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

[Ingo Molnar]

Cc:-ed other gents who touched the mutex code recently. Mail quoted below.

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
> 
>