On Sat, Apr 25, 2015 at 11:52:29AM -0400, Mathieu Desnoyers wrote:
> The transitive dependency between:
>
> RCU read-side C.S. -> synchronize_rcu -> rcu_gp_lock -> rcu_register_thread
>
> and the dependency:
>
> pthread_join -> awaiting for thread completion
>
> Can block a thread on join, and thus have the side-effect of deadlocking
> a thread doing a pthread_join while within a RCU read-side critical
> section. This join would be awaiting for completion of register_thread or
> rcu_unregister_thread, which may never complete because the rcu_gp_lock
> is held by synchronize_rcu executed from another thread.
>
> One solution to fix this is to add a new lock, rcu_registry_lock. This
> lock now protects the thread registry. It is released between iterations
> on the registry by synchronize_rcu, thus allowing thread
> registration/unregistration to complete even though synchronize_rcu is
> awaiting for RCU read-side critical sections to complete.
>
> Changes since v1:
> - Hold both rcu_gp_lock and rcu_registry_lock across fork in urcu-bp.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
Reviewed-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> CC: Eugene Ivanov <eugene.iva...@orc-group.com>
> CC: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> CC: Lai Jiangshan <la...@cn.fujitsu.com>
> CC: Stephen Hemminger <step...@networkplumber.org>
> ---
> urcu-bp.c | 49 +++++++++++++++++++++++++++++++++++++++--------
> urcu-qsbr.c | 38 +++++++++++++++++++++++++++++++++----
> urcu.c | 63
> ++++++++++++++++++++++++++++++++++++++++++++++++-------------
> 3 files changed, 125 insertions(+), 25 deletions(-)
>
> diff --git a/urcu-bp.c b/urcu-bp.c
> index 6b2875d..4dc4028 100644
> --- a/urcu-bp.c
> +++ b/urcu-bp.c
> @@ -99,7 +99,21 @@ void __attribute__((constructor)) rcu_bp_init(void);
> static
> void __attribute__((destructor)) rcu_bp_exit(void);
>
> +/*
> + * rcu_gp_lock ensures mutual exclusion between threads calling
> + * synchronize_rcu().
> + */
> static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
> +/*
> + * rcu_registry_lock ensures mutual exclusion between threads
> + * registering and unregistering themselves to/from the registry, and
> + * with threads reading that registry from synchronize_rcu(). However,
> + * this lock is not held all the way through the completion of awaiting
> + * for the grace period. It is sporadically released between iterations
> + * on the registry.
> + * rcu_registry_lock may nest inside rcu_gp_lock.
> + */
> +static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
>
> static pthread_mutex_t init_lock = PTHREAD_MUTEX_INITIALIZER;
> static int initialized;
> @@ -160,6 +174,10 @@ static void mutex_unlock(pthread_mutex_t *mutex)
> urcu_die(ret);
> }
>
> +/*
> + * Always called with rcu_registry lock held. Releases this lock between
> + * iterations and grabs it again. Holds the lock when it returns.
> + */
> static void wait_for_readers(struct cds_list_head *input_readers,
> struct cds_list_head *cur_snap_readers,
> struct cds_list_head *qsreaders)
> @@ -202,10 +220,14 @@ static void wait_for_readers(struct cds_list_head
> *input_readers,
> if (cds_list_empty(input_readers)) {
> break;
> } else {
> + /* Temporarily unlock the registry lock. */
> + mutex_unlock(&rcu_registry_lock);
> if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS)
> (void) poll(NULL, 0, RCU_SLEEP_DELAY_MS);
> else
> caa_cpu_relax();
> + /* Re-lock the registry lock before the next loop. */
> + mutex_lock(&rcu_registry_lock);
> }
> }
> }
> @@ -224,6 +246,8 @@ void synchronize_rcu(void)
>
> mutex_lock(&rcu_gp_lock);
>
> + mutex_lock(&rcu_registry_lock);
> +
> if (cds_list_empty(®istry))
> goto out;
>
> @@ -234,6 +258,8 @@ void synchronize_rcu(void)
>
> /*
> * Wait for readers to observe original parity or be quiescent.
> + * wait_for_readers() can release and grab again rcu_registry_lock
> + * interally.
> */
> wait_for_readers(®istry, &cur_snap_readers, &qsreaders);
>
> @@ -263,6 +289,8 @@ void synchronize_rcu(void)
>
> /*
> * Wait for readers to observe new parity or be quiescent.
> + * wait_for_readers() can release and grab again rcu_registry_lock
> + * interally.
