Fix: urcu-wait: futex wait: handle spurious futex wakeups

Observed issue
==============

The urcu-wait urcu_adaptative_busy_wait() implements a futex wait/wakeup
scheme similar to the workqueue code, which has an issue with spurious
wakeups.

A spurious wakeup on urcu_adaptative_busy_wait can cause
urcu_adaptative_busy_wait to reach label skip_futex_wait with a
wait->state state of URCU_WAIT_WAITING, which is unexpected. It would
cause busy-waiting on URCU_WAIT_TEARDOWN state to start early. The
wait-teardown stage is done with URCU_WAIT_ATTEMPTS active attempts,
following by attempts spaced by 10ms sleeps. I do not expect that these
spurious wakeups will cause user-observable effects other than being
slightly less efficient that it should be.

urcu-wait is used by all urcu flavor's synchronize_rcu() to implement
the grace period batching scheme.

This issue will cause spurious unexpected high CPU use, but will not
lead to data corruption.

Cause
=====

From futex(5):

       FUTEX_WAIT
              Returns 0 if the caller was woken up.  Note that a  wake-up  can
              also  be caused by common futex usage patterns in unrelated code
              that happened to have previously used the  futex  word's  memory
              location  (e.g., typical futex-based implementations of Pthreads
              mutexes can cause this under some conditions).  Therefore, call‐
              ers should always conservatively assume that a return value of 0
              can mean a spurious wake-up, and  use  the  futex  word's  value
              (i.e.,  the user-space synchronization scheme) to decide whether
              to continue to block or not.

Solution
========

We therefore need to validate whether the value differs from
URCU_WAIT_WAITING in user-space after the call to FUTEX_WAIT returns 0.

Known drawbacks
===============

None.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I8e9586597f091efc633f3310a68d18b0bd8de1e0

diff --git a/src/urcu-wait.h b/src/urcu-wait.h
index c586ec9da8a007519cc7d6a793a9b6598cea46be..ef5f7ede318c2b3f0aaddcde7aec189aa1bccf60 100644 (file)
--- a/src/urcu-wait.h
+++ b/src/urcu-wait.h
@@ -154,15 +154,26 @@ void urcu_adaptative_busy_wait(struct urcu_wait_node *wait)
                        goto skip_futex_wait;
                caa_cpu_relax();
        }
-       while (futex_noasync(&wait->state, FUTEX_WAIT, URCU_WAIT_WAITING,
-                       NULL, NULL, 0)) {
+       while (uatomic_read(&wait->state) == URCU_WAIT_WAITING) {
+               if (!futex_noasync(&wait->state, FUTEX_WAIT, URCU_WAIT_WAITING, NULL, NULL, 0)) {
+                       /*
+                        * Prior queued wakeups queued by unrelated code
+                        * using the same address can cause futex wait to
+                        * return 0 even through the futex value is still
+                        * URCU_WAIT_WAITING (spurious wakeups). Check
+                        * the value again in user-space to validate
+                        * whether it really differs from
+                        * URCU_WAIT_WAITING.
+                        */
+                       continue;
+               }
                switch (errno) {
-               case EWOULDBLOCK:
+               case EAGAIN:
                        /* Value already changed. */
                        goto skip_futex_wait;
                case EINTR:
                        /* Retry if interrupted by signal. */
-                       break;  /* Get out of switch. */
+                       break;  /* Get out of switch. Check again. */
                default:
                        /* Unexpected error. */
                        urcu_die(errno);