Re: [PATCH] locking/mutex: Reduce chance of setting HANDOFF bit on unlocked mutex

[Waiman Long <llong@redhat.com>]

On 6/30/21 6:21 AM, Peter Zijlstra wrote:
> On Tue, Jun 29, 2021 at 04:11:38PM -0400, Waiman Long wrote:
>
>> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
>> index d2df5e68b503..472ab21b5b8e 100644
>> --- a/kernel/locking/mutex.c
>> +++ b/kernel/locking/mutex.c
>> @@ -118,9 +118,9 @@ static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock)
>>   		}
>>   
>>   		/*
>> -		 * We set the HANDOFF bit, we must make sure it doesn't live
>> -		 * past the point where we acquire it. This would be possible
>> -		 * if we (accidentally) set the bit on an unlocked mutex.
>> +		 * Always clear the HANDOFF bit before acquiring the lock.
>> +		 * Note that if the bit is accidentally set on an unlocked
>> +		 * mutex, anyone can acquire it.
>>   		 */
>>   		flags &= ~MUTEX_FLAG_HANDOFF;
>>   
>> @@ -180,6 +180,11 @@ static inline void __mutex_set_flag(struct mutex *lock, unsigned long flag)
>>   	atomic_long_or(flag, &lock->owner);
>>   }
>>   
>> +static inline long __mutex_fetch_set_flag(struct mutex *lock, unsigned long flag)
>> +{
>> +	return atomic_long_fetch_or_relaxed(flag, &lock->owner);
>> +}
>> +
>>   static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag)
>>   {
> Hurmph, so we already have a cmpxchg loop in trylock, might as well make
> that do exactly what we want without holes on.
>
> How's something like the below? Boot tested, but please verify.
>
> ---
>   kernel/locking/mutex.c | 89 ++++++++++++++++++++++++++------------------------
>   1 file changed, 46 insertions(+), 43 deletions(-)

The code looks good to me. It is an even better approach to make sure 
that the HANDOFF will never be set on an unlocked mutex.

Reviewed-by: Waiman Long <longman@redhat.com>