Re: [Xen-devel] [PATCH v2 6/9] spinlock: Introduce spin_lock_cb()

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/18/2017 02:49 AM, Jan Beulich wrote:
>>>> On 13.04.17 at 18:55, <boris.ostrovsky@xxxxxxxxxx> wrote:
>> On 04/13/2017 11:46 AM, Jan Beulich wrote:
>>>>>> On 03.04.17 at 18:50, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>> While waiting for a lock we may want to periodically run some
>>>> code. We could use spin_trylock() but since it doesn't take lock
>>>> ticket it may take a long time until the lock is taken.
>>>>
>>>> Add spin_lock_cb() that allows us to execute a callback while waiting.
>>> You don't add any user(s) of this new interface and you also don't
>>> outline under what conditions you think using this might be a good
>>> idea. On that basis I don't think this makes much sense. I am
>>> particularly worried of undue latencies use of this function may
>>> incur.
>> There is (currently) only one user of this interface and it is
>> introduced in the next patch.
>>
>> If you don't think explanation above is sufficient I can add
>>
>>     "This code may, for example, allow the caller to release resources
>>       held by it that are no longer needed in the critical section protected
>>       by the lock."
>>
>> after the first sentence.
>>
>> As for latency, the fast path is not affected, it's only if the lock is
>> already taken do we make the callback.
> That's a rather relevant aspect, which I think needs calling out
> explicitly. As you may have noticed, my initial understanding of
> the basic idea here was that the callback would be invoked while
> spinning (i.e. to use to spinning time to do something useful),
> not while holding the lock.

The callback *is* invoked when we are spinning waiting for the lock. I
probably should have said "only if the lock is already taken by someone
else". However, on the fast path, where noone is holding the lock and
the caller can grab it right away, the callback is not invoked.

-boris




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