Date:   Mon, 7 Oct 2019 20:33:53 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Marco Elver <elver@...gle.com>,
        syzbot <syzbot+134336b86f728d6e55a0@...kaller.appspotmail.com>,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        rcu@...r.kernel.org, a@...table.cc,
        b.a.t.m.a.n@...ts.open-mesh.org, davem@...emloft.net,
        LKML <linux-kernel@...r.kernel.org>, mareklindner@...mailbox.ch,
        netdev@...r.kernel.org, sw@...onwunderlich.de,
        syzkaller-bugs@...glegroups.com
Subject: Re: KCSAN: data-race in find_next_bit / rcu_report_exp_cpu_mult

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@...lfernandes.org>
> > 
> > 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@...kaller.appspotmail.com
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
>  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

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