> */
> wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
>
> @@ -277,6 +305,7 @@ void synchronize_rcu(void)
> */
> cmm_smp_mb();
> out:
> + mutex_unlock(&rcu_registry_lock);
> mutex_unlock(&rcu_gp_lock);
> ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
> assert(!ret);
> @@ -485,9 +514,9 @@ void rcu_bp_register(void)
> */
> rcu_bp_init();
>
> - mutex_lock(&rcu_gp_lock);
> + mutex_lock(&rcu_registry_lock);
> add_thread();
> - mutex_unlock(&rcu_gp_lock);
> + mutex_unlock(&rcu_registry_lock);
> end:
> ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
> if (ret)
> @@ -508,9 +537,9 @@ void rcu_bp_unregister(struct rcu_reader *rcu_reader_reg)
> if (ret)
> abort();
>
> - mutex_lock(&rcu_gp_lock);
> + mutex_lock(&rcu_registry_lock);
> remove_thread(rcu_reader_reg);
> - mutex_unlock(&rcu_gp_lock);
> + mutex_unlock(&rcu_registry_lock);
> ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
> if (ret)
> abort();
> @@ -564,9 +593,10 @@ void rcu_bp_exit(void)
> }
>
> /*
> - * Holding the rcu_gp_lock across fork will make sure we fork() don't race
> with
> - * a concurrent thread executing with this same lock held. This ensures that
> the
> - * registry is in a coherent state in the child.
> + * Holding the rcu_gp_lock and rcu_registry_lock across fork will make
> + * sure we fork() don't race with a concurrent thread executing with
> + * any of those locks held. This ensures that the registry and data
> + * protected by rcu_gp_lock are in a coherent state in the child.
> */
> void rcu_bp_before_fork(void)
> {
> @@ -578,6 +608,7 @@ void rcu_bp_before_fork(void)
> ret = pthread_sigmask(SIG_BLOCK, &newmask, &oldmask);
> assert(!ret);
> mutex_lock(&rcu_gp_lock);
> + mutex_lock(&rcu_registry_lock);
> saved_fork_signal_mask = oldmask;
> }
>
> @@ -587,6 +618,7 @@ void rcu_bp_after_fork_parent(void)
> int ret;
>
> oldmask = saved_fork_signal_mask;
> + mutex_unlock(&rcu_registry_lock);
> mutex_unlock(&rcu_gp_lock);
> ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
> assert(!ret);
> @@ -594,7 +626,7 @@ void rcu_bp_after_fork_parent(void)
>
> /*
> * Prune all entries from registry except our own thread. Fits the Linux
> - * fork behavior. Called with rcu_gp_lock held.
> + * fork behavior. Called with rcu_gp_lock and rcu_registry_lock held.
> */
> static
> void urcu_bp_prune_registry(void)
> @@ -622,6 +654,7 @@ void rcu_bp_after_fork_child(void)
>
> urcu_bp_prune_registry();
> oldmask = saved_fork_signal_mask;
> + mutex_unlock(&rcu_registry_lock);
> mutex_unlock(&rcu_gp_lock);
> ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
> assert(!ret);
> diff --git a/urcu-qsbr.c b/urcu-qsbr.c
> index a35dcfc..3e77f98 100644
> --- a/urcu-qsbr.c
> +++ b/urcu-qsbr.c
> @@ -52,7 +52,21 @@
>
> void __attribute__((destructor)) rcu_exit(void);
>
> +/*
> + * rcu_gp_lock ensures mutual exclusion between threads calling
> + * synchronize_rcu().
> + */
> static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
> +/*
> + * rcu_registry_lock ensures mutual exclusion between threads
> + * registering and unregistering themselves to/from the registry, and
> + * with threads reading that registry from synchronize_rcu(). However,
> + * this lock is not held all the way through the completion of awaiting
> + * for the grace period. It is sporadically released between iterations
> + * on the registry.
> + * rcu_registry_lock may nest inside rcu_gp_lock.
> + */
> +static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
> struct rcu_gp rcu_gp = { .ctr = RCU_GP_ONLINE };
>
> /*
> @@ -171,6 +185,8 @@ static void wait_for_readers(struct cds_list_head
> *input_readers,
> }
> break;
> } else {
> + /* Temporarily unlock the registry lock. */
> + mutex_unlock(&rcu_registry_lock);
> if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
> wait_gp();
> } else {
> @@ -180,6 +196,8 @@ static void wait_for_readers(struct cds_list_head
> *input_readers,
> cmm_smp_mb();
> #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
> }
> + /* Re-lock the registry lock before the next loop. */
> + mutex_lock(&rcu_registry_lock);
> }
> }
> }
> @@ -233,11 +251,15 @@ void synchronize_rcu(void)
> */
> urcu_move_waiters(&waiters, &gp_waiters);
>
> + mutex_lock(&rcu_registry_lock);
> +
> if (cds_list_empty(®istry))
> goto out;
>
> /*
> * Wait for readers to observe original parity or be quiescent.
