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! ;-) > Some comments below: > > > On Mon, Oct 07, 2019 at 12:04:16PM +0200, Marco Elver wrote: > > > +RCU maintainers > > > This might be a data-race in RCU itself. > > > > > > On Mon, 7 Oct 2019 at 12:01, syzbot > > > <syzbot+134336b86f728d6e55a0@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > Hello, > > > > > > > > syzbot found the following crash on: > > > > > > > > HEAD commit: b4bd9343 x86, kcsan: Enable KCSAN for x86 > > > > git tree: https://github.com/google/ktsan.git kcsan > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=11edb20d600000 > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=c0906aa620713d80 > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=134336b86f728d6e55a0 > > > > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > > > > > > > > Unfortunately, I don't have any reproducer for this crash yet. > > > > > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > > > > Reported-by: syzbot+134336b86f728d6e55a0@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > > > ================================================================== > > > > 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 > > > > This is the second for_each_leaf_node_cpu_mask() loop in > > sync_rcu_exp_select_node_cpus(), the first loop is for collecting which > > CPU blocks current grace period (IOW, which CPU need to be sent an IPI > > to), and the second loop does the real work of sending IPI. The first > > loop is protected by proper lock (rcu node lock), so there is no race > > there. But the second one can't hold rcu node lock, because the IPI > > handler (rcu_exp_handler) needs to acquire the same lock, so rcu node > > lock has to be dropped before the second loop to avoid deadlock. > > > > Now for the racy find_next_bit() on rnp->expmask: > > > > 1) if an extra bit appears: it's OK since there is checking on whether > > the bit exists in mask_ofl_ipi (the result of the first loop). > > > > 2) if a bit is missing: it will be problematic, because the second loop > > will skip the CPU, and the rest of the code will treat the CPU as > > offline but hasn't reported a quesient state, and the > > rcu_report_exp_cpu_mult() will report the qs for it, even though the CPU > > may currenlty run inside a RCU read-side critical section. > > > > Note both "appears" and "missing" means some intermediate state of a > > plain unset for expmask contributed by compiler magic. > > > > Please see below for a compile-test-only patch: > > > > > > 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 > > > > > > > > Reported by Kernel Concurrency Sanitizer on: > > > > CPU: 1 PID: 7251 Comm: kworker/1:4 Not tainted 5.3.0+ #0 > > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > > > > Google 01/01/2011 > > > > Workqueue: rcu_gp wait_rcu_exp_gp > > > > ================================================================== > > > > > > > > > > > > Regards, > > Boqun > > > > ------------------->8 > > Subject: [PATCH] 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, take a snapshot of mask_ofl_ipi, and make the second loop > > iterate on the snapshot's bits, 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. > > > > Reported-by: syzbot+134336b86f728d6e55a0@xxxxxxxxxxxxxxxxxxxxxxxxx > > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx> > > --- > > kernel/rcu/tree_exp.h | 6 +++--- > > 1 file changed, 3 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index af7e7b9c86af..7f3e19d0275e 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -335,6 +335,7 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) > > unsigned long flags; > > unsigned long mask_ofl_test; > > unsigned long mask_ofl_ipi; > > + unsigned long mask_ofl_ipi_snap; > > int ret; > > struct rcu_exp_work *rewp = > > container_of(wp, struct rcu_exp_work, rew_work); > > @@ -371,13 +372,12 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) > > rnp->exp_tasks = rnp->blkd_tasks.next; > > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > > + 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? > 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 > thanks, > > - Joel > > > 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; > > -- > > 2.23.0 > >
