On Wed, Nov 30, 2022 at 11:34:59AM +0800, Pingfan Liu wrote: > From: Pingfan Liu <kernelfans@xxxxxxxxx> > > The state SRCU_SIZE_WAIT_CALL is only used in srcu_gp_start_if_needed(). > And it is not needed. Because counter_wrap_check has guarantee that both > srcu_gp_seq_needed and srcu_gp_seq_needed_exp are not far behind > srcu_gp_seq and no false alarm will be raised by the statement in > srcu_gp_start_if_needed() > > ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s) > > As a result, once if SRCU_SIZE_WAIT_BARRIER is seen, the tree snp can be > used immediately, not need to wait for another srcu_gp_end() to update > the srcu_gp_seq_needed and srcu_gp_seq_needed_exp to avoid false alarm. During the SRCU_SIZE_WAIT_BARRIER state, all callbacks are queued onto the boot CPU's callback list, but the srcu_barrier() function scans all of the CPUs' callback lists. So can't this cause srcu_gp_start_if_needed() to rcu_segcblist_advance() and rcu_segcblist_accelerate() a callback list that is guaranteed to be empty? Without this change, the might-not-be-empty boot CPU's callback list is dealt with. What am I missing here? More specifically, what benefits does this change provide? I am not yet seeing any. Thanx, Paul > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx> > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > Cc: "Zhang, Qiang1" <qiang1.zhang@xxxxxxxxx> > To: rcu@xxxxxxxxxxxxxxx > --- > v1 -> v2: > rebase onto dev branch > kernel/rcu/srcutree.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > index b25f77df9e5e..b0278d1e9c1f 100644 > --- a/kernel/rcu/srcutree.c > +++ b/kernel/rcu/srcutree.c > @@ -1165,7 +1165,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp, > */ > idx = __srcu_read_lock_nmisafe(ssp); > ss_state = smp_load_acquire(&ssp->srcu_size_state); > - if (ss_state < SRCU_SIZE_WAIT_CALL) > + if (ss_state < SRCU_SIZE_WAIT_BARRIER) > sdp = per_cpu_ptr(ssp->sda, get_boot_cpu_id()); > else > sdp = raw_cpu_ptr(ssp->sda); > -- > 2.31.1 >
