From: Douglas Anderson dianders@chromium.org
[ Upstream commit 440ab9e10e2e6e5fd677473ee6f9e3af0f6904d6 ]
At times when I'm using kgdb I see a splat on my console about suspicious RCU usage. I managed to come up with a case that could reproduce this that looked like this:
WARNING: suspicious RCU usage 5.7.0-rc4+ #609 Not tainted ----------------------------- kernel/pid.c:395 find_task_by_pid_ns() needs rcu_read_lock() protection!
other info that might help us debug this:
rcu_scheduler_active = 2, debug_locks = 1 3 locks held by swapper/0/1: #0: ffffff81b6b8e988 (&dev->mutex){....}-{3:3}, at: __device_attach+0x40/0x13c #1: ffffffd01109e9e8 (dbg_master_lock){....}-{2:2}, at: kgdb_cpu_enter+0x20c/0x7ac #2: ffffffd01109ea90 (dbg_slave_lock){....}-{2:2}, at: kgdb_cpu_enter+0x3ec/0x7ac
stack backtrace: CPU: 7 PID: 1 Comm: swapper/0 Not tainted 5.7.0-rc4+ #609 Hardware name: Google Cheza (rev3+) (DT) Call trace: dump_backtrace+0x0/0x1b8 show_stack+0x1c/0x24 dump_stack+0xd4/0x134 lockdep_rcu_suspicious+0xf0/0x100 find_task_by_pid_ns+0x5c/0x80 getthread+0x8c/0xb0 gdb_serial_stub+0x9d4/0xd04 kgdb_cpu_enter+0x284/0x7ac kgdb_handle_exception+0x174/0x20c kgdb_brk_fn+0x24/0x30 call_break_hook+0x6c/0x7c brk_handler+0x20/0x5c do_debug_exception+0x1c8/0x22c el1_sync_handler+0x3c/0xe4 el1_sync+0x7c/0x100 rpmh_rsc_probe+0x38/0x420 platform_drv_probe+0x94/0xb4 really_probe+0x134/0x300 driver_probe_device+0x68/0x100 __device_attach_driver+0x90/0xa8 bus_for_each_drv+0x84/0xcc __device_attach+0xb4/0x13c device_initial_probe+0x18/0x20 bus_probe_device+0x38/0x98 device_add+0x38c/0x420
If I understand properly we should just be able to blanket kgdb under one big RCU read lock and the problem should go away. We'll add it to the beast-of-a-function known as kgdb_cpu_enter().
With this I no longer get any splats and things seem to work fine.
Signed-off-by: Douglas Anderson dianders@chromium.org Link: https://lore.kernel.org/r/20200602154729.v2.1.I70e0d4fd46d5ed2aaf0c98a355e8e... Signed-off-by: Daniel Thompson daniel.thompson@linaro.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/debug/debug_core.c | 4 ++++ 1 file changed, 4 insertions(+)
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c index 159a53ff27162..694fcd0492827 100644 --- a/kernel/debug/debug_core.c +++ b/kernel/debug/debug_core.c @@ -489,6 +489,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs, arch_kgdb_ops.disable_hw_break(regs);
acquirelock: + rcu_read_lock(); /* * Interrupts will be restored by the 'trap return' code, except when * single stepping. @@ -545,6 +546,7 @@ return_normal: atomic_dec(&slaves_in_kgdb); dbg_touch_watchdogs(); local_irq_restore(flags); + rcu_read_unlock(); return 0; } cpu_relax(); @@ -563,6 +565,7 @@ return_normal: raw_spin_unlock(&dbg_master_lock); dbg_touch_watchdogs(); local_irq_restore(flags); + rcu_read_unlock();
goto acquirelock; } @@ -682,6 +685,7 @@ kgdb_restore: raw_spin_unlock(&dbg_master_lock); dbg_touch_watchdogs(); local_irq_restore(flags); + rcu_read_unlock();
return kgdb_info[cpu].ret_state; }