----- On Jul 28, 2021, at 3:45 PM, paulmck paulmck@xxxxxxxxxx wrote: [...] > > And how about like this? > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit cb8914dcc6443cca15ce48d937a93c0dfdb114d3 > Author: Paul E. McKenney <paulmck@xxxxxxxxxx> > Date: Wed Jul 28 12:38:42 2021 -0700 > > rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting() > > The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks > counter of an incoming CPU if required. It is currently is invoked "is currently is" -> "is currently" > from rcutree_prepare_cpu(), which runs before the incoming CPU is > running, and thus on some other CPU. This makes the per-CPU accesses in > rcu_dynticks_eqs_online() iffy at best, and it all "works" only because > the running CPU cannot possibly be in dyntick-idle mode, which means > that rcu_dynticks_eqs_online() never has any effect. One could argue > that this means that rcu_dynticks_eqs_online() is unnecessary, however, > removing it makes the CPU-online process vulnerable to slight changes > in the CPU-offline process. Why favor moving this from the prepare_cpu to the cpu_starting hotplug step, rather than using the target cpu's rdp from rcutree_prepare_cpu ? Maybe there was a good reason for having this very early in the prepare_cpu step ? Also, the commit message refers to this bug as having no effect because the running CPU cannot possibly be in dyntick-idle mode. I understand that calling this function was indeed effect-less, but then why is it OK for the CPU coming online to skip this call in the first place ? This commit message hints at "slight changes in the CPU-offline process" which could break it, but therer is no explanation of what makes this not an actual bug fix. Thanks, Mathieu > > This commit therefore moves the call to rcu_dynticks_eqs_online() from > rcutree_prepare_cpu() to rcu_cpu_starting(), this latter being guaranteed > to be running on the incoming CPU. The call to this function must of > course be placed before this rcu_cpu_starting() announces this CPU's > presence to RCU. > > Reported-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 0172a5fd6d8de..aa00babdaf544 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -4129,7 +4129,6 @@ int rcutree_prepare_cpu(unsigned int cpu) > rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs); > rdp->blimit = blimit; > rdp->dynticks_nesting = 1; /* CPU not up, no tearing. */ > - rcu_dynticks_eqs_online(); > raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */ > > /* > @@ -4249,6 +4248,7 @@ void rcu_cpu_starting(unsigned int cpu) > mask = rdp->grpmask; > WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1); > WARN_ON_ONCE(!(rnp->ofl_seq & 0x1)); > + rcu_dynticks_eqs_online(); > smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier(). > raw_spin_lock_irqsave_rcu_node(rnp, flags); > WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask); -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
