Messages in this thread | | | Date | Tue, 13 Oct 2015 16:41:41 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt |
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On 10/13/2015 02:23 PM, Peter Zijlstra wrote: > On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote: >> for (;; waitcnt++) { >> + loop = SPIN_THRESHOLD; >> + while (loop) { >> + /* >> + * Spin until the lock is free >> + */ >> + for (; loop&& READ_ONCE(l->locked); loop--) >> + cpu_relax(); >> + /* >> + * Seeing the lock is free, this queue head vCPU is >> + * the rightful next owner of the lock. However, the >> + * lock may have just been stolen by another task which >> + * has entered the slowpath. So we need to use atomic >> + * operation to make sure that we really get the lock. >> + * Otherwise, we have to wait again. >> + */ >> + if (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0) >> + goto gotlock; >> } > for (loop = SPIN_THRESHOLD; loop; --loop) { > if (!READ_ONCE(l->locked)&& > cmpxchg(&l->locked, 0, _Q_LOCKED_VA) == 0) > goto gotlock; > > cpu_relax(); > } >
This was the code that I used in my original patch, but it seems to confuse you about doing too many lock stealing. So I separated it out to make my intention more explicit. I will change it back to the old code.
Cheers, Longman
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