* Re: [PATCH] Fix: deadlock when thread join is issued in read-side C.S. (v2)
[not found] <1429977149-3219-1-git-send-email-mathieu.desnoyers@efficios.com>
@ 2015-04-25 16:31 ` Paul E. McKenney
[not found] ` <20150425163130.GP5561@linux.vnet.ibm.com>
1 sibling, 0 replies; 3+ messages in thread
From: Paul E. McKenney @ 2015-04-25 16:31 UTC (permalink / raw)
To: Mathieu Desnoyers; +Cc: lttng-dev, Lai Jiangshan
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.desnoyers@efficios.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: Eugene Ivanov <Eugene.Ivanov@orc-group.com>
> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: Lai Jiangshan <laijs@cn.fujitsu.com>
> CC: Stephen Hemminger <stephen@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
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] Fix: deadlock when thread join is issued in read-side C.S. (v2)
[not found] ` <20150425163130.GP5561@linux.vnet.ibm.com>
@ 2015-04-25 19:59 ` Mathieu Desnoyers
0 siblings, 0 replies; 3+ messages in thread
From: Mathieu Desnoyers @ 2015-04-25 19:59 UTC (permalink / raw)
To: paulmck; +Cc: lttng-dev, Lai Jiangshan
Merged into master, stable-0.8, stable-0.7, thanks !
Mathieu
----- Original Message -----
> 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.desnoyers@efficios.com>
>
> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> > CC: Eugene Ivanov <Eugene.Ivanov@orc-group.com>
> > CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > CC: Lai Jiangshan <laijs@cn.fujitsu.com>
> > CC: Stephen Hemminger <stephen@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
> >
>
>
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
^ permalink raw reply [flat|nested] 3+ messages in thread
* [PATCH] Fix: deadlock when thread join is issued in read-side C.S. (v2)
@ 2015-04-25 15:52 Mathieu Desnoyers
0 siblings, 0 replies; 3+ messages in thread
From: Mathieu Desnoyers @ 2015-04-25 15:52 UTC (permalink / raw)
To: Paul E . McKenney; +Cc: lttng-dev, Lai Jiangshan
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.desnoyers@efficios.com>
CC: Eugene Ivanov <Eugene.Ivanov@orc-group.com>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
CC: Stephen Hemminger <stephen@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
^ permalink raw reply related [flat|nested] 3+ messages in thread
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-- links below jump to the message on this page --
[not found] <1429977149-3219-1-git-send-email-mathieu.desnoyers@efficios.com>
2015-04-25 16:31 ` [PATCH] Fix: deadlock when thread join is issued in read-side C.S. (v2) Paul E. McKenney
[not found] ` <20150425163130.GP5561@linux.vnet.ibm.com>
2015-04-25 19:59 ` Mathieu Desnoyers
2015-04-25 15:52 Mathieu Desnoyers
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