linux-kernel - Re: [RFC PATCH v2] posix timers: allocate timer id per task

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Date:	Wed, 17 Oct 2012 18:26:13 +0400
From:	Stanislav Kinsbursky <skinsbursky@...allels.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...nvz.org" <devel@...nvz.org>
Subject: Re: [RFC PATCH v2] posix timers: allocate timer id per task

17.10.2012 17:57, Eric Dumazet пишет:
> On Wed, 2012-10-17 at 17:18 +0400, Stanislav Kinsbursky wrote:
>
>> +static int posix_timer_add(struct k_itimer *timer)
>> +{
>> +	struct signal_struct *sig = current->signal;
>> +	int next_free_id = sig->posix_timer_id;
>> +	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_rcu(&timer->t_hash, head);
>
> Hmm...
>
>> +			ret = sig->posix_timer_id++;
>> +		} else {
>> +			if (++sig->posix_timer_id < 0)
>> +				sig->posix_timer_id = 0;
>> +			if (sig->posix_timer_id == next_free_id)
>> +				ret = -EAGAIN;
>> +		}
>> +		spin_unlock(&hash_lock);
>> +	} while (ret == -ENOENT);
>> +	return ret;
>> +}
>> +
>
> You probably need to add a rcu_assign_pointer() or smp_wmb() before
> the :
>
> new_timer->it_signal = current->signal;
>
> in the following block :
>
>          spin_lock_irq(&current->sighand->siglock);
>          new_timer->it_signal = current->signal;
>          list_add(&new_timer->list, &current->signal->posix_timers);
>          spin_unlock_irq(&current->sighand->siglock);
>
> Or else another thread can read outdated informations...
>
>
>          spin_lock_irq(&current->sighand->siglock);
>          list_add(&new_timer->list, &current->signal->posix_timers);
>          spin_unlock_irq(&current->sighand->siglock);
> 	smp_wmb();
>          new_timer->it_signal = current->signal;
>
>

Sorry, but I don't understand what races you are talking about.
Could you specify, please?

-- 
Best regards,
Stanislav Kinsbursky
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