On Thu, Apr 28, 2016 at 09:47:23AM -0400, Yuxin Ren wrote: > Hi Boqun and Paul, > > Thank you so much for your help. > > I found one reason to use that lock. > In the slow path, a thread will move all waiters to a local queue. > https://github.com/urcu/userspace-rcu/blob/master/urcu.c#L406 > After this, following thread can also perform grace period, as the > global waiter queue is empty. > Thus the rcu_gp_lock is used to ensure mutual exclusion. > > However, from real time aspect, such blocking will cause priority > inversion: higher priority writer can be blocked by low priority > writer. > Is there a way to modify the code to allow multiple threads to perform > grace period concurrently?
If a thread has real-time requirements, you shouldn't have it do synchronous grace periods, just as you shouldn't have it do (say) sleep(10). You should instead either (1) have some other non-realtime thread do the cleanup activities involving synchronize_rcu() or (2) have the real-time thread use the asynchronous call_rcu(). Thanx, Paul > Thanks again!! > Yuxin > > On Thu, Apr 28, 2016 at 8:44 AM, Boqun Feng <boqun.f...@gmail.com> wrote: > > Hi Paul and Yuxin, > > > > On Wed, Apr 27, 2016 at 09:23:27PM -0700, Paul E. McKenney wrote: > >> Try building without it and see what happens when you run the tests. > >> > > > > I've run a 'regtest' with the following modification out of curiousity: > > > > diff --git a/urcu.c b/urcu.c > > index a5568bdbd075..9dc3c9feae56 100644 > > --- a/urcu.c > > +++ b/urcu.c > > @@ -398,8 +398,6 @@ void synchronize_rcu(void) > > /* We won't need to wake ourself up */ > > urcu_wait_set_state(&wait, URCU_WAIT_RUNNING); > > > > - mutex_lock(&rcu_gp_lock); > > - > > /* > > * Move all waiters into our local queue. > > */ > > @@ -480,7 +478,6 @@ void synchronize_rcu(void) > > smp_mb_master(); > > out: > > mutex_unlock(&rcu_registry_lock); > > - mutex_unlock(&rcu_gp_lock); > > > > /* > > * Wakeup waiters only after we have completed the grace period > > > > > > And guess what, the result of the test was: > > > > Test Summary Report > > ------------------- > > ./run-urcu-tests.sh 1 (Wstat: 0 Tests: 979 Failed: 18) > > Failed tests: 30, 45, 60, 75, 90, 103, 105, 120, 135 > > 150, 165, 180, 195, 210, 225, 240, 253 > > 255 > > Files=2, Tests=996, 1039 wallclock secs ( 0.55 usr 0.04 sys + 8981.02 cusr > > 597.64 csys = 9579.25 CPU) > > Result: FAIL > > > > And test case 30 for example is something like: > > > > tests/benchmark/test_urcu_mb <nreaders> <nwriters> 1 -d 0 -b 32768 > > > > and it failed because: > > > > lt-test_urcu_mb: test_urcu.c:183: thr_reader: Assertion `*local_ptr == 8' > > failed. > > zsh: abort (core dumped) ~/userspace-rcu/tests/benchmark/test_urcu_mb 4 4 > > 1 -d 0 -b 32768 > > > > So I think what was going on here was: > > > > CPU 0 (reader) CPU 1 (writer) > > CPU 2 (writer) > > =================== ==================== > > ====================== > > rcu_read_lock(); > > new = malloc(sizeof(int)); > > local_ptr = rcu_dereference(test_rcu_pointer); // local_ptr == old > > *new = 8; > > > > old = rcu_xchg_pointer(&test_rcu_pointer, new); > > synchronize_rcu(): > > urcu_wait_add(); // return 0 > > urcu_move_waiters(); // @gp_waiters is > > empty, > > // the next > > urcu_wait_add() will return 0 > > > > > > synchronize_rcu(): > > > > urcu_wait_add(); // return 0 > > > > mutex_lock(&rcu_register_lock); > > wait_for_readers(); // remove registered > > reader from @registery, > > // release > > rcu_register_lock and wait via poll() > > > > > > mutex_lock(&rcu_registry_lock); > > > > wait_for_readers(); // @regsitery is empty! we are so lucky > > > > return; > > > > > > if (old) > > > > free(old) > > rcu_read_unlock(); > > assert(*local_ptr==8); // but local_ptr(i.e. old) is already freed. > > > > > > So the point is there could be two writers calling synchronize_rcu() but > > not returning early(both of them enter the slow path to perform a grace > > period), so the rcu_gp_lock is necessary in this case. > > > > (Cc Mathieu) > > > > But this is only my understanding and I'm learning the URCU code too ;-) > > > > Regards, > > Boqun > > > > > >> Might well be that it is unnecessary, but I will defer to Mathieu > >> on that point. > >> > >> Thanx, Paul > >> > >> On Wed, Apr 27, 2016 at 10:18:04PM -0400, Yuxin Ren wrote: > >> > As they don't currently perform grace period, why do we use the > >> > rcu_gp_lock? > >> > > >> > Thank you. > >> > Yuxin > >> > > >> > On Wed, Apr 27, 2016 at 10:08 PM, Paul E. McKenney > >> > <paul...@linux.vnet.ibm.com> wrote: > >> > > On Wed, Apr 27, 2016 at 09:34:16PM -0400, Yuxin Ren wrote: > >> > >> Hi, > >> > >> > >> > >> I am learning the URCU code. > >> > >> > >> > >> Why do we need rcu_gp_lock in synchronize_rcu? > >> > >> https://github.com/urcu/userspace-rcu/blob/master/urcu.c#L401 > >> > >> > >> > >> In the comment, it says this lock ensures mutual exclusion between > >> > >> threads calling synchronize_rcu(). > >> > >> But only the first thread added to waiter queue can proceed to detect > >> > >> grace period. > >> > >> How can multiple threads currently perform the grace thread? > >> > > > >> > > They don't concurrently perform grace periods, and it would be wasteful > >> > > for them to do so. Instead, the first one performs the grace period, > >> > > and all that were waiting at the time it started get the benefit of > >> > > that > >> > > same grace period. > >> > > > >> > > Any that arrived after the first grace period performs the first > >> > > grace period are served by whichever of them performs the second > >> > > grace period. > >> > > > >> > > Thanx, Paul > >> > > > >> > > >> > >> _______________________________________________ > >> lttng-dev mailing list > >> lttng-dev@lists.lttng.org > >> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev > _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev