On 5/25/21 11:33 AM, Paul E. McKenney wrote:
[Please note: This e-mail is from an EXTERNAL e-mail address]
On Tue, May 25, 2021 at 10:31:55AM +0800, Xu, Yanfei wrote:
On 5/25/21 6:46 AM, Paul E. McKenney wrote:
[Please note: This e-mail is from an EXTERNAL e-mail address]
On Sun, May 23, 2021 at 09:13:50PM -0700, Paul E. McKenney wrote:
On Sun, May 23, 2021 at 08:51:56AM +0200, Dmitry Vyukov wrote:
On Fri, May 21, 2021 at 7:29 PM syzbot
<syzbot+7b2b13f4943374609532@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Hello,
syzbot found the following issue on:
HEAD commit: f18ba26d libbpf: Add selftests for TC-BPF management API
git tree: bpf-next
console output: https://syzkaller.appspot.com/x/log.txt?x=17f50d1ed00000
kernel config: https://syzkaller.appspot.com/x/.config?x=8ff54addde0afb5d
dashboard link: https://syzkaller.appspot.com/bug?extid=7b2b13f4943374609532
Unfortunately, I don't have any reproducer for this issue yet.
IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7b2b13f4943374609532@xxxxxxxxxxxxxxxxxxxxxxxxx
This looks rcu-related. +rcu mailing list
I think I see a possible cause for this, and will say more after some
testing and after becoming more awake Monday morning, Pacific time.
No joy. From what I can see, within RCU Tasks Trace, the calls to
get_task_struct() are properly protected (either by RCU or by an earlier
get_task_struct()), and the calls to put_task_struct() are balanced by
those to get_task_struct().
I could of course have missed something, but at this point I am suspecting
an unbalanced put_task_struct() has been added elsewhere.
As always, extra eyes on this code would be a good thing.
If it were reproducible, I would of course suggest bisection. :-/
Thanx, Paul
Hi Paul,
Could it be?
CPU1 CPU2
trc_add_holdout(t, bhp)
//t->usage==2
release_task
put_task_struct_rcu_user
delayed_put_task_struct
......
put_task_struct(t)
//t->usage==1
check_all_holdout_tasks_trace
->trc_wait_for_one_reader
->trc_del_holdout
->put_task_struct(t)
//t->usage==0 and task_struct freed
READ_ONCE(t->trc_reader_checked)
//ops， t had been freed.
So, after excuting trc_wait_for_one_reader（）, task might had been removed
from holdout list and the corresponding task_struct was freed.
And we shouldn't do READ_ONCE(t->trc_reader_checked).
I was suspicious of that call to trc_del_holdout() from within
trc_wait_for_one_reader(), but the only time it executes is in the
context of the current running task, which means that CPU 2 had better
not be invoking release_task() on it just yet.
Or am I missing your point?
Two times.
1. the task is current.
trc_wait_for_one_reader
->trc_del_holdout
2. task isn't current.
trc_wait_for_one_reader
->get_task_struct
->try_invoke_on_locked_down_task（trc_inspect_reader）
->trc_del_holdout
->put_task_struct
Of course, if you can reproduce it, the following patch might be
Sorry...I can't reproduce it, just analyse syzbot's log. :(
Thanks,
Yanfei
an interesting thing to try, my doubts notwithstanding. But more
important, please check the patch to make sure that we are both
talking about the same call to trc_del_holdout()!
If we are talking about the same call to trc_del_holdout(), are you
actually seeing that code execute except when rcu_tasks_trace_pertask()
calls trc_wait_for_one_reader()?
I investigate the trc_wait_for_one_reader（） and found before we excute
trc_del_holdout, there is always set t->trc_reader_checked=true. How about
we just set the checked flag and unified excute trc_del_holdout()
in check_all_holdout_tasks_trace with checking the flag?
The problem is that we cannot execute trc_del_holdout() except in
the context of the RCU Tasks Trace grace-period kthread. So it is
necessary to manipulate ->trc_reader_checked separately from the list
in order to safely synchronize with IPIs and with the exit code path
for any reader tasks, see for example trc_read_check_handler() and
exit_tasks_rcu_finish_trace().
Or are you thinking of some other approach?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index efb8127f3a36..2a0d4bdd619a 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -987,7 +987,6 @@ static void trc_wait_for_one_reader(struct task_struct *t,
// The current task had better be in a quiescent state.
if (t == current) {
t->trc_reader_checked = true;
- trc_del_holdout(t);
WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
return;
}
