[Ocfs2-devel] [PATCH 1/1] ocfs2 fix o2dlm dlm run purgelist

[Sunil Mushran <sunil.mushran@oracle.com>]

On 06/19/2010 12:56 PM, Srinivas Eeda wrote:
> There are two problems in dlm_run_purgelist
>
> 1. If a lockres is found to be in use, dlm_run_purgelist keeps trying to purge
> the same lockres instead of trying the next lockres.
>
> 2. When a lockres is found unused, dlm_run_purgelist releases lockres spinlock
> before setting DLM_LOCK_RES_DROPPING_REF and calls dlm_purge_lockres.
> spinlock is reacquired but in this window lockres can get reused. This leads
> to BUG.
>
> This patch modifies dlm_run_purgelist to skip lockres if it's in use and purge
>   next lockres. It also sets DLM_LOCK_RES_DROPPING_REF before releasing the
> lockres spinlock protecting it from getting reused.
>
> Signed-off-by: Srinivas Eeda<srinivas.eeda@oracle.com>
> ---
>   fs/ocfs2/dlm/dlmthread.c |   55 +++++++++++++++++++++------------------------
>   1 files changed, 26 insertions(+), 29 deletions(-)
>
> diff --git a/fs/ocfs2/dlm/dlmthread.c b/fs/ocfs2/dlm/dlmthread.c
> index 11a6d1f..79d1ef6 100644
> --- a/fs/ocfs2/dlm/dlmthread.c
> +++ b/fs/ocfs2/dlm/dlmthread.c
> @@ -158,15 +158,6 @@ static int dlm_purge_lockres(struct dlm_ctxt *dlm,
>   	int master;
>   	int ret = 0;
>
> -	spin_lock(&res->spinlock);
> -	if (!__dlm_lockres_unused(res)) {
> -		mlog(0, "%s:%.*s: tried to purge but not unused\n",
> -		     dlm->name, res->lockname.len, res->lockname.name);
> -		__dlm_print_one_lock_resource(res);
> -		spin_unlock(&res->spinlock);
> -		BUG();
> -	}
> -

Always have assert_spin_locked() if the function expects the
spin locks to have been taken by the caller.

>   	if (res->state&  DLM_LOCK_RES_MIGRATING) {
>   		mlog(0, "%s:%.*s: Delay dropref as this lockres is "
>   		     "being remastered\n", dlm->name, res->lockname.len,
> @@ -184,13 +175,13 @@ static int dlm_purge_lockres(struct dlm_ctxt *dlm,
>
>   	if (!master)
>   		res->state |= DLM_LOCK_RES_DROPPING_REF;
> -	spin_unlock(&res->spinlock);
>
>   	mlog(0, "purging lockres %.*s, master = %d\n", res->lockname.len,
>   	     res->lockname.name, master);
>
>   	if (!master) {
>   		/* drop spinlock...  retake below */
> +		spin_unlock(&res->spinlock);
>   		spin_unlock(&dlm->spinlock);
>
>   		spin_lock(&res->spinlock);
> @@ -208,30 +199,34 @@ static int dlm_purge_lockres(struct dlm_ctxt *dlm,
>   		mlog(0, "%s:%.*s: dlm_deref_lockres returned %d\n",
>   		     dlm->name, res->lockname.len, res->lockname.name, ret);
>   		spin_lock(&dlm->spinlock);
> +		spin_lock(&res->spinlock);
>   	}
>
> -	spin_lock(&res->spinlock);
>   	if (!list_empty(&res->purge)) {
>   		mlog(0, "removing lockres %.*s:%p from purgelist, "
>   		     "master = %d\n", res->lockname.len, res->lockname.name,
>   		     res, master);
>   		list_del_init(&res->purge);
> -		spin_unlock(&res->spinlock);
>   		dlm_lockres_put(res);
>   		dlm->purge_count--;
> -	} else
> -		spin_unlock(&res->spinlock);
> +	}
>
> -	__dlm_unhash_lockres(res);
> +	if (__dlm_lockres_unused(res))
> +		__dlm_unhash_lockres(res);
> +	else {
> +		mlog(ML_ERROR, "found lockres %s:%.*s: in use after deref\n",
> +		     dlm->name, res->lockname.len, res->lockname.name);
> +		__dlm_print_one_lock_resource(res);
> +	}

Why do we need this? If we run into this condition, the only
safe response is a BUG_ON.

>
>   	/* lockres is not in the hash now.  drop the flag and wake up
>   	 * any processes waiting in dlm_get_lock_resource. */
>   	if (!master) {
> -		spin_lock(&res->spinlock);
>   		res->state&= ~DLM_LOCK_RES_DROPPING_REF;
>   		spin_unlock(&res->spinlock);
>   		wake_up(&res->wq);
> -	}
> +	} else
> +		spin_unlock(&res->spinlock);
>   	return 0;
>   }
>
> @@ -251,17 +246,7 @@ static void dlm_run_purge_list(struct dlm_ctxt *dlm,
>   		lockres = list_entry(dlm->purge_list.next,
>   				     struct dlm_lock_resource, purge);
>
> -		/* Status of the lockres *might* change so double
> -		 * check. If the lockres is unused, holding the dlm
> -		 * spinlock will prevent people from getting and more
> -		 * refs on it -- there's no need to keep the lockres
> -		 * spinlock. */
>   		spin_lock(&lockres->spinlock);
> -		unused = __dlm_lockres_unused(lockres);
> -		spin_unlock(&lockres->spinlock);
> -
> -		if (!unused)
> -			continue;
>
>   		purge_jiffies = lockres->last_used +
>   			msecs_to_jiffies(DLM_PURGE_INTERVAL_MS);
> @@ -273,15 +258,27 @@ static void dlm_run_purge_list(struct dlm_ctxt *dlm,
>   			 * in tail order, we can stop at the first
>   			 * unpurgable resource -- anyone added after
>   			 * him will have a greater last_used value */
> +			spin_unlock(&lockres->spinlock);
>   			break;
>   		}
>
> +		/* Status of the lockres *might* change so double
> +		 * check. If the lockres is unused, holding the dlm
> +		 * spinlock will prevent people from getting and more
> +		 * refs on it -- there's no need to keep the lockres
> +		 * spinlock. */
> +		unused = __dlm_lockres_unused(lockres);
> +		if (!unused) {
> +			list_move_tail(&dlm->purge_list,&lockres->purge);
> +			spin_unlock(&lockres->spinlock);
> +			continue;
> +		}
> +

+		if (!unused ||
+		    lockres->state&  DLM_LOCK_RES_MIGRATING) {
+			list_move_tail(&dlm->purge_list,&lockres->purge);
+			spin_unlock(&lockres->spinlock);
+			continue;
+		}


You can move the MIGRATING check from atop dlm_purge_lockres() to here.


>   		dlm_lockres_get(lockres);
>
>   		/* This may drop and reacquire the dlm spinlock if it
>   		 * has to do migration. */
> -		if (dlm_purge_lockres(dlm, lockres))
> -			BUG();
> +		dlm_purge_lockres(dlm, lockres);
>

Make the function a void.

>   		dlm_lockres_put(lockres);
>