Date:   Fri, 6 Jan 2017 14:07:10 +0800
From:   Joseph Qi <jiangqi903@...il.com>
To:     Eric Ren <zren@...e.com>, ocfs2-devel@....oracle.com
Cc:     akpm@...ux-foundation.org, mfasheh@...sity.com, jlbec@...lplan.org,
        ghe@...e.com, junxiao.bi@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] ocfs2/dlmglue: prepare tracking logic to avoid
 recursive cluster lock

Hi Eric,


On 17/1/5 23:31, Eric Ren wrote:
> We are in the situation that we have to avoid recursive cluster locking,
> but there is no way to check if a cluster lock has been taken by a
> precess already.
>
> Mostly, we can avoid recursive locking by writing code carefully.
> However, we found that it's very hard to handle the routines that
> are invoked directly by vfs code. For instance:
>
> const struct inode_operations ocfs2_file_iops = {
>      .permission     = ocfs2_permission,
>      .get_acl        = ocfs2_iop_get_acl,
>      .set_acl        = ocfs2_iop_set_acl,
> };
>
> Both ocfs2_permission() and ocfs2_iop_get_acl() call ocfs2_inode_lock(PR):
> do_sys_open
>   may_open
>    inode_permission
>     ocfs2_permission
>      ocfs2_inode_lock() <=== first time
>       generic_permission
>        get_acl
>         ocfs2_iop_get_acl
> 	ocfs2_inode_lock() <=== recursive one
>
> A deadlock will occur if a remote EX request comes in between two
> of ocfs2_inode_lock(). Briefly describe how the deadlock is formed:
>
> On one hand, OCFS2_LOCK_BLOCKED flag of this lockres is set in
> BAST(ocfs2_generic_handle_bast) when downconvert is started
> on behalf of the remote EX lock request. Another hand, the recursive
> cluster lock (the second one) will be blocked in in __ocfs2_cluster_lock()
> because of OCFS2_LOCK_BLOCKED. But, the downconvert never complete, why?
> because there is no chance for the first cluster lock on this node to be
> unlocked - we block ourselves in the code path.
>
> The idea to fix this issue is mostly taken from gfs2 code.
> 1. introduce a new field: struct ocfs2_lock_res.l_holders, to
> keep track of the processes' pid  who has taken the cluster lock
> of this lock resource;
> 2. introduce a new flag for ocfs2_inode_lock_full: OCFS2_META_LOCK_GETBH;
> it means just getting back disk inode bh for us if we've got cluster lock.
> 3. export a helper: ocfs2_is_locked_by_me() is used to check if we
> have got the cluster lock in the upper code path.
>
> The tracking logic should be used by some of the ocfs2 vfs's callbacks,
> to solve the recursive locking issue cuased by the fact that vfs routines
> can call into each other.
>
> The performance penalty of processing the holder list should only be seen
> at a few cases where the tracking logic is used, such as get/set acl.
>
> You may ask what if the first time we got a PR lock, and the second time
> we want a EX lock? fortunately, this case never happens in the real world,
> as far as I can see, including permission check, (get|set)_(acl|attr), and
> the gfs2 code also do so.
>
> Signed-off-by: Eric Ren <zren@...e.com>
> ---
>   fs/ocfs2/dlmglue.c | 47 ++++++++++++++++++++++++++++++++++++++++++++---
>   fs/ocfs2/dlmglue.h | 18 ++++++++++++++++++
>   fs/ocfs2/ocfs2.h   |  1 +
>   3 files changed, 63 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
> index 83d576f..500bda4 100644
> --- a/fs/ocfs2/dlmglue.c
> +++ b/fs/ocfs2/dlmglue.c
> @@ -532,6 +532,7 @@ void ocfs2_lock_res_init_once(struct ocfs2_lock_res *res)
>   	init_waitqueue_head(&res->l_event);
>   	INIT_LIST_HEAD(&res->l_blocked_list);
>   	INIT_LIST_HEAD(&res->l_mask_waiters);
> +	INIT_LIST_HEAD(&res->l_holders);
>   }
>   
>   void ocfs2_inode_lock_res_init(struct ocfs2_lock_res *res,
> @@ -749,6 +750,45 @@ void ocfs2_lock_res_free(struct ocfs2_lock_res *res)
>   	res->l_flags = 0UL;
>   }
>   
> +inline void ocfs2_add_holder(struct ocfs2_lock_res *lockres,
> +				   struct ocfs2_holder *oh)
> +{
> +	INIT_LIST_HEAD(&oh->oh_list);
> +	oh->oh_owner_pid =  get_pid(task_pid(current));
> +
> +	spin_lock(&lockres->l_lock);
> +	list_add_tail(&oh->oh_list, &lockres->l_holders);
> +	spin_unlock(&lockres->l_lock);
> +}
> +
> +inline void ocfs2_remove_holder(struct ocfs2_lock_res *lockres,
> +				       struct ocfs2_holder *oh)
> +{
> +	spin_lock(&lockres->l_lock);
> +	list_del(&oh->oh_list);
> +	spin_unlock(&lockres->l_lock);
> +
> +	put_pid(oh->oh_owner_pid);
> +}
> +
> +inline struct ocfs2_holder *ocfs2_is_locked_by_me(struct ocfs2_lock_res *lockres)
> +{
> +	struct ocfs2_holder *oh;
> +	struct pid *pid;
> +
> +	/* look in the list of holders for one with the current task as owner */
> +	spin_lock(&lockres->l_lock);
> +	pid = task_pid(current);
> +	list_for_each_entry(oh, &lockres->l_holders, oh_list) {
> +		if (oh->oh_owner_pid == pid)
> +			goto out;
> +	}
> +	oh = NULL;
> +out:
> +	spin_unlock(&lockres->l_lock);
> +	return oh;
> +}
Since this ocfs2_holder won't be used in the caller, I suggest just 
return a bool value here.
Something like:
spin_lock();
list_for_each_entry() {
     if (oh->oh_owner_pid == pid) {
         spin_unlock();
         return 1;
     }
}
spin_unlock();
return 0;

