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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
Kernel Hackers Manual 3.8. June 2014 WQ_HAS_SLEEPER(9)
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