From: Mathieu Desnoyers Date: Mon, 12 Nov 2012 02:44:59 +0000 (-0500) Subject: urcu-bp: improve 2-phase wait scheme X-Git-Tag: v0.8.0~144 X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=dd61d0772f8dda35aadf9636ebabfaebf24add6a;p=userspace-rcu.git urcu-bp: improve 2-phase wait scheme In the single-bit, 2-phase grace period scheme, all we need to do is to observe each reader going through a quiescent state while we are in the grace period. We therefore only need to perform one global counter update, surrounded by 2 iterations on readers to observe change in their snapshot. We can therefore remove the first counter update (prior to the first iteration on readers): it was useless and was only slowing down the grace period. CC: Paul E. McKenney CC: Lai Jiangshan CC: Alan Stern Signed-off-by: Mathieu Desnoyers --- diff --git a/urcu-bp.c b/urcu-bp.c index b07a1bb..4b3cf01 100644 --- a/urcu-bp.c +++ b/urcu-bp.c @@ -164,31 +164,16 @@ static void mutex_unlock(pthread_mutex_t *mutex) urcu_die(ret); } -static void update_counter_and_wait(void) +static void wait_for_readers(void) { CDS_LIST_HEAD(qsreaders); int wait_loops = 0; struct rcu_reader *index, *tmp; - /* Switch parity: 0 -> 1, 1 -> 0 */ - CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE); - - /* - * Must commit qparity update to memory before waiting for other parity - * quiescent state. Failure to do so could result in the writer waiting - * forever while new readers are always accessing data (no progress). - * Ensured by CMM_STORE_SHARED and CMM_LOAD_SHARED. - */ - - /* - * Adding a cmm_smp_mb() which is _not_ formally required, but makes the - * model easier to understand. It does not have a big performance impact - * anyway, given this is the write-side. - */ - cmm_smp_mb(); - /* - * Wait for each thread rcu_reader.ctr count to become 0. + * Wait for each thread URCU_TLS(rcu_reader).ctr to either + * indicate quiescence (not nested), or observe the current + * rcu_gp_ctr value. */ for (;;) { wait_loops++; @@ -234,9 +219,26 @@ void synchronize_rcu(void) rcu_gc_registry(); /* - * Wait for previous parity to be empty of readers. + * Wait for readers to observe original parity or be quiescent. + */ + wait_for_readers(); + + /* + * Adding a cmm_smp_mb() which is _not_ formally required, but makes the + * model easier to understand. It does not have a big performance impact + * anyway, given this is the write-side. + */ + cmm_smp_mb(); + + /* Switch parity: 0 -> 1, 1 -> 0 */ + CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE); + + /* + * Must commit qparity update to memory before waiting for other parity + * quiescent state. Failure to do so could result in the writer waiting + * forever while new readers are always accessing data (no progress). + * Ensured by CMM_STORE_SHARED and CMM_LOAD_SHARED. */ - update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */ /* * Adding a cmm_smp_mb() which is _not_ formally required, but makes the @@ -246,9 +248,9 @@ void synchronize_rcu(void) cmm_smp_mb(); /* - * Wait for previous parity to be empty of readers. + * Wait for readers to observe new parity or be quiescent. */ - update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */ + wait_for_readers(); /* * Finish waiting for reader threads before letting the old ptr being