KCSAN: data-race in __rcu_read_unlock / sync_rcu_exp_select_cpus

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 05f22368 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=1296ff50e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=87d111955f40591f
dashboard link: https://syzkaller.appspot.com/bug?extid=99f4ddade3c22ab0cf23
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+99f4dd...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __rcu_read_unlock / sync_rcu_exp_select_cpus

read to 0xffffffff85a7d440 of 8 bytes by task 7624 on cpu 0:
rcu_read_unlock_special kernel/rcu/tree_plugin.h:615 [inline]
__rcu_read_unlock+0x381/0x3c0 kernel/rcu/tree_plugin.h:383
rcu_read_unlock include/linux/rcupdate.h:652 [inline]
filemap_map_pages+0x5b3/0x990 mm/filemap.c:2687
do_fault_around mm/memory.c:3450 [inline]
do_read_fault mm/memory.c:3484 [inline]
do_fault mm/memory.c:3618 [inline]
handle_pte_fault mm/memory.c:3849 [inline]
__handle_mm_fault+0x2554/0x2cb0 mm/memory.c:3973
handle_mm_fault+0x21b/0x530 mm/memory.c:4010
do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
__do_page_fault+0x3fb/0x9e0 arch/x86/mm/fault.c:1506
do_page_fault+0x54/0x233 arch/x86/mm/fault.c:1530
page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1202

write to 0xffffffff85a7d440 of 8 bytes by task 7353 on cpu 1:
sync_exp_reset_tree kernel/rcu/tree_exp.h:137 [inline]
sync_rcu_exp_select_cpus+0xd5/0x590 kernel/rcu/tree_exp.h:427
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: 7353 Comm: kworker/1:0 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: rcu_gp wait_rcu_exp_gp
==================================================================
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 7353 Comm: kworker/1:0 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: rcu_gp wait_rcu_exp_gp
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0xf5/0x159 lib/dump_stack.c:113
panic+0x210/0x640 kernel/panic.c:221
kcsan_report.cold+0xc/0x10 kernel/kcsan/report.c:302
__kcsan_setup_watchpoint+0x32e/0x4a0 kernel/kcsan/core.c:411
__tsan_write8 kernel/kcsan/kcsan.c:36 [inline]
__tsan_write8+0x32/0x40 kernel/kcsan/kcsan.c:36
sync_exp_reset_tree kernel/rcu/tree_exp.h:137 [inline]
sync_rcu_exp_select_cpus+0xd5/0x590 kernel/rcu/tree_exp.h:427
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
Shutting down cpus with NMI
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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[Marco Elver]

+RCU folks

[Paul E. McKenney]

Again, good catch! Does the patch shown below fix this?

Thanx, Paul

------------------------------------------------------------------------

commit c3c182aa501c13faccf5f2c168004a5c98a89d74
Author: Paul E. McKenney <pau...@kernel.org>
Date: Mon Nov 4 08:22:45 2019 -0800

rcu: Use READ_ONCE() for ->expmask in rcu_read_unlock_special()

The rcu_node structure's ->expmask field is updated only when holding the
->lock, but is also accessed locklessly. This means that all ->expmask
updates must use WRITE_ONCE() and all reads carried out without holding
->lock must use READ_ONCE(). This commit therefore changes the lockless
->expmask read in rcu_read_unlock_special() to use READ_ONCE().

Signed-off-by: Paul E. McKenney <pau...@kernel.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 6d54fd4..9a1b4b2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -601,7 +601,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
struct rcu_node *rnp = rdp->mynode;

exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) ||
- (rdp->grpmask & rnp->expmask) ||
+ (rdp->grpmask & READ_ONCE(rnp->expmask)) ||
tick_nohz_full_cpu(rdp->cpu);
// Need to defer quiescent state until everything is enabled.
if (irqs_were_disabled && use_softirq &&

[Marco Elver]

Assuming writers will follow in another patch:

Acked-by: Marco Elver <el...@google.com>

Thanks,
-- Marco

[Paul E. McKenney]

Applied, thank you!

And this time actually remembered to include the Reported-by.

Thanx, Paul