Messages in this thread | | | Subject | Re: [PATCH 1/2] ocfs2/dlmglue: prepare tracking logic to avoid recursive cluster lock | From | Joseph Qi <> | Date | Fri, 6 Jan 2017 15:24:37 +0800 |
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On 17/1/6 15:03, Eric Ren wrote: > On 01/06/2017 02:07 PM, Joseph Qi wrote: >> 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@suse.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; > > Aha, you have the point. However, it is also reasonable to return the > lock holder by the way. > When debugging with printk or crash, it's easy to get the pid of the > holder without any further > analysis. So, I tend to keep it;-) IC, but we can also get the ocfs2_holder from ocfs2_lock_res in case of coredump, right? I think it is indirectly in your way, and mismatch with the function name:)
Thanks ,Joseph > > Thanks very much for your review! > Eric > >> >> 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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