[sheepdog] [PATCH v4] sheep/cluster: add the distributed lock implemented by zookeeper for object-storage

Robin Dong robin.k.dong at gmail.com
Mon Dec 2 02:57:56 CET 2013


2013/12/1 Liu Yuan <namei.unix at gmail.com>

> On Sun, Dec 01, 2013 at 03:48:04PM +0800, Robin Dong wrote:
> > Implement the distributed lock by zookeeper (refer:
> http://zookeeper.apache.org/doc/trunk/recipes.html)
> > The routine is:
> >         1. create a seq-ephemeral znode in lock directory (use lock-id
> as dir name)
> >         2. get smallest file path as owner of the lock; the other thread
> wait on a pthread_mutex_t (cluster_lock->wait)
> >         3. if owner of the lock release it (or the owner is killed by
> accident), zookeeper will
> >            trigger zk_watch() which will wake up all waiting threads to
> compete new owner of the lock
> >
> > We use dlock_array to store pointers of cluster_locks in this sheep
> daemon so when receiving the event of ZOO_DELETED_EVENT
> > the program will wake up all waiters (in this sheep daemon) who is
> sleeping on the lock id and let them compete for new
> > owner.
> > dlock_array is just a normal array using lock-id as index, so imaging a
> scenario: two threads (A and B) in one sheep daemon
> > call zk_lock() for same lock-id, they will create two znodes in
> zookeeper but set dlock_array[lock_id] to only one of
> > them (for example, set to B). After that, when ZOO_DELETED_EVENT comes,
> the zk_waiter() will only wake up thread B and thread A
> > will sleep on '->wait' forever becuase no one could wakeup him.
> > We have two method to solve this problem:
> >       A. using more complicated structure instead of dlock_array to
> store both A and B 's lock handle.
> >       B. adding a lock to avoid A and B call zk_lock() in the same time.
> > We prefer method B because it also avoid creating too many files in a
> directory of zookeeper which will take too much pressure
> > on zookeeper server if the number of sheep deamons are huge. Therefore
> we add 'local_lock' in 'struct cluster_lock'.
> >
> > Signed-off-by: Robin Dong <sanbai at taobao.com>
> > ---
> >  include/sheep.h           |    8 +++
> >  sheep/cluster.h           |   34 +++++++++++
> >  sheep/cluster/corosync.c  |   16 +++++
> >  sheep/cluster/local.c     |   16 +++++
> >  sheep/cluster/zookeeper.c |  140
> ++++++++++++++++++++++++++++++++++++++++++++-
> >  5 files changed, 213 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/sheep.h b/include/sheep.h
> > index 293e057..fd7258b 100644
> > --- a/include/sheep.h
> > +++ b/include/sheep.h
> > @@ -255,6 +255,14 @@ static inline void nodes_to_buffer(struct rb_root
> *nroot, void *buffer)
> >
> >  #define MAX_NODE_STR_LEN 256
> >
> > +/* structure for distributed lock */
> > +struct cluster_lock {
> > +     uint64_t id;            /* id of this mutex */
> > +     pthread_mutex_t wait;
> > +     pthread_mutex_t local_lock;
> > +     char ephemeral_path[MAX_NODE_STR_LEN];
> > +};
> > +
> >  static inline const char *node_to_str(const struct sd_node *id)
> >  {
> >       static __thread char str[MAX_NODE_STR_LEN];
> > diff --git a/sheep/cluster.h b/sheep/cluster.h
> > index 81b5ae4..f0950ac 100644
> > --- a/sheep/cluster.h
> > +++ b/sheep/cluster.h
> > @@ -109,6 +109,40 @@ struct cluster_driver {
> >       int (*unblock)(void *msg, size_t msg_len);
> >
> >       /*
> > +      * Init a distributed mutually exclusive lock to avoid race
> condition
> > +      * when the whole sheepdog cluster process one exclusive resource.
> > +      *
> > +      * This function use 'lock_id' as the id of this distributed lock.
> > +      * A thread can create many locks in one sheep daemon.
> > +      *
> > +      * Returns SD_RES_XXX
> > +      */
> > +     int (*init_lock)(struct cluster_lock *lock, uint64_t lock_id);
> > +
> > +     /*
> > +      * Acquire the distributed lock.
> > +      *
> > +      * The cluster_lock referenced by 'lock' shall be locked by calling
> > +      * cluster->lock(). If the cluster_lock is already locked, the
> calling
> > +      * thread shall block until the cluster_lock becomes available.
> > +      *
> > +      * This operation will create a seq-ephemeral znode in lock
> directory
> > +      * of zookeeper (use lock-id as dir name). The smallest file path
> in
> > +      * this directory wil be the owner of the lock; the other threads
> will
> > +      * wait on a pthread_mutex_t (cluster_lock->wait)
> > +      */
> >
>
> The second paragraph should be moved to zookeeper.c since it is only zk
> related.
>
> >
> > +     void (*lock)(struct cluster_lock *lock);
> > +
> > +     /*
> > +      * Release the distributed lock.
> > +      *
> > +      * If the owner of the cluster_lock release it (or the owner is
> > +      * killed by accident), zookeeper will trigger zk_watch() which
> will
> > +      * wake up all waiting threads to compete new owner of the lock
> > +      */
> > +     void (*unlock)(struct cluster_lock *lock);
> > +
> > +     /*
> >        * Update the specific node in the driver's private copy of nodes
> >        *
