linux-kernel - Re: a competition when some threads acquire futex

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Date:   Wed, 6 Sep 2017 08:27:31 +0800
From:   "chengjian (D)" <cj.chengjian@...wei.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     <huawei.libin@...wei.com>, <mingo@...hat.com>,
        <peterz@...radead.org>, <dvhart@...radead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: a competition when some threads acquire futex

>> diff --git a/kernel/futex.c b/kernel/futex.c
>> index 3d38eaf..0b2d17a 100644
>> --- a/kernel/futex.c
>> +++ b/kernel/futex.c
>> @@ -1545,6 +1545,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval,
>> struct futex_pi_state *pi_
>>    	spin_unlock(&hb->lock);
>>   	wake_up_q(&wake_q);
>> +        _cond_resched( );
>

Hi Thomas,

I wrote _cond_resched( ) in futex_wake( ) which will be called to wake 
up waiters
when the process release the futex.


But the patch producted by git format-patch displayed in wake_futex_pi( ).


```c

/*
  * Wake up waiters matching bitset queued on this futex (uaddr).
  */
static int
futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
{
     struct futex_hash_bucket *hb;
     struct futex_q *this, *next;
     union futex_key key = FUTEX_KEY_INIT;
     int ret;
     DEFINE_WAKE_Q(wake_q);

     if (!bitset)
         return -EINVAL;

     ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, VERIFY_READ);
     if (unlikely(ret != 0))
         goto out;

     hb = hash_futex(&key);

     /* Make sure we really have tasks to wakeup */
     if (!hb_waiters_pending(hb))
         goto out_put_key;

     spin_lock(&hb->lock);

     plist_for_each_entry_safe(this, next, &hb->chain, list) {
         if (match_futex (&this->key, &key)) {
             if (this->pi_state || this->rt_waiter) {
                 ret = -EINVAL;
                 break;
             }

             /* Check if one of the bits is set in both bitsets */
             if (!(this->bitset & bitset))
                 continue;

             mark_wake_futex(&wake_q, this);
             if (++ret >= nr_wake)
                 break;
         }
     }

     spin_unlock(&hb->lock);
     wake_up_q(&wake_q);

+ _cond_resched( );
out_put_key:
     put_futex_key(&key);
out:
     return ret;
}
```



Thanks.

Cheng Jian