On Mon, Nov 04, 2019 at 12:32:57PM +0100, Marco Elver wrote: > +RCU folks > > On Mon, 4 Nov 2019 at 12:30, syzbot > <syzbot+99f4ddade3c22ab0cf23@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > 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+99f4ddade3c22ab0cf23@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > ================================================================== > > 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.. > > > > > > --- > > This bug is generated by a bot. It may contain errors. > > See https://goo.gl/tpsmEJ for more information about syzbot. > > syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx. > > > > syzbot will keep track of this bug report. See: > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot. Again, good catch! Does the patch shown below fix this? Thanx, Paul ------------------------------------------------------------------------ commit c3c182aa501c13faccf5f2c168004a5c98a89d74 Author: Paul E. McKenney <paulmck@xxxxxxxxxx> 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 <paulmck@xxxxxxxxxx> 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 &&
