Subject: Re: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult

From: Joel Fernandes <[email-masked]>
Date: Mon, 7 Oct 2019 20:11:31 -0400

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 ;-)

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
> > <[email-masked]> 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: [email-masked]
> > >
> > > ==================================================================
> > > 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: [email-masked]
> Signed-off-by: Boqun Feng <[email-masked]>
> ---
>  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) {

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.

Reviewed-by: Joel Fernandes (Google) <[email-masked]>

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
>