Re: [RFC PATCH 2/2 with seqcount v3] reservation: add suppport for read-only access using rcu

Thomas Hellstrom
2014-05-19T14:44:20Z

On 05/19/2014 03:13 PM, Maarten Lankhorst wrote:
> op 19-05-14 15:42, Thomas Hellstrom schreef:
>> Hi, Maarten!
>>
>> Some nitpicks, and that krealloc within rcu lock still worries me.
>> Otherwise looks good.
>>
>> /Thomas
>>
>>
>>
>> On 04/23/2014 12:15 PM, Maarten Lankhorst wrote:
>>> @@ -55,8 +60,8 @@ int reservation_object_reserve_shared(struct
>>> reservation_object *obj)
>>> kfree(obj->staged);
>>> obj->staged = NULL;
>>> return 0;
>>> - }
>>> - max = old->shared_max * 2;
>>> + } else
>>> + max = old->shared_max * 2;
>> Perhaps as a separate reformatting patch?
> I'll fold it in to the patch that added
> reservation_object_reserve_shared.
>>> +
>>> +int reservation_object_get_fences_rcu(struct reservation_object *obj,
>>> + struct fence **pfence_excl,
>>> + unsigned *pshared_count,
>>> + struct fence ***pshared)
>>> +{
>>> + unsigned shared_count = 0;
>>> + unsigned retry = 1;
>>> + struct fence **shared = NULL, *fence_excl = NULL;
>>> + int ret = 0;
>>> +
>>> + while (retry) {
>>> + struct reservation_object_list *fobj;
>>> + unsigned seq;
>>> +
>>> + seq = read_seqcount_begin(&obj->seq);
>>> +
>>> + rcu_read_lock();
>>> +
>>> + fobj = rcu_dereference(obj->fence);
>>> + if (fobj) {
>>> + struct fence **nshared;
>>> +
>>> + shared_count = ACCESS_ONCE(fobj->shared_count);
>> ACCESS_ONCE() shouldn't be needed inside the seqlock?
> Yes it is, shared_count may be increased, leading to potential
> different sizes for krealloc and memcpy
> if the ACCESS_ONCE is removed. I could use shared_max here instead,
> which stays the same,
> but it would waste more memory.

OK.
>
>>> + nshared = krealloc(shared, sizeof(*shared) *
>>> shared_count, GFP_KERNEL);
>> Again, krealloc should be a sleeping function, and not suitable within a
>> RCU read lock? I still think this krealloc should be moved to the start
>> of the retry loop, and we should start with a suitable guess of
>> shared_count (perhaps 0?) It's not like we're going to waste a lot of
>> memory....
> But shared_count is only known when holding the rcu lock.
>
> What about this change?

Sure. That should work.

/Thomas

>
> @@ -254,16 +254,27 @@ int reservation_object_get_fences_rcu(struct
> reservation_object *obj,
> fobj = rcu_dereference(obj->fence);
> if (fobj) {
> struct fence **nshared;
> + size_t sz;
>
> shared_count = ACCESS_ONCE(fobj->shared_count);
> - nshared = krealloc(shared, sizeof(*shared) *
> shared_count, GFP_KERNEL);
> + sz = sizeof(*shared) * shared_count;
> +
> + nshared = krealloc(shared, sz,
> + GFP_NOWAIT | __GFP_NOWARN);
> if (!nshared) {
> + rcu_read_unlock();
> + nshared = krealloc(shared, sz, GFP_KERNEL)
> + if (nshared) {
> + shared = nshared;
> + continue;
> + }
> +
> ret = -ENOMEM;
> - shared_count = retry = 0;
> - goto unlock;
> + shared_count = 0;
> + break;
> }
> shared = nshared;
> - memcpy(shared, fobj->shared, sizeof(*shared) *
> shared_count);
> + memcpy(shared, fobj->shared, sz);
> } else
> shared_count = 0;
> fence_excl = rcu_dereference(obj->fence_excl);
>
>
>>> +
>>> + /*
>>> + * There could be a read_seqcount_retry here, but nothing
>>> cares
>>> + * about whether it's the old or newer fence pointers that are
>>> + * signale. That race could still have happened after checking
>> Typo.
> Oops