On Tue, Oct 08, 2019 at 10:50:56AM +0800, Boqun Feng wrote: > On Mon, Oct 07, 2019 at 07:12:33PM -0700, Paul E. McKenney wrote: > > On Mon, Oct 07, 2019 at 08:11:31PM -0400, Joel Fernandes wrote: > > > On Mon, Oct 07, 2019 at 09:43:04PM +0800, Boqun Feng wrote: > > > > Hi Marco, > > > > > > Hi Boqun, Steve and Paul, fun times! > > > > > > Marco, good catch ;-) > > > > Indeed! ;-) > > > [...] > > > > + mask_ofl_ipi_snap = mask_ofl_ipi; > > > > /* IPI the remaining CPUs for expedited quiescent state. */ > > > > - for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) { > > > > + for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi_snap) { > > > > Why can't we just use mask_ofl_ipi? The bits removed are only those > > bits just now looked at, right? Also, the test of mask_ofl_ipi can be > > dropped, since that branch will never be taken, correct? > > > > You're correct. But I think we can further simplify the code a little > bit so that we won't need to modify the mask_ofl_ipi: > > In the second loop: > > 1) if the target CPU is online and response the IPI we do nothing. > > 2) if the target CPU is offline but it doesn't block current GP, we do > nothing. > > 3) otherwise, the target CPU is offline and blocks current GP, we add > the corresponding bit in mask_ofl_test. > > Thoughts? > > Please see the end of email for a patch. > > > > This looks good to me. Just a nit, I prefer if the comment to IPI the > > > remaining CPUs is before the assignment to mask_ofl_ipi_snap since the > > > new assignment is done for consumption by the for_each..(..) loop itself. > > > > > > Steve's patch looks good as well and I was thinking along the same lines but > > > Boqun's patch is slightly better because he doesn't need to snapshot exp_mask > > > inside the locked section. > > > > There are also similar lockless accesses to ->expmask in the stall-warning > > code. > > > > > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > > > But thank all three of you for looking this over! My original patch > > was overly ornate. ;-) > > > > Thanx, Paul > > > > -------------------->8 > Subject: [PATCH v2] rcu: exp: Avoid race on lockless rcu_node::expmask loop > > KCSAN reported an issue: > > | BUG: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult > | > | write to 0xffffffff85a7f140 of 8 bytes by task 7 on cpu 0: > | rcu_report_exp_cpu_mult+0x4f/0xa0 kernel/rcu/tree_exp.h:244 > | rcu_report_exp_rdp+0x6c/0x90 kernel/rcu/tree_exp.h:254 > | rcu_preempt_deferred_qs_irqrestore+0x3bb/0x580 kernel/rcu/tree_plugin.h:475 > | rcu_read_unlock_special+0xec/0x370 kernel/rcu/tree_plugin.h:659 > | __rcu_read_unlock+0xcf/0xe0 kernel/rcu/tree_plugin.h:394 > | rcu_read_unlock include/linux/rcupdate.h:645 [inline] > | batadv_nc_purge_orig_hash net/batman-adv/network-coding.c:411 [inline] > | batadv_nc_worker+0x13a/0x390 net/batman-adv/network-coding.c:718 > | process_one_work+0x3d4/0x890 kernel/workqueue.c:2269 > | worker_thread+0xa0/0x800 kernel/workqueue.c:2415 > | kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253 > | ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352 > | > | read to 0xffffffff85a7f140 of 8 bytes by task 7251 on cpu 1: > | _find_next_bit lib/find_bit.c:39 [inline] > | find_next_bit+0x57/0xe0 lib/find_bit.c:70 > | sync_rcu_exp_select_node_cpus+0x28e/0x510 kernel/rcu/tree_exp.h:375 > | sync_rcu_exp_select_cpus+0x30c/0x590 kernel/rcu/tree_exp.h:439 > | rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:575 [inline] > | wait_rcu_exp_gp+0x25/0x40 kernel/rcu/tree_exp.h:589 > | process_one_work+0x3d4/0x890 kernel/workqueue.c:2269 > | worker_thread+0xa0/0x800 kernel/workqueue.c:2415 > | kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253 > | ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352 > > The root cause of this is the second for_each_leaf_node_cpu_mask() loop > in sync_rcu_exp_select_node_cpus() accesses the rcu_node::expmask > without holding rcu_node's lock. This is by design, because the second > loop may issue IPIs to other CPUs, and the IPI handler (rcu_exp_handler) > may acquire the same rcu_node's lock. So the rcu_node's lock has to be > dropped before the second loop. > > The problem will occur when the normal unsetting of rcu_node::expmask > results into some intermediate state (because it's a plain access), > where an extra bit gets zeroed. The second loop will skip the > corrensponding CPU, but treat it as offline and in quesient state. This > will cause trouble because that CPU may be in a RCU read-side critical > section. > > To fix this, make the second loop iterate on mask_ofl_ipi, as a result, > the find_next_bit() of the second loop doesn't access any variables that > may get changed in parallel, so the race is avoided. While we are at it, > remove the unset of mask_ofl_ipi to improve the readiblity, because we > can always use mask_ofl_test to record which CPU's QS should be > reported. Good point on the second loop! > Reported-by: syzbot+134336b86f728d6e55a0@xxxxxxxxxxxxxxxxxxxxxxxxx > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx> > --- > kernel/rcu/tree_exp.h | 17 +++++++---------- > 1 file changed, 7 insertions(+), 10 deletions(-) > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > index af7e7b9c86af..fb51752ac9a6 100644 > --- a/kernel/rcu/tree_exp.h > +++ b/kernel/rcu/tree_exp.h > @@ -372,12 +372,10 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > /* IPI the remaining CPUs for expedited quiescent state. */ > - for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) { > + for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) { > unsigned long mask = leaf_node_cpu_bit(rnp, cpu); > struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); > > - if (!(mask_ofl_ipi & mask)) > - continue; > retry_ipi: > if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) { > mask_ofl_test |= mask; This part I have already on -rcu branch "dev". > @@ -389,10 +387,10 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) > } > ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0); > put_cpu(); > - if (!ret) { > - mask_ofl_ipi &= ~mask; > + /* the CPU responses the IPI, and it will report QS itself */ > + if (!ret) > continue; > - } > + > /* Failed, raced with CPU hotplug operation. */ > raw_spin_lock_irqsave_rcu_node(rnp, flags); > if ((rnp->qsmaskinitnext & mask) && > @@ -403,13 +401,12 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) > schedule_timeout_uninterruptible(1); > goto retry_ipi; > } > - /* CPU really is offline, so we can ignore it. */ > - if (!(rnp->expmask & mask)) > - mask_ofl_ipi &= ~mask; > + /* CPU really is offline, and we need its QS. */ > + if (rnp->expmask & mask) > + mask_ofl_test |= mask; > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > } > /* Report quiescent states for those that went offline. */ > - mask_ofl_test |= mask_ofl_ipi; > if (mask_ofl_test) > rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false); > } Would you be willing to port this optimization on top of current -rcu branch "dev" with an suitably modified commit message? Thanx, Paul