[Devel] Re: [RFC PATCH v2] posix timers: allocate timer id per task

[Stanislav Kinsbursky]

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Or else another thread can read outdated informations...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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