x86_virt_invoke_kvm_emergency_callback() reaches rcu_dereference()
through machine_crash_shutdown() with IRQs disabled but with RCU not
necessarily watching, which triggers a suspicious RCU usage splat on
debug kernels (CONFIG_PROVE_RCU=y) during panic/kdump:
WARNING: suspicious RCU usage
arch/x86/virt/hw.c:52 suspicious rcu_dereference_check() usage!
other info that might help us debug this:
rcu_scheduler_active = 2, debug_locks = 1
1 lock held by tee/11119:
#0: ffff8881fa32c440 (sb_writers#3){.+.+}-{0:0}, at: ksys_write
Call Trace:
dump_stack_lvl+0x84/0xd0
lockdep_rcu_suspicious.cold+0x37/0x8f
x86_virt_invoke_kvm_emergency_callback+0x5f/0x70
x86_svm_emergency_disable_virtualization_cpu+0x2a/0x30
x86_virt_emergency_disable_virtualization_cpu+0x6b/0x90
native_machine_crash_shutdown+0x72/0x170
__crash_kexec+0x137/0x280
panic+0xce/0xd0
sysrq_handle_crash+0x1f/0x20
__handle_sysrq.cold+0x192/0x335
write_sysrq_trigger+0x8c/0xc0
proc_reg_write+0x1c3/0x3c0
vfs_write+0x1d0/0xf80
ksys_write+0x116/0x250
do_syscall_64+0x11c/0x1480
entry_SYSCALL_64_after_hwframe+0x76/0x7e
The RCU usage is correct: writers
(x86_virt_{register,unregister}_emergency_callback()) serialize via
rcu_assign_pointer() + synchronize_rcu(), while the reader on the
emergency path runs with IRQs disabled (the only caller is
x86_virt_emergency_disable_virtualization_cpu(), which has
lockdep_assert_irqs_disabled()), which is a valid classic-RCU read-side
critical section.
Use rcu_dereference_check() with irqs_disabled() to silence the splat
without weakening the protection.
Reproducible on a debug kernel (CONFIG_PROVE_LOCKING=y, CONFIG_PROVE_RCU=y)
with kvm_amd or kvm_intel loaded by triggering kdump:
echo c > /proc/sysrq-trigger
Fixes: 428afac5a8ea ("KVM: x86: Move bulk of emergency virtualizaton logic to virt subsystem")
Signed-off-by: Mikhail Gavrilov
---
arch/x86/virt/hw.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/arch/x86/virt/hw.c b/arch/x86/virt/hw.c
index f647557d38ac..57eebc99299d 100644
--- a/arch/x86/virt/hw.c
+++ b/arch/x86/virt/hw.c
@@ -49,7 +49,13 @@ static void x86_virt_invoke_kvm_emergency_callback(void)
{
cpu_emergency_virt_cb *kvm_callback;
- kvm_callback = rcu_dereference(kvm_emergency_callback);
+ /*
+ * Callers invoke this with IRQs disabled (see
+ * x86_virt_emergency_disable_virtualization_cpu()), which is a valid
+ * RCU read-side critical section. Tell lockdep so it doesn't complain
+ * during panic/reboot paths.
+ */
+ kvm_callback = rcu_dereference_check(kvm_emergency_callback, irqs_disabled());
if (kvm_callback)
kvm_callback();
}
--
2.54.0