Understanding locks in networking

lock_sock and release_sock do not hold a normal spinlock directly but instead hold the owner field and do other housework as well.

lock_sock grabs the lock sk->sk_lock.slock, disables local bottom halves and then it checks to see if there is an owner. If it does it spins until this releases, sets the owner and then releases sk->sk_lock.slock. This means bh_lock_sock can still execute even if the socket is "locked" provided of course that the lock_sock call isn't in execution at that very point in time.

release_sock grabs the sk_lock.slock, processes any receive backlog, clears the owner, wakes up any wait queue on sk_lock.wq and then releases sk_lock.slock and enables bottom halves.

bh_lock_sock and bh_release_sock just grab and release sk->sk_lock.slock

Below are code samples to help illustrate the points.

lock_sock

in include/net/sock.h

extern void FASTCALL(lock_sock(struct sock *sk));
extern void FASTCALL(release_sock(struct sock *sk));

in net/core/sock.c

void fastcall lock_sock(struct sock *sk)
{
	might_sleep();
	spin_lock_bh(&(sk->sk_lock.slock));
	if (sk->sk_lock.owner)
		__lock_sock(sk);
	sk->sk_lock.owner = (void *)1;
	spin_unlock_bh(&(sk->sk_lock.slock));
}

EXPORT_SYMBOL(lock_sock);

void fastcall release_sock(struct sock *sk)
{
	spin_lock_bh(&(sk->sk_lock.slock));
	if (sk->sk_backlog.tail)
		__release_sock(sk);
	sk->sk_lock.owner = NULL;
        if (waitqueue_active(&(sk->sk_lock.wq)))
		wake_up(&(sk->sk_lock.wq));
	spin_unlock_bh(&(sk->sk_lock.slock));
}
EXPORT_SYMBOL(release_sock);

and

static void __lock_sock(struct sock *sk)
{
	DEFINE_WAIT(wait);

	for(;;) {
		prepare_to_wait_exclusive(&sk->sk_lock.wq, &wait,
					TASK_UNINTERRUPTIBLE);
		spin_unlock_bh(&sk->sk_lock.slock);
		schedule();
		spin_lock_bh(&sk->sk_lock.slock);
		if(!sock_owned_by_user(sk))
			break;
	}
	finish_wait(&sk->sk_lock.wq, &wait);
}

static void __release_sock(struct sock *sk)
{
	struct sk_buff *skb = sk->sk_backlog.head;

	do {
		sk->sk_backlog.head = sk->sk_backlog.tail = NULL;
		bh_unlock_sock(sk);

		do {
			struct sk_buff *next = skb->next;

			skb->next = NULL;
			sk->sk_backlog_rcv(sk, skb);

			/*
			 * We are in process context here with softirqs
			 * disabled, use cond_resched_softirq() to preempt.
			 * This is safe to do because we've taken the backlog
			 * queue private:
			 */
			cond_resched_softirq();

			skb = next;
		} while (skb != NULL);

		bh_lock_sock(sk);
	} while((skb = sk->sk_backlog.head) != NULL);
}

bh_lock_sock

in include/net/sock.h

#define bh_lock_sock(__sk)	spin_lock(&((__sk)->sk_lock.slock))
#define bh_unlock_sock(__sk)	spin_unlock(&((__sk)->sk_lock.slock))

general locking

in include/linux/spinlock.h:

#define spin_lock_bh(lock)             _spin_lock_bh(lock)

in include/linux/spinlock_api_smp.h

void __lockfunc _spin_lock(spinlock_t *lock)           __acquires(spinlock_t);
void __lockfunc _spin_lock_bh(spinlock_t *lock)        __acquires(spinlock_t);

in include/linux/spinlock_api_up.h:

#define _spin_lock_bh(lock)                     __LOCK_BH(lock)
#define _spin_lock(lock)                        __LOCK(lock)

#define __LOCK(lock) \
  do { preempt_disable(); __acquire(lock); (void)(lock); } while (0)

#define __LOCK_BH(lock) \
  do { local_bh_disable(); __LOCK(lock); } while (0)

#define __LOCK_IRQ(lock) \
  do { local_irq_disable(); __LOCK(lock); } while (0)

#define __LOCK_IRQSAVE(lock, flags) \
  do { local_irq_save(flags); __LOCK(lock); } while (0)

#define __UNLOCK(lock) \
  do { preempt_enable(); __release(lock); (void)(lock); } while (0)

#define __UNLOCK_BH(lock) \
  do { preempt_enable_no_resched(); local_bh_enable(); __release(lock); (void)(lock); } while (0)

#define __UNLOCK_IRQ(lock) \
  do { local_irq_enable(); __UNLOCK(lock); } while (0)

#define __UNLOCK_IRQRESTORE(lock, flags) \
  do { local_irq_restore(flags); __UNLOCK(lock); } while (0)

in include/linux/spinlock.h

#define spin_lock(lock)                        _spin_lock(lock)

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