Subject: [v2 PATCH] arm64: replace read_lock to rcu lock in call_break_hook

From: [email-masked] (Shi, Yang)
Date: Thu, 01 Oct 2015 13:53:51 -0700

On 10/1/2015 10:08 AM, Steven Rostedt wrote:
> On Thu,  1 Oct 2015 09:37:37 -0700
> Yang Shi <[email-masked]> wrote:
>
>> BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:917
>> in_atomic(): 0, irqs_disabled(): 128, pid: 342, name: perf
>> 1 lock held by perf/342:
>>   #0:  (break_hook_lock){+.+...}, at: [<ffffffc0000851ac>] call_break_hook+0x34/0xd0
>> irq event stamp: 62224
>> hardirqs last  enabled at (62223): [<ffffffc00010b7bc>] __call_rcu.constprop.59+0x104/0x270
>> hardirqs last disabled at (62224): [<ffffffc0000fbe20>] vprintk_emit+0x68/0x640
>> softirqs last  enabled at (0): [<ffffffc000097928>] copy_process.part.8+0x428/0x17f8
>> softirqs last disabled at (0): [<          (null)>]           (null)
>> CPU: 0 PID: 342 Comm: perf Not tainted 4.1.6-rt5 #4
>> Hardware name: linux,dummy-virt (DT)
>> Call trace:
>> [<ffffffc000089968>] dump_backtrace+0x0/0x128
>> [<ffffffc000089ab0>] show_stack+0x20/0x30
>> [<ffffffc0007030d0>] dump_stack+0x7c/0xa0
>> [<ffffffc0000c878c>] ___might_sleep+0x174/0x260
>> [<ffffffc000708ac8>] __rt_spin_lock+0x28/0x40
>> [<ffffffc000708db0>] rt_read_lock+0x60/0x80
>> [<ffffffc0000851a8>] call_break_hook+0x30/0xd0
>> [<ffffffc000085a70>] brk_handler+0x30/0x98
>> [<ffffffc000082248>] do_debug_exception+0x50/0xb8
>> Exception stack(0xffffffc00514fe30 to 0xffffffc00514ff50)
>> fe20:                                     00000000 00000000 c1594680 0000007f
>> fe40: ffffffff ffffffff 92063940 0000007f 0550dcd8 ffffffc0 00000000 00000000
>> fe60: 0514fe70 ffffffc0 000be1f8 ffffffc0 0514feb0 ffffffc0 0008948c ffffffc0
>> fe80: 00000004 00000000 0514fed0 ffffffc0 ffffffff ffffffff 9282a948 0000007f
>> fea0: 00000000 00000000 9282b708 0000007f c1592820 0000007f 00083914 ffffffc0
>> fec0: 00000000 00000000 00000010 00000000 00000064 00000000 00000001 00000000
>> fee0: 005101e0 00000000 c1594680 0000007f c1594740 0000007f ffffffd8 ffffff80
>> ff00: 00000000 00000000 00000000 00000000 c1594770 0000007f c1594770 0000007f
>> ff20: 00665e10 00000000 7f7f7f7f 7f7f7f7f 01010101 01010101 00000000 00000000
>> ff40: 928e4cc0 0000007f 91ff11e8 0000007f
>>
>> call_break_hook is called in atomic context (hard irq disabled), so replace
>> the sleepable lock to rcu lock and replace relevant list operations to rcu
>> version.
>>
>> Signed-off-by: Yang Shi <[email-masked]>
>> ---
>> v1-> v2
>> Replace list operations to rcu version.
>>
>>   arch/arm64/kernel/debug-monitors.c | 10 +++++-----
>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
>> index cebf786..cf0e4fc 100644
>> --- a/arch/arm64/kernel/debug-monitors.c
>> +++ b/arch/arm64/kernel/debug-monitors.c
>> @@ -276,14 +276,14 @@ static DEFINE_RWLOCK(break_hook_lock);
>>   void register_break_hook(struct break_hook *hook)
>>   {
>>   	write_lock(&break_hook_lock);
>> -	list_add(&hook->node, &break_hook);
>> +	list_add_rcu(&hook->node, &break_hook);
>>   	write_unlock(&break_hook_lock);
>>   }
>>
>>   void unregister_break_hook(struct break_hook *hook)
>>   {
>>   	write_lock(&break_hook_lock);
>> -	list_del(&hook->node);
>> +	list_del_rcu(&hook->node);
>>   	write_unlock(&break_hook_lock);
>>   }
>
> Shouldn't there be a synchronize_rcu() somewhere?

So far kgdb is the only user of unregister_break_hook in mainline kernel.

Just read Documentation/RCU/checklist.txt, it says:

Note that synchronize_rcu() -only- guarantees to wait until
all currently executing rcu_read_lock()-protected RCU read-side critical 
sections complete.

For kgdb, the unregister is just called in kgdb_arch_exit by 
kgdb_unregister_io_module, which is called when rmmod kgdb module.

The break point handler is done synchronously. So, it sounds should be 
not a problem without calling synchronize_rcu().

Yang

> -- Steve
>
>>
>> @@ -292,11 +292,11 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
>>   	struct break_hook *hook;
>>   	int (*fn)(struct pt_regs *regs, unsigned int esr) = NULL;
>>
>> -	read_lock(&break_hook_lock);
>> -	list_for_each_entry(hook, &break_hook, node)
>> +	rcu_read_lock();
>> +	list_for_each_entry_rcu(hook, &break_hook, node)
>>   		if ((esr & hook->esr_mask) == hook->esr_val)
>>   			fn = hook->fn;
>> -	read_unlock(&break_hook_lock);
>> +	rcu_read_unlock();
>>
>>   	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
>>   }
>