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WQ_HAS_SLEEPER(9) | Linux Networking | WQ_HAS_SLEEPER(9) |
NAME¶
wq_has_sleeper - check if there are any waiting processesSYNOPSIS¶
bool
wq_has_sleeper(struct socket_wq * wq);
ARGUMENTS¶
wqstruct socket_wq
DESCRIPTION¶
Returns true if socket_wq has waiting processes The purpose of the wq_has_sleeper and sock_poll_wait is to wrap the memory barrier call. They were added due to the race found within the tcp code.CONSIDER FOLLOWING TCP CODE PATHS¶
CPU1 CPU2 sys_select receive packet ... ... __add_wait_queue update tp->rcv_nxt ... ... tp->rcv_nxt check sock_def_readable ... { schedule rcu_read_lock; wq = rcu_dereference(sk->sk_wq); if (wq && waitqueue_active(wq->wait)) wake_up_interruptible(wq->wait) ... } The race for tcp fires when the __add_wait_queue changes done by CPU1 stay in its cache, and so does the tp->rcv_nxt update on CPU2 side. The CPU1 could then endup calling schedule and sleep forever if there are no more data on the socket.COPYRIGHT¶
May 2018 | Kernel Hackers Manual 3.16 |