> + * wait_for_readers() can release and grab again rcu_registry_lock
> + * interally.
> */
> wait_for_readers(®istry, &cur_snap_readers, &qsreaders);
>
> @@ -279,6 +301,8 @@ void synchronize_rcu(void)
>
> /*
> * Wait for readers to observe new parity or be quiescent.
> + * wait_for_readers() can release and grab again rcu_registry_lock
> + * interally.
> */
> wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
>
> @@ -287,6 +311,7 @@ void synchronize_rcu(void)
> */
> cds_list_splice(&qsreaders, ®istry);
> out:
> + mutex_unlock(&rcu_registry_lock);
> mutex_unlock(&rcu_gp_lock);
> urcu_wake_all_waiters(&waiters);
> gp_end:
> @@ -339,6 +364,8 @@ void synchronize_rcu(void)
> */
> urcu_move_waiters(&waiters, &gp_waiters);
>
> + mutex_lock(&rcu_registry_lock);
> +
> if (cds_list_empty(®istry))
> goto out;
>
> @@ -363,6 +390,8 @@ void synchronize_rcu(void)
>
> /*
> * Wait for readers to observe new count of be quiescent.
> + * wait_for_readers() can release and grab again rcu_registry_lock
> + * interally.
> */
> wait_for_readers(®istry, NULL, &qsreaders);
>
> @@ -371,6 +400,7 @@ void synchronize_rcu(void)
> */
> cds_list_splice(&qsreaders, ®istry);
> out:
> + mutex_unlock(&rcu_registry_lock);
> mutex_unlock(&rcu_gp_lock);
> urcu_wake_all_waiters(&waiters);
> gp_end:
> @@ -420,9 +450,9 @@ void rcu_register_thread(void)
> URCU_TLS(rcu_reader).tid = pthread_self();
> assert(URCU_TLS(rcu_reader).ctr == 0);
>
> - mutex_lock(&rcu_gp_lock);
> + mutex_lock(&rcu_registry_lock);
> cds_list_add(&URCU_TLS(rcu_reader).node, ®istry);
> - mutex_unlock(&rcu_gp_lock);
> + mutex_unlock(&rcu_registry_lock);
> _rcu_thread_online();
> }
>
> @@ -433,9 +463,9 @@ void rcu_unregister_thread(void)
> * with a waiting writer.
> */
> _rcu_thread_offline();
> - mutex_lock(&rcu_gp_lock);
> + mutex_lock(&rcu_registry_lock);
> cds_list_del(&URCU_TLS(rcu_reader).node);
> - mutex_unlock(&rcu_gp_lock);
> + mutex_unlock(&rcu_registry_lock);
> }
>
> void rcu_exit(void)
> diff --git a/urcu.c b/urcu.c
> index ae3490f..1429f6d 100644
> --- a/urcu.c
> +++ b/urcu.c
> @@ -100,7 +100,21 @@ void __attribute__((constructor)) rcu_init(void);
> void __attribute__((destructor)) rcu_exit(void);
> #endif
>
> +/*
> + * rcu_gp_lock ensures mutual exclusion between threads calling
> + * synchronize_rcu().
> + */
> static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
> +/*
> + * rcu_registry_lock ensures mutual exclusion between threads
> + * registering and unregistering themselves to/from the registry, and
> + * with threads reading that registry from synchronize_rcu(). However,
> + * this lock is not held all the way through the completion of awaiting
> + * for the grace period. It is sporadically released between iterations
> + * on the registry.
> + * rcu_registry_lock may nest inside rcu_gp_lock.
> + */
> +static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
> struct rcu_gp rcu_gp = { .ctr = RCU_GP_COUNT };
>
> /*
> @@ -226,6 +240,10 @@ static void wait_gp(void)
> NULL, NULL, 0);
> }
>
> +/*
> + * Always called with rcu_registry lock held. Releases this lock between
> + * iterations and grabs it again. Holds the lock when it returns.
