Re: [PATCH] tty/sysrq: Convert show_lock to raw_spinlock_t

[Julien Grall <Julien.Grall@arm.com>]

Hello Sebastian,

On 04/03/2019 22:01, Sebastian Andrzej Siewior wrote:
> On 2019-03-04 17:21:57 [+0000], Julien Grall wrote:
>> (CC correctly linux-rt-users)
>>
>> On 04/03/2019 17:20, Julien Grall wrote:
>>> At the moment show_lock is implemented using spin_lock_t and called from
>>> an interrupt context on Arm64. The following backtrace was triggered by:
>>>
>>> 42sh# echo l > /proc/sysrq-trigger
>>>
>>> [ 4432.073756] sysrq: SysRq : Show backtrace of all active CPUs
>>> [ 4432.403422] BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:974
>>> [ 4432.403424] sysrq: CPU6:
>>> [ 4432.403426] in_atomic(): 1, irqs_disabled(): 128, pid: 2410, name: kworker/u16:2
>>>
>>> [...]
>>>
>>> [ 4432.403581] Call trace:
>>> [ 4432.403584]  dump_backtrace+0x0/0x148
>>> [ 4432.403586]  show_stack+0x14/0x20
>>> [ 4432.403588]  dump_stack+0x9c/0xd4
>>> [ 4432.403592]  ___might_sleep+0x1cc/0x298
>>> [ 4432.403595]  rt_spin_lock+0x5c/0x70
>>> [ 4432.403596]  showacpu+0x34/0x68
>>> [ 4432.403599]  flush_smp_call_function_queue+0xd4/0x278
>>> [ 4432.403602]  generic_smp_call_function_single_interrupt+0x10/0x18
>>> [ 4432.403605]  handle_IPI+0x26c/0x668
>>> [ 4432.403607]  gic_handle_irq+0x9c/0xa0
>>> [ 4432.403609]  el1_irq+0xb4/0x13c
>>>
>>> With RT-patches, spin_lock can now sleep and therefore cannot be used from
>>> interrupt context. Use a raw_spin_lock instead to prevent the lock to
>>> sleep.
>>>
>>> Signed-off-by: Julien Grall <julien.grall@arm.com>
> 
> I don't have to look at this properly but this looks is wrong.

May I ask why does it look wrong? On Arm64, this code is called from an 
IRQ disabled context (see the check in flush_smp_call_function_queue).

So it seems to me that it makes sense to switch to a raw spin lock here.
I would be happy to implement differently if you have a better solution 
here.

> Please CC the RT developers if you plan any changes that affects primary
> only the RT tree. This change is a nop for the !RT tree.

Sorry, I was not sure whether CCing linux-rt-users was enough here.

> 
>>>    drivers/tty/sysrq.c | 6 +++---
>>>    1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
>>> index 1f03078ec352..8473557c7ab2 100644
>>> --- a/drivers/tty/sysrq.c
>>> +++ b/drivers/tty/sysrq.c
>>> @@ -208,7 +208,7 @@ static struct sysrq_key_op sysrq_showlocks_op = {
>>>    #endif
>>>    #ifdef CONFIG_SMP
>>> -static DEFINE_SPINLOCK(show_lock);
>>> +static DEFINE_RAW_SPINLOCK(show_lock);
>>>    static void showacpu(void *dummy)
>>>    {
>>> @@ -218,10 +218,10 @@ static void showacpu(void *dummy)
>>>    	if (idle_cpu(smp_processor_id()))
>>>    		return;
>>> -	spin_lock_irqsave(&show_lock, flags);
>>> +	raw_spin_lock_irqsave(&show_lock, flags);
>>>    	pr_info("CPU%d:\n", smp_processor_id());
>>>    	show_stack(NULL, NULL);
>>> -	spin_unlock_irqrestore(&show_lock, flags);
>>> +	raw_spin_unlock_irqrestore(&show_lock, flags);
>>>    }
>>>    static void sysrq_showregs_othercpus(struct work_struct *dummy)
>>>

Best regards,

-- 
Julien Grall