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WQ_HAS_SLEEPER(9) Linux Networking WQ_HAS_SLEEPER(9)

NAME

wq_has_sleeper - check if there are any waiting processes

SYNOPSIS

bool wq_has_sleeper(struct socket_wq * wq);

ARGUMENTS

wq
struct 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