On Thu, Jul 29, 2021 at 01:41:41PM -0400, Mathieu Desnoyers wrote: > ----- On Jul 29, 2021, at 11:57 AM, paulmck paulmck@xxxxxxxxxx wrote: > > > On Thu, Jul 29, 2021 at 10:41:18AM -0400, Mathieu Desnoyers wrote: > >> ----- On Jul 28, 2021, at 4:28 PM, paulmck paulmck@xxxxxxxxxx wrote: > >> > >> > On Wed, Jul 28, 2021 at 04:03:02PM -0400, Mathieu Desnoyers wrote: > >> >> ----- 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" > >> > > >> > Good catch, fixed! > >> > > >> >> > 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 ? > >> > > >> > Some years back, there was a good reason. This reason was that > >> > rcutree_prepare_cpu() marked the CPU as being online from an RCU > >> > viewpoint. But now rcu_cpu_starting() is the one that marks the CPU as > >> > being online, so the ->dynticks check can be deferred to this function. > >> > > >> >> 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. > >> > > >> > Because rcutorture would not have suffered in silence had this > >> > situation ever arisen. > >> > >> Testing can usually prove the presence of a bug, but it's rather tricky to prove > >> the absence of bug. > > > > In general, true enough. > > > > But in this particular case, a WARN would have deterministically triggered > > the very next time that this CPU found its way either to the idle loop > > or to nohz_full usermode execution. > > > >> > I have updated the commit log to answer these questions as shown > >> > below. Thoughts? > >> > >> I'm still concerned about one scenario wrt moving rcu_dynticks_eqs_online() > >> from rcutree_prepare_cpu to rcu_cpu_starting. What happens if an interrupt > >> handler, or a NMI handler, nests early over the CPU-online startup code ? > >> AFAIU, this interrupt handler could contain RCU read-side critical sections, > >> but if the eqs state does not show the CPU as "online", I wonder whether it > >> will work as expected. > > > > Interrupts are still disabled at this point in the onlining process, > > my _irqsave() locks notwithstanding. > > > > You are right about NMI handlers, but there would be much more damage > > from an early NMI handler than mere RCU issues. And this can be handled > > as described in the next paragraph. > > > > Now, there are architectures (including x86) that need RCU readers > > before notify_cpu_starting() time (which is where rcu_cpu_starting() > > is invoked by default, and those architectures can (and do) simply > > place a call to rcu_cpu_starting() at an earlier appropriate point in > > the architecture-specific CPU-bringup code. And this is in fact the > > reason for the ->cpu_started check at the beginning of rcu_cpu_starting(). > > So an architecture using NMIs early in the CPU-bringup code should > > invoke rcu_cpu_starting() before enabling NMIs. > > > > So why not just be safe and mark the CPU online early in the process? > > > > Because that could result in unbounded grace periods and strange > > deadlocks. These deadlocks were broken earlier by code that assumed that > > a CPU could not possibly take more than one jiffy to come online, but that > > assumption is clearly broken on todays systems, even the bare-metal ones. > > > > In theory, I could change the raw_spin_lock_irqsave_rcu_node() to > > raw_spin_lock_rcu_node(), rely on the lockdep_assert_irqs_disabled() > > in the matching raw_spin_unlock_rcu_node(), and ditch the "flags" > > local variable, but rcu_report_qs_rnp() needs that "flags" argument. > > > > OK, I guess one approach is to get the "flags" value from local_save_flags() > > and get rid of the _irqsave and _irq restore. Assuming lockdep is > > functional that early in CPU bringup. > > > > But would that really be better than status quo? > > I'm OK with your explanation about the fact that interrupts and NMIs scenarios > are correctly handled, so moving this call from prepare_cpu to cpu_starting > is fine with me. I will add a "Link:" to this conversation. May I also add your "Acked-by" or similar? Thanx, Paul > Thanks, > > Mathieu > > > > > Thanx, Paul > > > >> Thanks, > >> > >> Mathieu > >> > >> > > >> > Thanx, Paul > >> > > >> > ------------------------------------------------------------------------ > >> > > >> > commit 516c8c4cc6fce62539f7e0182739812db4591c1d > >> > 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 when required. It is currently invoked > >> > 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. > >> > > >> > It is currently OK for rcu_dynticks_eqs_online() to have no effect, but > >> > only because the CPU-offline process just happens to leave ->dynticks in > >> > the correct state. After all, if ->dynticks were in the wrong state on a > >> > just-onlined CPU, rcutorture would complain bitterly the next time that > >> > CPU went idle, at least in kernels built with CONFIG_RCU_EQS_DEBUG=y, > >> > for example, those built by rcutorture scenario TREE04. One could > >> > argue that this means that rcu_dynticks_eqs_online() is unnecessary, > >> > however, removing it would make the CPU-online process vulnerable to > >> > slight changes in the CPU-offline process. > >> > > >> > One could also ask why it is safe to move the rcu_dynticks_eqs_online() > >> > call so late in the CPU-online process. Indeed, there was a time when it > >> > would not have been safe, which does much to explain its current location. > >> > However, the marking of a CPU as online from an RCU perspective has long > >> > since moved from rcutree_prepare_cpu() to rcu_cpu_starting(), and all > >> > that is required is that ->dynticks be set correctly by the time that > >> > the CPU is marked as online from an RCU perspective. After all, the RCU > >> > grace-period kthread does not check to see if offline CPUs are also idle. > >> > (In case you were curious, this is one reason why there is quiescent-state > >> > reporting as part of the offlining process.) > >> > > >> > 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 > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com
