clocksource/drivers/sh_cmt: Address race condition for clock events

There is a race condition in the CMT interrupt handler. In the interrupt
handler the driver sets a driver private flag, FLAG_IRQCONTEXT. This
flag is used to indicate any call to set_next_event() should not be
directly propagated to the device, but instead cached. This is done as
the interrupt handler itself reprograms the device when needed before it
completes and this avoids this operation to take place twice.

It is unclear why this design was chosen, my suspicion is to allow the
struct clock_event_device.event_handler callback, which is called while
the FLAG_IRQCONTEXT is set, can update the next event without having to
write to the device twice.

Unfortunately there is a race between when the FLAG_IRQCONTEXT flag is
set and later cleared where the interrupt handler have already started to
write the next event to the device. If set_next_event() is called in
this window the value is only cached in the driver but not written. This
leads to the board to misbehave, or worse lockup and produce a splat.

rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: 0-...!: (0 ticks this GP) idle=f5e0/0/0x0 softirq=519/519 fqs=0 (false positive?)
rcu: (detected by 1, t=6502 jiffies, g=-595, q=77 ncpus=2)
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.10.0-rc5-arm64-renesas-00019-g74a6f86eaf1c-dirty #20
Hardware name: Renesas Salvator-X 2nd version board based on r8a77965 (DT)
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : tick_check_broadcast_expired+0xc/0x40
lr : cpu_idle_poll.isra.0+0x8c/0x168
sp : ffff800081c63d70
x29: ffff800081c63d70 x28: 00000000580000c8 x27: 00000000bfee5610
x26: 0000000000000027 x25: 0000000000000000 x24: 0000000000000000
x23: ffff00007fbb9100 x22: ffff8000818f1008 x21: ffff8000800ef07c
x20: ffff800081c79ec0 x19: ffff800081c70c28 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000 x15: 0000ffffc2c717d8
x14: 0000000000000000 x13: ffff000009c18080 x12: ffff8000825f7fc0
x11: 0000000000000000 x10: ffff8000818f3cd4 x9 : 0000000000000028
x8 : ffff800081c79ec0 x7 : ffff800081c73000 x6 : 0000000000000000
x5 : 0000000000000000 x4 : ffff7ffffe286000 x3 : 0000000000000000
x2 : ffff7ffffe286000 x1 : ffff800082972900 x0 : ffff8000818f1008
Call trace:
tick_check_broadcast_expired+0xc/0x40
do_idle+0x9c/0x280
cpu_startup_entry+0x34/0x40
kernel_init+0x0/0x11c
do_one_initcall+0x0/0x260
__primary_switched+0x80/0x88
rcu: rcu_preempt kthread timer wakeup didn't happen for 6501 jiffies! g-595 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
rcu: Possible timer handling issue on cpu=0 timer-softirq=262
rcu: rcu_preempt kthread starved for 6502 jiffies! g-595 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 ->cpu=0
rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
rcu: RCU grace-period kthread stack dump:
task:rcu_preempt state:I stack:0 pid:15 tgid:15 ppid:2 flags:0x00000008
Call trace:
__switch_to+0xbc/0x100
__schedule+0x358/0xbe0
schedule+0x48/0x148
schedule_timeout+0xc4/0x138
rcu_gp_fqs_loop+0x12c/0x764
rcu_gp_kthread+0x208/0x298
kthread+0x10c/0x110
ret_from_fork+0x10/0x20

The design have been part of the driver since it was first merged in
early 2009. It becomes increasingly harder to trigger the issue the
older kernel version one tries. It only takes a few boots on v6.10-rc5,
while hundreds of boots are needed to trigger it on v5.10.

Close the race condition by using the CMT channel lock for the two
competing sections. The channel lock was added to the driver after its
initial design.

Signed-off-by: Niklas Söderlund <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Daniel Lezcano <[e-mail]>

drivers/clocksource/sh_cmt.c

···
 static irqreturn_t sh_cmt_interrupt(int irq, void *dev_id)
 {
 	struct sh_cmt_channel *ch = dev_id;
+	unsigned long flags;
 
 	/* clear flags */
 	sh_cmt_write_cmcsr(ch, sh_cmt_read_cmcsr(ch) &
···
 
 	ch->flags &= ~FLAG_SKIPEVENT;
 
+	raw_spin_lock_irqsave(&ch->lock, flags);
+
 	if (ch->flags & FLAG_REPROGRAM) {
 		ch->flags &= ~FLAG_REPROGRAM;
 		sh_cmt_clock_event_program_verify(ch, 1);
···
 	}
 
 	ch->flags &= ~FLAG_IRQCONTEXT;
+
+	raw_spin_unlock_irqrestore(&ch->lock, flags);
 
 	return IRQ_HANDLED;
 }
···
 				   struct clock_event_device *ced)
 {
 	struct sh_cmt_channel *ch = ced_to_sh_cmt(ced);
+	unsigned long flags;
 
 	BUG_ON(!clockevent_state_oneshot(ced));
+
+	raw_spin_lock_irqsave(&ch->lock, flags);
+
 	if (likely(ch->flags & FLAG_IRQCONTEXT))
 		ch->next_match_value = delta - 1;
 	else
-		sh_cmt_set_next(ch, delta - 1);
+		__sh_cmt_set_next(ch, delta - 1);
+
+	raw_spin_unlock_irqrestore(&ch->lock, flags);
 
 	return 0;
 }