The irq_lock() usage here is incompatible with SMP systems, and one's
first reaction might be to convert it to a spinlock.
But are those irq_lock() instances really necessary?
Commit 6161ea2542 ("net: socket: socketpair: mitigate possible race
condition") doesn't say much:
> There was a possible race condition between sock_is_nonblock()
> and k_sem_take() in spair_read() and spair_write() that was
> mitigated.
A possible race without the irq_lock would be:
thread A thread B
| |
+ spair_write(): |
+ is_nonblock = sock_is_nonblock(spair); [false]
* [preemption here] |
| + spair_ioctl():
| + res = k_sem_take(&spair->sem, K_FOREVER);
| + [...]
| + spair->flags |= SPAIR_FLAG_NONBLOCK;
| * [preemption here]
+ res = k_sem_take(&spair->sem, K_NO_WAIT); [-1]
+ if (res < 0) { |
+ if (is_nonblock) { [skipped] }
* res = k_sem_take(&spair->sem, K_FOREVER); [blocks here]
| + [...]
But the version with irq_lock() isn't much better:
thread A thread B
| |
| + spair_ioctl():
| + res = k_sem_take(&spair->sem, K_FOREVER);
| + [...]
| * [preemption here]
+ spair_write(): |
+ irq_lock(); |
+ is_nonblock = sock_is_nonblock(spair); [false]
+ res = k_sem_take(&spair->sem, K_NO_WAIT); [-1]
+ irq_unlock(); |
* [preemption here] |
| + spair->flags |= SPAIR_FLAG_NONBLOCK;
| + [...]
| + k_sem_give(&spair->sem);
| + spair_read():
| + res = k_sem_take(&spair->sem, K_NO_WAIT);
| * [preemption here]
+ if (res < 0) { |
+ if (is_nonblock) { [skipped] }
* res = k_sem_take(&spair->sem, K_FOREVER); [blocks here]
In both cases the last k_sem_take(K_FOREVER) will block despite
SPAIR_FLAG_NONBLOCK being set at that moment. Other race scenarios
exist too, and on SMP they are even more likely.
The only guarantee provided by the irq_lock() is to make sure that
whenever the semaphore is acquired then the is_nonblock value is always
current. A better way to achieve that and be SMP compatible is to simply
move the initial sock_is_nonblock() *after* the k_sem_take() and remove
those irq_locks().
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
cf49699b0d