[PATCH AUTOSEL 4.9 13/13] kgdb: Avoid suspicious RCU usage warning

Sasha Levin
2020-07-02T01:28:05Z

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.I70e0d4fd46d5ed2aaf0c98a355e8e1b7a5bb7e4e@changeid
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 9c939c6bf21cb..321ccdbb73649 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -488,6 +488,7 @@ 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.
@@ -542,6 +543,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
atomic_dec(&slaves_in_kgdb);
dbg_touch_watchdogs();
local_irq_restore(flags);
+ rcu_read_unlock();
return 0;
}
cpu_relax();
@@ -560,6 +562,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
raw_spin_unlock(&dbg_master_lock);
dbg_touch_watchdogs();
local_irq_restore(flags);
+ rcu_read_unlock();

goto acquirelock;
}
@@ -677,6 +680,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
raw_spin_unlock(&dbg_master_lock);
dbg_touch_watchdogs();
local_irq_restore(flags);
+ rcu_read_unlock();

return kgdb_info[cpu].ret_state;
}
--
2.25.1