[lttng-dev] Deadlock between call_rcu thread and RCU-bp thread doing registration in rcu_read_lock()

[Eugene Ivanov]

Same with QSBR. It seems that by design I can't register new threads
while synchronize_rcu() is in progress.

On 04/01/2015 01:01 PM, Eugene Ivanov wrote:
> Hi,
>
> I use rcu-bp (0.8.6) and get deadlock between call_rcu thread and
> threads willing to do rcu_read_lock():
> 1. Some thread is in read-side critical section.
> 2. call_rcu thread waits for readers in stack of rcu_bp_register(), i.e.
> holds mutex.
> 3. Another thread enters into critical section via rcu_read_lock() and
> blocks on the mutex taken by thread 2.
>
> Such deadlock is quite unexpected for me. Especially if RCU is used for
> reference counting.
>
> Originally it happened with rculfhash, below is minimized reproducer:
>
> #include <pthread.h>
> #include <urcu-bp.h>
>
> struct Node
> {
>         struct rcu_head rcu_head;
> };
>
> static void free_node(struct rcu_head * head)
> {
>         struct Node *node = caa_container_of(head, struct Node,
> rcu_head);
>         free(node);
> }
>
> static void * reader_thread(void * arg)
> {
>     rcu_read_lock();
>     rcu_read_unlock();
>     return NULL;
> }
>
> int main(int argc, char * argv[])
> {
>     rcu_read_lock();
>     struct Node * node = malloc(sizeof(*node));
>     call_rcu(&node->rcu_head, free_node);
>
>     pthread_t read_thread_info;
>     pthread_create(&read_thread_info, NULL, reader_thread, NULL);
>     pthread_join(read_thread_info, NULL);
>
>     rcu_read_unlock();
>
>     return 0;
> }
>
>
> Stacks:
>
> Thread 3 (Thread 0x7f8e2ab05700 (LWP 7386)):
> #0  0x00000035cacdf343 in *__GI___poll (fds=<optimized out>,
> nfds=<optimized out>, timeout=<optimized out>) at
> ../sysdeps/unix/sysv/linux/poll.c:87
> #1  0x000000383880233e in wait_for_readers
> (input_readers=0x7f8e2ab04cf0, cur_snap_readers=0x0,
> qsreaders=0x7f8e2ab04ce0) at urcu-bp.c:211
> #2  0x0000003838802af2 in synchronize_rcu_bp () at urcu-bp.c:272
> #3  0x00000038388043a3 in call_rcu_thread (arg=0x1f7f030) at
> urcu-call-rcu-impl.h:320
> #4  0x00000035cb0079d1 in start_thread (arg=0x7f8e2ab05700) at
> pthread_create.c:301
> #5  0x00000035cace8b6d in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:115
> Thread 2 (Thread 0x7f8e2a304700 (LWP 7387)):
> #0  __lll_lock_wait () at
> ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:136
> #1  0x00000035cb009508 in _L_lock_854 () from /lib64/libpthread.so.0
> #2  0x00000035cb0093d7 in __pthread_mutex_lock (mutex=0x3838a05ca0
> <rcu_gp_lock>) at pthread_mutex_lock.c:61
> #3  0x0000003838801ed9 in mutex_lock (mutex=<optimized out>) at
> urcu-bp.c:147
> #4  0x000000383880351e in rcu_bp_register () at urcu-bp.c:493
> #5  0x000000383880382e in _rcu_read_lock_bp () at
> urcu/static/urcu-bp.h:159
> #6  rcu_read_lock_bp () at urcu-bp.c:296
> #7  0x0000000000400801 in reader_thread ()
> #8  0x00000035cb0079d1 in start_thread (arg=0x7f8e2a304700) at
> pthread_create.c:301
> #9  0x00000035cace8b6d in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:115
> Thread 1 (Thread 0x7f8e2ab06740 (LWP 7385)):
> #0  0x00000035cb00822d in pthread_join (threadid=140248569890560,
> thread_return=0x0) at pthread_join.c:89
> #1  0x000000000040088f in main ()
>
>
> --
> Eugene Ivanov
>
>
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--
Eugene Ivanov


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