Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition

[Peter Hurley <peter@hurleysoftware.com>]

On 06/24/2013 05:58 PM, Tim Chen wrote:
> On Sat, 2013-06-22 at 03:57 -0400, Peter Hurley wrote:
>> Will this spin for full scheduler value on a reader-owned lock?
>>
>>> +		/* wait_lock will be acquired if write_lock is obtained */
>>> +		if (rwsem_try_write_lock(sem->count, true, sem)) {
>>> +			ret = 1;
>>> +			goto out;
>>> +		}
>>> +
>>> +		/*
>>> +		 * When there's no owner, we might have preempted between the
>>                                                           ^^^^^^^^
>>
>> Isn't pre-emption disabled?
>>
>
> Peter, on further review, this code is needed.  This code guard against
> the case of this thread preempting another thread in the middle
> of setting the  owner field.  Disabling preemption does not prevent this
> thread from preempting others, even though others cannot preempt
> this thread.

Yep; so the "we" in the quoted comment really refers to another thread
executing down_write_xxxx().

Thanks for the clarification.

Regards,
Peter Hurley

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