skwq_has_sleeper (9) Linux Manual Page

skwq_has_sleeper – check if there are any waiting processes

Synopsis

bool skwq_has_sleeper(struct socket_wq * wq);

Arguments

wq

struct socket_wq

Description

Returns true if socket_wq has waiting processes The purpose of the skwq_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.

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