Re: "WARNING in rcu_sr_normal_complete" in Linux Kernel v6.12-rc4

[Uladzislau Rezki]

On Thu, Jan 16, 2025 at 03:03:56PM +0100, Uladzislau Rezki wrote:
> Hello, Cheung Wall!
> 
> > 
> > I am writing to report a potential vulnerability identified in the
> > Linux Kernel version v6.12-rc4. This vulnerability was discovered
> > while i was testing the kernel.
> > 
> > Linux Kernel Repository Git Commit:
> > 42f7652d3eb527d03665b09edac47f85fb600924 (tag: v6.12-rc4)
> > 
> > Bug Location: 0010:rcu_sr_normal_complete+0xd0/0x110 kernel/rcu/tree.c:1638
> > 
> > Bug report: https://pastebin.com/kuJ7wnPe
> > 
> > Entire Log: https://pastebin.com/fT42ya0T
> > 
> > Thank you for your time and attention.
> > 
> Do you have a reproducer of this splat?
> 
The reproducer is below:

for (( i=0; i<$LOOPS; i++ )); do
	tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --configs \
	'16*TREE05' --memory 10G --bootargs 'rcutorture.fwd_progress=1' \
	--kconfig "CONFIG_NR_CPUS=4"

	echo "Done $i"
done

Please note, it also requires to modify TREE05:

+# This part is for synchronize_rcu() testing
+rcutorture.nfakewriters=-1
+rcutorture.gp_sync=1
+rcupdate.rcu_normal=1
+rcutree.rcu_normal_wake_from_gp=1

and:

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index d26fb1d33ed9..6bc161e1e8ac 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -4050,6 +4050,10 @@ rcu_torture_init(void)
                                          writer_task);
        if (torture_init_error(firsterr))
                goto unwind;
+
+       if (nfakewriters < 0)
+               nfakewriters = (int) num_possible_cpus();
+
        if (nfakewriters > 0) {
                fakewriter_tasks = kcalloc(nfakewriters,
                                           sizeof(fakewriter_tasks[0]),

--
Uladzislau Rezki