From: Mathieu Desnoyers Date: Mon, 12 Nov 2012 02:40:23 +0000 (-0500) Subject: urcu-qsbr: improve 2-phase wait scheme X-Git-Tag: v0.8.0~146 X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=f6b42f9c22f265a7c071a8b60c546a76faa5f69b;p=urcu.git urcu-qsbr: 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. Suggested-by: Alan Stern CC: Paul E. McKenney CC: Lai Jiangshan Signed-off-by: Mathieu Desnoyers --- diff --git a/urcu-qsbr.c b/urcu-qsbr.c index c6a1b18..eb167d1 100644 --- a/urcu-qsbr.c +++ b/urcu-qsbr.c @@ -116,38 +116,16 @@ static void wait_gp(void) NULL, NULL, 0); } -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; -#if (CAA_BITS_PER_LONG < 64) - /* Switch parity: 0 -> 1, 1 -> 0 */ - CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); -#else /* !(CAA_BITS_PER_LONG < 64) */ - /* Increment current G.P. */ - CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR); -#endif /* !(CAA_BITS_PER_LONG < 64) */ - - /* - * Must commit rcu_gp_ctr update to memory before waiting for - * quiescent state. Failure to do so could result in the writer - * waiting forever while new readers are always accessing data - * (no progress). Enforce compiler-order of store to rcu_gp_ctr - * before load URCU_TLS(rcu_reader).ctr. - */ - cmm_barrier(); - - /* - * 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_qs_gp count to become 0. + * Wait for each thread URCU_TLS(rcu_reader).ctr to either + * indicate quiescence (offline), or for them to observe the + * current rcu_gp_ctr value. */ for (;;) { wait_loops++; @@ -223,17 +201,17 @@ void synchronize_rcu(void) goto out; /* - * Wait for previous parity to be empty of readers. + * Wait for readers to observe original parity or be quiescent. */ - update_counter_and_wait(); /* 0 -> 1, wait readers in parity 0 */ + wait_for_readers(); /* - * Must finish waiting for quiescent state for parity 0 before - * committing next rcu_gp_ctr update to memory. Failure to - * do so could result in the writer waiting forever while new + * Must finish waiting for quiescent state for original parity + * before committing next rcu_gp_ctr update to memory. Failure + * to do so could result in the writer waiting forever while new * readers are always accessing data (no progress). Enforce - * compiler-order of load URCU_TLS(rcu_reader).ctr before store to - * rcu_gp_ctr. + * compiler-order of load URCU_TLS(rcu_reader).ctr before store + * to rcu_gp_ctr. */ cmm_barrier(); @@ -244,10 +222,29 @@ void synchronize_rcu(void) */ cmm_smp_mb(); + /* Switch parity: 0 -> 1, 1 -> 0 */ + CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR); + /* - * Wait for previous parity to be empty of readers. + * Must commit rcu_gp_ctr update to memory before waiting for + * quiescent state. Failure to do so could result in the writer + * waiting forever while new readers are always accessing data + * (no progress). Enforce compiler-order of store to rcu_gp_ctr + * before load URCU_TLS(rcu_reader).ctr. */ - update_counter_and_wait(); /* 1 -> 0, wait readers in parity 1 */ + cmm_barrier(); + + /* + * 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 readers to observe new parity or be quiescent. + */ + wait_for_readers(); out: mutex_unlock(&rcu_gp_lock); @@ -280,7 +277,30 @@ void synchronize_rcu(void) mutex_lock(&rcu_gp_lock); if (cds_list_empty(®istry)) goto out; - update_counter_and_wait(); + + /* Increment current G.P. */ + CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR); + + /* + * Must commit rcu_gp_ctr update to memory before waiting for + * quiescent state. Failure to do so could result in the writer + * waiting forever while new readers are always accessing data + * (no progress). Enforce compiler-order of store to rcu_gp_ctr + * before load URCU_TLS(rcu_reader).ctr. + */ + cmm_barrier(); + + /* + * 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 readers to observe new count of be quiescent. + */ + wait_for_readers(); out: mutex_unlock(&rcu_gp_lock);