Re: [PATCH 1/2] ocfs2/dlmglue: prepare tracking logic to avoid recursive cluster lock

[Junxiao Bi]

On 01/13/2017 02:12 PM, Eric Ren wrote:
> Hi Junxiao!
> 
> On 01/13/2017 11:59 AM, Junxiao Bi wrote:
>> On 01/05/2017 11:31 PM, 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@xxxxxxxx>
>>> ---
>>>   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));
>> struct pid(oh->oh_owner_pid) looks complicated here, why not use
>> task_struct(current) or pid_t(current->pid) directly? Also i didn't see
>> the ref count needs to be considered.
> 
> This is learned from gfs2 code, which is tested by practice. So, I think
> it's not bad
> to keep it;-)
> 
>>
>>> +
>>> +    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);
>> same the above
>>
>>> +}
>>> +
>>> +inline struct ocfs2_holder *ocfs2_is_locked_by_me(struct
>>> ocfs2_lock_res *lockres)
>> Agree with Joseph, return bool looks better. I didn't see how that help
>> debug since the return value is not used.
>>
>>
>>> +{
>>> +    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;
>>> +}
>>> +
>>>   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 {
>> will ocfs2_lock_holder more clearly? The same to the function name.
> OK, good suggestion. Hrm, but in order to align with "ocfs2_inc_holders", I
> think it's good to keep those function names as it is;-)
that name is also not very clear. Maybe you can make another patch to
clear it.

Thanks,
Junxiao.


> 
> Thanks for your review!
> Eric
> 
>>
>> Thanks,
>> Junxiao.
>>
>>> +    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];
>>>
>>
>