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Date:   Mon,  4 Mar 2019 17:20:53 +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.ort, Julien Grall <julien.grall@....com>
Subject: [PATCH] tty/sysrq: Convert show_lock to raw_spinlock_t

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)
-- 
2.11.0

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