----- On Jul 29, 2021, at 2:05 PM, paulmck paulmck@xxxxxxxxxx wrote: > 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? Of course, feel free to add my Acked-by. Thanks, Mathieu > > 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 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
