Re: [PATCH RFC ] locking/mutexes: don't spin on owner when wait list is not NULL.

[Ding Tianhong]

On 2016/1/21 15:29, Ingo Molnar wrote:
> 
> Cc:-ed other gents who touched the mutex code recently. Mail quoted below.
> 

Ok, thanks.

Ding

> Thanks,
> 
> 	Ingo
> 
> * Ding Tianhong <dingtianhong@xxxxxxxxxx> wrote:
> 
>> I build a script to create several process for ioctl loop calling,
>> the ioctl will calling the kernel function just like:
>> xx_ioctl {
>> ...
>> rtnl_lock();
>> function();
>> rtnl_unlock();
>> ...
>> }
>> The function may sleep several ms, but will not halt, at the same time
>> another user service may calling ifconfig to change the state of the
>> ethernet, and after several hours, the hung task thread report this problem:
>>
>> ========================================================================
>> 149738.039038] INFO: task ifconfig:11890 blocked for more than 120 seconds.
>> [149738.040597] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [149738.042280] ifconfig D ffff88061ec13680 0 11890 11573 0x00000080
>> [149738.042284] ffff88052449bd40 0000000000000082 ffff88053a33f300 ffff88052449bfd8
>> [149738.042286] ffff88052449bfd8 ffff88052449bfd8 ffff88053a33f300 ffffffff819e6240
>> [149738.042288] ffffffff819e6244 ffff88053a33f300 00000000ffffffff ffffffff819e6248
>> [149738.042290] Call Trace:
>> [149738.042300] [<ffffffff8160d219>] schedule_preempt_disabled+0x29/0x70
>> [149738.042303] [<ffffffff8160af65>] __mutex_lock_slowpath+0xc5/0x1c0
>> [149738.042305] [<ffffffff8160a3cf>] mutex_lock+0x1f/0x2f
>> [149738.042309] [<ffffffff8150d945>] rtnl_lock+0x15/0x20
>> [149738.042311] [<ffffffff81514e3a>] dev_ioctl+0xda/0x590
>> [149738.042314] [<ffffffff816121cc>] ? __do_page_fault+0x21c/0x560
>> [149738.042318] [<ffffffff814e42c5>] sock_do_ioctl+0x45/0x50
>> [149738.042320] [<ffffffff814e49d0>] sock_ioctl+0x1f0/0x2c0
>> [149738.042324] [<ffffffff811dc9b5>] do_vfs_ioctl+0x2e5/0x4c0
>> [149738.042327] [<ffffffff811e6a00>] ? fget_light+0xa0/0xd0
>>
>> ================================ cut here ================================
>>
>> I got the vmcore and found that the ifconfig is already in the wait_list of the
>> rtnl_lock for 120 second, but my process could get and release the rtnl_lock
>> normally several times in one second, so it means that my process jump the
>> queue and the ifconfig couldn't get the rtnl all the time, I check the mutex lock
>> slow path and found that the mutex may spin on owner ignore whether the  wait list
>> is empty, it will cause the task in the wait list always be cut in line, so add
>> test for wait list in the mutex_can_spin_on_owner and avoid this problem.
>>
>> Signed-off-by: Ding Tianhong <dingtianhong@xxxxxxxxxx>
>> ---
>>  kernel/locking/mutex.c | 11 ++++++-----
>>  1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
>> index 0551c21..596b341 100644
>> --- a/kernel/locking/mutex.c
>> +++ b/kernel/locking/mutex.c
>> @@ -256,7 +256,7 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
>>  	struct task_struct *owner;
>>  	int retval = 1;
>>  
>> -	if (need_resched())
>> +	if (need_resched() || atomic_read(&lock->count) == -1)
>>  		return 0;
>>  
>>  	rcu_read_lock();
>> @@ -283,10 +283,11 @@ static inline bool mutex_try_to_acquire(struct mutex *lock)
>>  /*
>>   * Optimistic spinning.
>>   *
>> - * We try to spin for acquisition when we find that the lock owner
>> - * is currently running on a (different) CPU and while we don't
>> - * need to reschedule. The rationale is that if the lock owner is
>> - * running, it is likely to release the lock soon.
>> + * We try to spin for acquisition when we find that there are no
>> + * pending waiters and the lock owner is currently running on a
>> + * (different) CPU and while we don't need to reschedule. The
>> + * rationale is that if the lock owner is running, it is likely
>> + * to release the lock soon.
>>   *
>>   * Since this needs the lock owner, and this mutex implementation
>>   * doesn't track the owner atomically in the lock field, we need to
>> -- 
>> 2.5.0
>>
>>
> 
> .
>