> + */
> static void wait_for_readers(struct cds_list_head *input_readers,
> struct cds_list_head *cur_snap_readers,
> struct cds_list_head *qsreaders)
> @@ -282,10 +300,14 @@ static void wait_for_readers(struct cds_list_head
> *input_readers,
> }
> break;
> } else {
> + /* Temporarily unlock the registry lock. */
> + mutex_unlock(&rcu_registry_lock);
> if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS)
> wait_gp();
> else
> caa_cpu_relax();
> + /* Re-lock the registry lock before the next loop. */
> + mutex_lock(&rcu_registry_lock);
> }
> #else /* #ifndef HAS_INCOHERENT_CACHES */
> /*
> @@ -305,12 +327,16 @@ static void wait_for_readers(struct cds_list_head
> *input_readers,
> smp_mb_master(RCU_MB_GROUP);
> wait_gp_loops = 0;
> }
> + /* Temporarily unlock the registry lock. */
> + mutex_unlock(&rcu_registry_lock);
> if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
> wait_gp();
> wait_gp_loops++;
> } else {
> caa_cpu_relax();
> }
> + /* Re-lock the registry lock before the next loop. */
> + mutex_lock(&rcu_registry_lock);
> }
> #endif /* #else #ifndef HAS_INCOHERENT_CACHES */
> }
> @@ -348,17 +374,23 @@ void synchronize_rcu(void)
> */
> urcu_move_waiters(&waiters, &gp_waiters);
>
> + mutex_lock(&rcu_registry_lock);
> +
> if (cds_list_empty(®istry))
> goto out;
>
> - /* All threads should read qparity before accessing data structure
> - * where new ptr points to. Must be done within rcu_gp_lock because it
> - * iterates on reader threads.*/
> + /*
> + * All threads should read qparity before accessing data structure
> + * where new ptr points to. Must be done within rcu_registry_lock
> + * because it iterates on reader threads.
> + */
> /* Write new ptr before changing the qparity */
> smp_mb_master(RCU_MB_GROUP);
>
> /*
> * Wait for readers to observe original parity or be quiescent.
> + * wait_for_readers() can release and grab again rcu_registry_lock
> + * interally.
> */
> wait_for_readers(®istry, &cur_snap_readers, &qsreaders);
>
> @@ -399,6 +431,8 @@ void synchronize_rcu(void)
>
> /*
> * Wait for readers to observe new parity or be quiescent.
> + * wait_for_readers() can release and grab again rcu_registry_lock
> + * interally.
> */
> wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
>
> @@ -407,11 +441,14 @@ void synchronize_rcu(void)
> */
> cds_list_splice(&qsreaders, ®istry);
>
> - /* Finish waiting for reader threads before letting the old ptr being
> - * freed. Must be done within rcu_gp_lock because it iterates on reader
> - * threads. */
> + /*
> + * Finish waiting for reader threads before letting the old ptr
> + * being freed. Must be done within rcu_registry_lock because it
> + * iterates on reader threads.
> + */
> smp_mb_master(RCU_MB_GROUP);
> out:
> + mutex_unlock(&rcu_registry_lock);
> mutex_unlock(&rcu_gp_lock);
>
> /*
> @@ -447,17 +484,17 @@ void rcu_register_thread(void)
> assert(URCU_TLS(rcu_reader).need_mb == 0);
> assert(!(URCU_TLS(rcu_reader).ctr & RCU_GP_CTR_NEST_MASK));
>
> - mutex_lock(&rcu_gp_lock);
> + mutex_lock(&rcu_registry_lock);
> rcu_init(); /* In case gcc does not support constructor attribute */
> cds_list_add(&URCU_TLS(rcu_reader).node, ®istry);
> - mutex_unlock(&rcu_gp_lock);
> + mutex_unlock(&rcu_registry_lock);
> }
>
> void rcu_unregister_thread(void)
> {
> - mutex_lock(&rcu_gp_lock);
> + mutex_lock(&rcu_registry_lock);
> cds_list_del(&URCU_TLS(rcu_reader).node);
> - mutex_unlock(&rcu_gp_lock);
> + mutex_unlock(&rcu_registry_lock);
> }
>
> #ifdef RCU_MEMBARRIER
> @@ -488,9 +525,9 @@ static void sigrcu_handler(int signo, siginfo_t *siginfo,
> void *context)
> * rcu_init constructor. Called when the library is linked, but also when
> * reader threads are calling rcu_register_thread().
> * Should only be called by a single thread at a given time. This is ensured
> by
> - * holing the rcu_gp_lock from rcu_register_thread() or by running at library
> - * load time, which should not be executed by multiple threads nor
> concurrently
> - * with rcu_register_thread() anyway.
> + * holing the rcu_registry_lock from rcu_register_thread() or by running
> + * at library load time, which should not be executed by multiple
> + * threads nor concurrently with rcu_register_thread() anyway.
> */
> void rcu_init(void)
> {
> --
> 2.1.4
>
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