arm64: replace read_lock to rcu lock in call_step_hook

BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:917
in_atomic(): 1, irqs_disabled(): 128, pid: 383, name: sh
Preemption disabled at:[<ffff800000124c18>] kgdb_cpu_enter+0x158/0x6b8

CPU: 3 PID: 383 Comm: sh Tainted: G        W       4.1.13-rt13 #2
Hardware name: Freescale Layerscape 2085a RDB Board (DT)
Call trace:
[<ffff8000000885e8>] dump_backtrace+0x0/0x128
[<ffff800000088734>] show_stack+0x24/0x30
[<ffff80000079a7c4>] dump_stack+0x80/0xa0
[<ffff8000000bd324>] ___might_sleep+0x18c/0x1a0
[<ffff8000007a20ac>] __rt_spin_lock+0x2c/0x40
[<ffff8000007a2268>] rt_read_lock+0x40/0x58
[<ffff800000085328>] single_step_handler+0x38/0xd8
[<ffff800000082368>] do_debug_exception+0x58/0xb8
Exception stack(0xffff80834a1e7c80 to 0xffff80834a1e7da0)
7c80: ffffff9c ffffffff 92c23ba0 0000ffff 4a1e7e40 ffff8083 001bfcc4 ffff8000
7ca0: f2000400 00000000 00000000 00000000 4a1e7d80 ffff8083 0049501c ffff8000
7cc0: 00005402 00000000 00aaa210 ffff8000 4a1e7ea0 ffff8083 000833f4 ffff8000
7ce0: ffffff9c ffffffff 92c23ba0 0000ffff 4a1e7ea0 ffff8083 001bfcc0 ffff8000
7d00: 4a0fc400 ffff8083 00005402 00000000 4a1e7d40 ffff8083 00490324 ffff8000
7d20: ffffff9c 00000000 92c23ba0 0000ffff 000a0000 00000000 00000000 00000000
7d40: 00000008 00000000 00080000 00000000 92c23b8b 0000ffff 92c23b8e 0000ffff
7d60: 00000038 00000000 00001cb2 00000000 00000005 00000000 92d7b498 0000ffff
7d80: 01010101 01010101 92be9000 0000ffff 00000000 00000000 00000030 00000000
[<ffff8000000833f4>] el1_dbg+0x18/0x6c

This issue is similar with 62c6c61a("arm64: replace read_lock to rcu lock in
call_break_hook"), but comes to single_step_handler.

This also solves kgdbts boot test silent hang issue on 4.4 -rt kernel.
Signed-off-by: Yang Shi <yang.shi@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/debug-monitors.c

@@ -186,20 +186,21 @@ static void clear_regs_spsr_ss(struct pt_regs *regs)
 
 /* EL1 Single Step Handler hooks */
 static LIST_HEAD(step_hook);
-static DEFINE_RWLOCK(step_hook_lock);
+static DEFINE_SPINLOCK(step_hook_lock);
 
 void register_step_hook(struct step_hook *hook)
 {
-	write_lock(&step_hook_lock);
-	list_add(&hook->node, &step_hook);
-	write_unlock(&step_hook_lock);
+	spin_lock(&step_hook_lock);
+	list_add_rcu(&hook->node, &step_hook);
+	spin_unlock(&step_hook_lock);
 }
 
 void unregister_step_hook(struct step_hook *hook)
 {
-	write_lock(&step_hook_lock);
-	list_del(&hook->node);
-	write_unlock(&step_hook_lock);
+	spin_lock(&step_hook_lock);
+	list_del_rcu(&hook->node);
+	spin_unlock(&step_hook_lock);
+	synchronize_rcu();
 }
 
 /*
@@ -213,15 +214,15 @@ static int call_step_hook(struct pt_regs *regs, unsigned int esr)
 	struct step_hook *hook;
 	int retval = DBG_HOOK_ERROR;
 
-	read_lock(&step_hook_lock);
+	rcu_read_lock();
 
-	list_for_each_entry(hook, &step_hook, node)	{
+	list_for_each_entry_rcu(hook, &step_hook, node)	{
 		retval = hook->fn(regs, esr);
 		if (retval == DBG_HOOK_HANDLED)
 			break;
 	}
 
-	read_unlock(&step_hook_lock);
+	rcu_read_unlock();
 
 	return retval;
 }