1SK_HAS_SLEEPER(9)              Linux Networking              SK_HAS_SLEEPER(9)
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NAME

6       sk_has_sleeper - check if there are any waiting processes
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SYNOPSIS

9       int sk_has_sleeper(struct sock * sk);
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ARGUMENTS

12       sk
13           socket
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DESCRIPTION

16       Returns true if socket has waiting processes
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18       The purpose of the sk_has_sleeper and sock_poll_wait is to wrap the
19       memory barrier call. They were added due to the race found within the
20       tcp code.
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CONSIDER FOLLOWING TCP CODE PATHS

23       CPU1 CPU2
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25       sys_select receive packet ... ... __add_wait_queue update tp->rcv_nxt
26       ... ... tp->rcv_nxt check sock_def_readable ... { schedule ... if
27       (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
28       wake_up_interruptible(sk->sk_sleep) ... }
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30       The race for tcp fires when the __add_wait_queue changes done by CPU1
31       stay in its cache, and so does the tp->rcv_nxt update on CPU2 side. The
32       CPU1 could then endup calling schedule and sleep forever if there are
33       no more data on the socket.
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35       The sk_has_sleeper is always called right after a call to read_lock, so
36       we can use smp_mb__after_lock barrier.
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39Kernel Hackers Manual 2.6.         June 2019                 SK_HAS_SLEEPER(9)
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