Thanks,
Joseph
> +
>   static inline void ocfs2_inc_holders(struct ocfs2_lock_res *lockres,
>   				     int level)
>   {
> @@ -2333,8 +2373,9 @@ int ocfs2_inode_lock_full_nested(struct inode *inode,
>   		goto getbh;
>   	}
>   
> -	if (ocfs2_mount_local(osb))
> -		goto local;
> +	if ((arg_flags & OCFS2_META_LOCK_GETBH) ||
> +	    ocfs2_mount_local(osb))
> +		goto update;
>   
>   	if (!(arg_flags & OCFS2_META_LOCK_RECOVERY))
>   		ocfs2_wait_for_recovery(osb);
> @@ -2363,7 +2404,7 @@ int ocfs2_inode_lock_full_nested(struct inode *inode,
>   	if (!(arg_flags & OCFS2_META_LOCK_RECOVERY))
>   		ocfs2_wait_for_recovery(osb);
>   
> -local:
> +update:
>   	/*
>   	 * We only see this flag if we're being called from
>   	 * ocfs2_read_locked_inode(). It means we're locking an inode
> diff --git a/fs/ocfs2/dlmglue.h b/fs/ocfs2/dlmglue.h
> index d293a22..d65ff1e 100644
> --- a/fs/ocfs2/dlmglue.h
> +++ b/fs/ocfs2/dlmglue.h
> @@ -70,6 +70,11 @@ struct ocfs2_orphan_scan_lvb {
>   	__be32	lvb_os_seqno;
>   };
>   
> +struct ocfs2_holder {
> +	struct list_head oh_list;
> +	struct pid *oh_owner_pid;
> +};
> +
>   /* ocfs2_inode_lock_full() 'arg_flags' flags */
>   /* don't wait on recovery. */
>   #define OCFS2_META_LOCK_RECOVERY	(0x01)
> @@ -77,6 +82,8 @@ struct ocfs2_orphan_scan_lvb {
>   #define OCFS2_META_LOCK_NOQUEUE		(0x02)
>   /* don't block waiting for the downconvert thread, instead return -EAGAIN */
>   #define OCFS2_LOCK_NONBLOCK		(0x04)
> +/* just get back disk inode bh if we've got cluster lock. */
> +#define OCFS2_META_LOCK_GETBH		(0x08)
>   
>   /* Locking subclasses of inode cluster lock */
>   enum {
> @@ -170,4 +177,15 @@ void ocfs2_put_dlm_debug(struct ocfs2_dlm_debug *dlm_debug);
>   
>   /* To set the locking protocol on module initialization */
>   void ocfs2_set_locking_protocol(void);
> +
> +/*
> + * Keep a list of processes who have interest in a lockres.
> + * Note: this is now only uesed for check recursive cluster lock.
> + */
> +inline void ocfs2_add_holder(struct ocfs2_lock_res *lockres,
> +			     struct ocfs2_holder *oh);
> +inline void ocfs2_remove_holder(struct ocfs2_lock_res *lockres,
> +			     struct ocfs2_holder *oh);
> +inline struct ocfs2_holder *ocfs2_is_locked_by_me(struct ocfs2_lock_res *lockres);
> +
>   #endif	/* DLMGLUE_H */
> diff --git a/fs/ocfs2/ocfs2.h b/fs/ocfs2/ocfs2.h
> index 7e5958b..0c39d71 100644
> --- a/fs/ocfs2/ocfs2.h
> +++ b/fs/ocfs2/ocfs2.h
> @@ -172,6 +172,7 @@ struct ocfs2_lock_res {
>   
>   	struct list_head         l_blocked_list;
>   	struct list_head         l_mask_waiters;
> +	struct list_head	 l_holders;
>   
>   	unsigned long		 l_flags;
>   	char                     l_name[OCFS2_LOCK_ID_MAX_LEN];

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