> >        * Returns SD_RES_XXX
> > diff --git a/sheep/cluster/corosync.c b/sheep/cluster/corosync.c
> > index ea4421b..35c95d3 100644
> > --- a/sheep/cluster/corosync.c
> > +++ b/sheep/cluster/corosync.c
> > @@ -774,6 +774,19 @@ again:
> >       return 0;
> >  }
> >
> > +static int corosync_init_lock(struct cluster_lock *dlock, uint32_t id)
> > +{
> > +     return -1;
> > +}
> > +
> > +static void corosync_lock(struct cluster_lock *dlock)
> > +{
> > +}
> > +
> > +static void corosync_unlock(struct cluster_lock *dlock)
> > +{
> > +}
> > +
> >  static int corosync_update_node(struct sd_node *node)
> >  {
> >       struct cpg_node cnode = this_node;
> > @@ -794,6 +807,9 @@ static struct cluster_driver cdrv_corosync = {
> >       .notify         = corosync_notify,
> >       .block          = corosync_block,
> >       .unblock        = corosync_unblock,
> > +     .init_lock      = corosync_init_lock,
> > +     .lock           = corosync_lock,
> > +     .unlock         = corosync_unlock,
> >       .update_node    = corosync_update_node,
> >  };
> >
> > diff --git a/sheep/cluster/local.c b/sheep/cluster/local.c
> > index b8cbb5c..69d9d6f 100644
> > --- a/sheep/cluster/local.c
> > +++ b/sheep/cluster/local.c
> > @@ -547,6 +547,19 @@ static int local_init(const char *option)
> >       return 0;
> >  }
> >
> > +static int local_init_lock(struct cluster_lock *dlock, uint64_t id)
> > +{
> > +     return -1;
> > +}
> > +
> > +static void local_lock(struct cluster_lock *dlock)
> > +{
> > +}
> > +
> > +static void local_unlock(struct cluster_lock *dlock)
> > +{
> > +}
> > +
> >  static int local_update_node(struct sd_node *node)
> >  {
> >       struct local_node lnode = this_node;
> > @@ -566,6 +579,9 @@ static struct cluster_driver cdrv_local = {
> >       .notify         = local_notify,
> >       .block          = local_block,
> >       .unblock        = local_unblock,
> > +     .init_lock      = local_init_lock,
> > +     .lock           = local_lock,
> > +     .unlock         = local_unlock,
> >       .update_node    = local_update_node,
> >  };
> >
> > diff --git a/sheep/cluster/zookeeper.c b/sheep/cluster/zookeeper.c
> > index fa89c46..01cb37c 100644
> > --- a/sheep/cluster/zookeeper.c
> > +++ b/sheep/cluster/zookeeper.c
> > @@ -30,6 +30,21 @@
> >  #define QUEUE_ZNODE BASE_ZNODE "/queue"
> >  #define MEMBER_ZNODE BASE_ZNODE "/member"
> >  #define MASTER_ZNONE BASE_ZNODE "/master"
> > +#define LOCK_ZNODE BASE_ZNODE "/lock"
> > +
> > +#define MAX_MUTEX_NR 4096
> > +#define WAIT_TIME    1               /* second */
> > +
> > +static struct cluster_lock **dlock_array;
> > +
> > +/*
> > + * Wait a while when create, delete or get_children fail on
> > + * zookeeper lock so it will not print too much loop log
> > + */
> > +static void zk_wait(void)
> > +{
> > +     sleep(WAIT_TIME);
> > +}
> >
> >  /* iterate child znodes */
> >  #define FOR_EACH_ZNODE(parent, path, strs)                          \
> > @@ -506,6 +521,7 @@ static void zk_watcher(zhandle_t *zh, int type, int
> state, const char *path,
> >  {
> >       struct zk_node znode;
> >       char str[MAX_NODE_STR_LEN], *p;
> > +     uint64_t lock_id;
> >       int ret;
> >
> >       if (type == ZOO_SESSION_EVENT && state ==
> ZOO_EXPIRED_SESSION_STATE) {
> > @@ -528,6 +544,14 @@ static void zk_watcher(zhandle_t *zh, int type, int
> state, const char *path,
> >       } else if (type == ZOO_DELETED_EVENT) {
> >               struct zk_node *n;
> >
> > +             /* process distributed lock */
> > +             ret = sscanf(path, LOCK_ZNODE "/%lu/%s", &lock_id, str);
> > +             if (ret == 2 && lock_id < MAX_MUTEX_NR &&
> > +                 dlock_array && dlock_array[lock_id]) {
> > +
> pthread_mutex_unlock(&(dlock_array[lock_id]->wait));
> > +                     sd_debug("release lock %lu %s", lock_id, str);
> > +             }
> > +
> >               ret = sscanf(path, MASTER_ZNONE "/%s", str);
> >               if (ret == 1) {
> >                       zk_compete_master();
> > @@ -1058,6 +1082,108 @@ kick_block_event:
> >       kick_block_event();
> >  }
> >
> > +static int zk_init_lock(struct cluster_lock *dlock, uint64_t lock_id)
>
> rename dlock as clock for better uniformity.
>

"clock" will be more confused than "dlock"....Never mind, I will use
"cluster_lock" which is more clearly.

>
> > +{
> > +     int rc = 0;
> > +     char path[MAX_NODE_STR_LEN];
> > +
> > +     if (lock_id > MAX_MUTEX_NR) {
>
> If we constraint lock_id into [0, MAX_MUTEX_NR] then uint64_t is
> meaningless.
> IIUC, lock_id is the unique identifier represent which lock is under
> operation. This
> means we can use vid + number to represent which lock the code is trying
> to hold.
> But with above check, apparently vid + number > MAX_MUTEX_NR as always,
> no? So
> it seems that different clients can't simply use 'vid+xxx' to represent a
> lock
> without extra communication.
>

I think it need to change dlock_array to hash table to support whole
uint64_t lock_id.


>
> Thanks
> Yuan
>



-- 
--
Best Regard
Robin Dong
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