Re: [PATCH] futex: Avoid wake_futex for a PI futex_q

[Darren Hart <dvhart@linux.intel.com>]

On 11/21/2012 02:34 PM, Andrew Morton wrote:
> On Tue, 20 Nov 2012 23:36:45 -0800
> Darren Hart <dvhart@linux.intel.com> wrote:
> 
>> Dave Jones reported a bug with futex_lock_pi() that his trinity test
>> exposed. Sometime between queue_me() and taking the q.lock_ptr, the
>> lock_ptr became NULL, resulting in a crash.
>>
>> While futex_wake() is careful to not call wake_futex() on futex_q's with
>> a pi_state or an rt_waiter (which are either waiting for a
>> futex_unlock_pi() or a PI futex_requeue()), futex_wake_op() and
>> futex_requeue() do not perform the same test.
>>
>> Update futex_wake_op() and futex_requeue() to test for q.pi_state and
>> q.rt_waiter and abort with -EINVAL if detected. To ensure any future
>> breakage is caught, add a WARN() to wake_futex() if the same condition
>> is true.
>>
>> This fix has seen 3 hours of testing with "trinity -c futex" on an
>> x86_64 VM with 4 CPUS.
>>
>> ...
>>
>> --- a/kernel/futex.c
>> +++ b/kernel/futex.c
>> @@ -840,6 +840,11 @@ static void wake_futex(struct futex_q *q)
>>  {
>>  	struct task_struct *p = q->task;
>>  
>> +	if (q->pi_state || q->rt_waiter) {
>> +		WARN(1, "%s: refusing to wake PI futex\n", __FUNCTION__);
>> +		return;
>> +	}
> 
> There's no need to display __FUNCTION__ because WARN() gives a
> backtrace, and we can more neatly use the WARN() return value:
> 
> --- a/kernel/futex.c~futex-avoid-wake_futex-for-a-pi-futex_q-fix
> +++ a/kernel/futex.c
> @@ -843,10 +843,8 @@ static void wake_futex(struct futex_q *q
>  {
>  	struct task_struct *p = q->task;
>  
> -	if (q->pi_state || q->rt_waiter) {
> -		WARN(1, "%s: refusing to wake PI futex\n", __FUNCTION__);
> +	if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
>  		return;
> -	}

Thanks, that's better. Duh. That block of code used to be a lot more
complex during my debug sessions, I should have caught that and boiled
it down one step further.

--
Darren


>  
>  	/*
>  	 * We set q->lock_ptr = NULL _before_ we wake up the task. If
> _
> 
>>
>> ...
>>

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel