linux-kernel - Re: [PATCH] tty/sysrq: Convert show_lock to raw_spinlock

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Message-ID: <545fb96e-0f07-758e-5c79-a725e74e3387@arm.com>
Date:   Mon, 4 Mar 2019 17:21:57 +0000
From:   Julien Grall <julien.grall@....com>
To:     linux-kernel@...r.kernel.org
Cc:     jslaby@...e.com, gregkh@...uxfoundation.org,
        "linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH] tty/sysrq: Convert show_lock to raw_spinlock_t

(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@....com>
> ---
>   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)
> 

-- 
Julien Grall