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Peter Anvin" , Dan Williams , Chao Gao , x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Mikhail Gavrilov Subject: [PATCH] x86/virt: Fix RCU lockdep splat in emergency virt callback path Date: Sun, 3 May 2026 22:45:34 +0500 Message-ID: <20260503174534.45699-1-mikhail.v.gavrilov@gmail.com> X-Mailer: git-send-email 2.54.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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=3Dy) 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 =3D 2, debug_locks =3D 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=3Dy, CONFIG_PROVE_RCU= =3Dy) 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; =20 - kvm_callback =3D 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 =3D rcu_dereference_check(kvm_emergency_callback, irqs_disab= led()); if (kvm_callback) kvm_callback(); } --=20 2.54.0