Messages in this thread | | | Subject | Re: [PATCH v3 1/2] ocfs2/dlmglue: prepare tracking logic to avoid recursive cluster lock | From | Joseph Qi <> | Date | Tue, 17 Jan 2017 15:39:16 +0800 |
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On 17/1/17 14:30, 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. > > Changes since v1: > - Let ocfs2_is_locked_by_me() just return true/false to indicate if the > process gets the cluster lock - suggested by: Joseph Qi <jiangqi903@gmail.com> > and Junxiao Bi <junxiao.bi@oracle.com>. > > - Change "struct ocfs2_holder" to a more meaningful name "ocfs2_lock_holder", > suggested by: Junxiao Bi. > > - Do not inline functions whose bodies are not in scope, changed by: > Stephen Rothwell <sfr@canb.auug.org.au>. > > Changes since v2: > - Wrap the tracking logic code of recursive locking into functions, > ocfs2_inode_lock_tracker() and ocfs2_inode_unlock_tracker(), > suggested by: Junxiao Bi. > > [sfr@canb.auug.org.au remove some inlines] > Signed-off-by: Eric Ren <zren@suse.com> > --- > fs/ocfs2/dlmglue.c | 105 +++++++++++++++++++++++++++++++++++++++++++++++++++-- > fs/ocfs2/dlmglue.h | 18 +++++++++ > fs/ocfs2/ocfs2.h | 1 + > 3 files changed, 121 insertions(+), 3 deletions(-) > > diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c > index 77d1632..c75b9e9 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 +750,50 @@ void ocfs2_lock_res_free(struct ocfs2_lock_res *res) > res->l_flags = 0UL; > } > > +/* > + * Keep a list of processes who have interest in a lockres. > + * Note: this is now only uesed for check recursive cluster locking. > + */ > +static inline void ocfs2_add_holder(struct ocfs2_lock_res *lockres, > + struct ocfs2_lock_holder *oh) > +{ > + INIT_LIST_HEAD(&oh->oh_list); > + oh->oh_owner_pid = get_pid(task_pid(current)); Trim the redundant space here. Others look good to me. Reviewed-by: Joseph Qi <jiangqi903@gmail.com> > + > + spin_lock(&lockres->l_lock); > + list_add_tail(&oh->oh_list, &lockres->l_holders); > + spin_unlock(&lockres->l_lock); > +} > + > +static inline void ocfs2_remove_holder(struct ocfs2_lock_res *lockres, > + struct ocfs2_lock_holder *oh) > +{ > + spin_lock(&lockres->l_lock); > + list_del(&oh->oh_list); > + spin_unlock(&lockres->l_lock); > + > + put_pid(oh->oh_owner_pid); > +} > + > +static inline int ocfs2_is_locked_by_me(struct ocfs2_lock_res *lockres) > +{ > + struct ocfs2_lock_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) { > + spin_unlock(&lockres->l_lock); > + return 1; > + } > + } > + spin_unlock(&lockres->l_lock); > + > + return 0; > +} > + > static inline void ocfs2_inc_holders(struct ocfs2_lock_res *lockres, > int level) > { > @@ -2333,8 +2378,9 @@ 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 +2409,7 @@ 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 > @@ -2497,6 +2543,59 @@ void ocfs2_inode_unlock(struct inode *inode, > ocfs2_cluster_unlock(OCFS2_SB(inode->i_sb), lockres, level); > } > > +/* > + * This _tracker variantes are introduced to deal with the recursive cluster > + * locking issue. The idea is to keep track of a lock holder on the stack of > + * the current process. If there's a lock holder on the stack, we know the > + * task context is already protected by cluster locking. Currently, they're > + * used in some VFS entry routines. > + * > + * return < 0 on error, return == 0 if there's no lock holder on the stack > + * before this call, return == 1 if this call would be a recursive locking. > + */ > +int ocfs2_inode_lock_tracker(struct inode *inode, > + struct buffer_head **ret_bh, > + int ex, > + struct ocfs2_lock_holder *oh) > +{ > + int status; > + int arg_flags = 0, has_locked; > + struct ocfs2_lock_res *lockres; > + > + lockres = &OCFS2_I(inode)->ip_inode_lockres; > + has_locked = ocfs2_is_locked_by_me(lockres); > + /* Just get buffer head if the cluster lock has been taken */ > + if (has_locked) > + arg_flags = OCFS2_META_LOCK_GETBH; > + > + if (likely(!has_locked || ret_bh)) { > + status = ocfs2_inode_lock_full(inode, ret_bh, ex, arg_flags); > + if (status < 0) { > + if (status != -ENOENT) > + mlog_errno(status); > + return status; > + } > + } > + if (!has_locked) > + ocfs2_add_holder(lockres, oh); > + > + return has_locked; > +} > + > +void ocfs2_inode_unlock_tracker(struct inode *inode, > + int ex, > + struct ocfs2_lock_holder *oh, > + int had_lock) > +{ > + struct ocfs2_lock_res *lockres; > + > + lockres = &OCFS2_I(inode)->ip_inode_lockres; > + if (!had_lock) { > + ocfs2_remove_holder(lockres, oh); > + ocfs2_inode_unlock(inode, ex); > + } > +} > + > int ocfs2_orphan_scan_lock(struct ocfs2_super *osb, u32 *seqno) > { > struct ocfs2_lock_res *lockres; > diff --git a/fs/ocfs2/dlmglue.h b/fs/ocfs2/dlmglue.h > index d293a22..a7fc18b 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_lock_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); > + > +/* The _tracker pair is used to avoid cluster recursive locking */ > +int ocfs2_inode_lock_tracker(struct inode *inode, > + struct buffer_head **ret_bh, > + int ex, > + struct ocfs2_lock_holder *oh); > +void ocfs2_inode_unlock_tracker(struct inode *inode, > + int ex, > + struct ocfs2_lock_holder *oh, > + int had_lock); > + > #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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