Re: [RCU] zombie task hung in synchronize_rcu_expedited

[Wei Fu <fuweid89@gmail.com>]

Hi!

> 
> On 06/07, Wei Fu wrote:
> >
> > All the kernels disable CONFIG_PREEMPT and PREEMPT_RCU.
> 
> Ah, this can explain both soft-lockup and synchronize_rcu() hang. If my theory
> is correct.
> 
> Can you try the patch I sent?
> 
> Oleg.
> 

Yes. I applied your patch on v5.15.160 and run reproducer for 5 hours.
I didn't see this issue. Currently, it looks good!. I will continue that test
on this weekend.

In last reply, you mentioned TIF_NOTIFY_SIGNAL related to busy-wait loop.
Would you please explain why flag-clear works here?

Thanks,
Wei

```
➜  linux git:(v5.15.160) ✗ git --no-pager show
commit c61bd26ae81a896c8660150b4e356153da30880a (HEAD, tag: v5.15.160, origin/linux-5.15.y)
Author: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date:   Sat May 25 16:20:19 2024 +0200

    Linux 5.15.160

    Link: https://lore.kernel.org/r/20240523130327.956341021@linuxfoundation.org
    Tested-by: SeongJae Park <sj@kernel.org>
    Tested-by: Mark Brown <broonie@kernel.org>
    Tested-by: Florian Fainelli <florian.fainelli@broadcom.com>
    Tested-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
    Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
    Tested-by: Shuah Khan <skhan@linuxfoundation.org>
    Tested-by: Ron Economos <re@w6rz.net>
    Tested-by: Kelsey Steele <kelseysteele@linux.microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Makefile b/Makefile
index 5cbfe2be72dd..bfc863d71978 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 VERSION = 5
 PATCHLEVEL = 15
-SUBLEVEL = 159
+SUBLEVEL = 160
 EXTRAVERSION =
 NAME = Trick or Treat

➜  linux git:(v5.15.160) ✗ git --no-pager diff .
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 259fc4ca0d9c..40b011f88067 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -214,6 +214,7 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
         */
        do {
                clear_thread_flag(TIF_SIGPENDING);
+               clear_thread_flag(TIF_NOTIFY_SIGNAL);
                rc = kernel_wait4(-1, NULL, __WALL, NULL);
        } while (rc != -ECHILD);

```