domain_lock and dlm->spinlock, we should take
> them in order of: dlm_domain_lock then dlm->spinlock.
> 
> There is pathes disobey this order. That is calling dlm_lockres_put() with
> dlm->spinlock held in dlm_run_purge_list. dlm_lockres_put() calls dlm_put() at
> the ref and dlm_put() locks on dlm_domain_lock.
> 
> Fix:
> In dlm_run_purge_list, if it could the last ref on the lockres, unlock
> dlm->spinlock before calling dlm_lockres_put() and lock it back after
> dlm_lockres_put().
> 
> Signed-off-by: Wengang Wang <wen.gang.wang at oracle.com>
> ---
>  fs/ocfs2/dlm/dlmthread.c |   27 +++++++++++++++++++--------
>  1 files changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/ocfs2/dlm/dlmthread.c b/fs/ocfs2/dlm/dlmthread.c
> index d4f73ca..b1bd624 100644
> --- a/fs/ocfs2/dlm/dlmthread.c
> +++ b/fs/ocfs2/dlm/dlmthread.c
> @@ -152,6 +152,7 @@ void dlm_lockres_calc_usage(struct dlm_ctxt *dlm,
>  	spin_unlock(&dlm->spinlock);
>  }
>  
> +/* returns 1 if the lockres is removed from purge list, 0 otherwise */
>  static int dlm_purge_lockres(struct dlm_ctxt *dlm,
>  			     struct dlm_lock_resource *res)
>  {
> @@ -217,7 +218,9 @@ static int dlm_purge_lockres(struct dlm_ctxt *dlm,
>  		     res, master);
>  		list_del_init(&res->purge);
>  		spin_unlock(&res->spinlock);
> +		/* not the last ref, dlm_run_purge_list() holds another */
>  		dlm_lockres_put(res);
> +		ret = 1;
>  		dlm->purge_count--;
>  	} else
>  		spin_unlock(&res->spinlock);
> @@ -232,13 +235,13 @@ static int dlm_purge_lockres(struct dlm_ctxt *dlm,
>  		spin_unlock(&res->spinlock);
>  		wake_up(&res->wq);
>  	}
> -	return 0;
> +	return ret;
>  }
>  
>  static void dlm_run_purge_list(struct dlm_ctxt *dlm,
>  			       int purge_now)
>  {
> -	unsigned int run_max, unused;
> +	unsigned int run_max, unused, removed;
>  	unsigned long purge_jiffies;
>  	struct dlm_lock_resource *lockres;
>  
> @@ -280,13 +283,21 @@ static void dlm_run_purge_list(struct dlm_ctxt *dlm,
>  
>  		/* This may drop and reacquire the dlm spinlock if it
>  		 * has to do migration. */
> -		if (dlm_purge_lockres(dlm, lockres))
> -			BUG();
> -
> +		removed = dlm_purge_lockres(dlm, lockres);
> +		/* If the lockres is removed from purge list, this could be
> +		 * the last ref. Unlock dlm->spinlock to avoid deadlock
> +		 * --dlm_lockres_put() does spin_lock on dlm_domain_lock on the
> +		 * last ref while there, in other path, could be lock requests
> +		 * in normal order: dlm_domain_lock then dlm->spinlock.
> +		 */
> +		if (removed)
> +			spin_unlock(&dlm->spinlock);
>  		dlm_lockres_put(lockres);
> -
> -		/* Avoid adding any scheduling latencies */
> -		cond_resched_lock(&dlm->spinlock);
> +		if (removed)
> +			spin_lock(&dlm->spinlock);
> +		else
> +			/* Avoid adding any scheduling latencies */
> +			cond_resched_lock(&dlm->spinlock);
>  	}
>  
>  	spin_unlock(&dlm->spinlock);
> -- 
> 1.6.6.1
> 
> 
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