[CRIU] Re: [RFC PATCH] posix timers: allocate timer id per task

[Pavel Emelyanov]

On 10/15/2012 07:24 PM, Stanislav Kinsbursky wrote:
> 15.10.2012 19:16, Pavel Emelyanov пишет:
>> On 10/15/2012 06:01 PM, Stanislav Kinsbursky wrote:
>>> This patch replaces global idr with global hash table for posix timers and
>>> makes timer ids unique not globally, but per task. Next free timer id is type
>>> of integer and stored on signal struct (posix_timer_id). If free timer id
>>> reaches negative value on timer creation, it will be dropped to zero and
>>> -EAGAIN will be returned to user.
>>>
>>> Hash table is size of page (4KB).
>>>
>>> Key is constructed as follows:
>>> key = hash_ptr(current->signal) ^ hash_32(posix_timer_id);
>>>
>>> ---
>>>   include/linux/posix-timers.h |    1
>>>   include/linux/sched.h        |    4 +-
>>>   kernel/posix-timers.c        |  112 ++++++++++++++++++++++++++++--------------
>>>   3 files changed, 78 insertions(+), 39 deletions(-)
>>>
>>
>>
>>> +{
>>> +	struct signal_struct *sig = current->signal;
>>> +	struct hlist_head *head;
>>> +	int ret = -ENOENT;
>>> +
>>> +	do {
>>> +		spin_lock(&hash_lock);
>>> +		head = &posix_timers_hashtable[hash(sig, sig->posix_timer_id)];
>>> +		if (__posix_timers_find(head, sig, sig->posix_timer_id) == NULL) {
>>> +			hlist_add_head(&timer->t_hash, head);
>>> +			ret = sig->posix_timer_id++;
>>> +		} else if (++sig->posix_timer_id < 0) {
>>> +			sig->posix_timer_id = 0;
>>> +			ret = -EAGAIN;
>>
>> What for? Overflow is OK, no need in restart.
>>
> 
> I was thinking, that we need some way to stop here in case of corner case, when 
> all id's are busy.
> I'm don't think that anyone can reach even INT_MAX timers in one task, so 
> INT_MAX looks enough. This "EGAIN" will be returned to user.

while (1) {
	add_timer()
	del_timer()
}

will overflow it very fast.

>>> +		}
>>> +		spin_unlock(&hash_lock);
>>> +	} while (ret == -ENOENT);
>>> +	return ret;
>>> +}
>>
> 
>