sk_wait_data

[Pranay Srivastava]

Hi

On Thu, Feb 19, 2015 at 3:45 PM, Grzegorz Dwornicki <gd1100 at gmail.com> wrote:
> Hello
>
> Proces have a established tcp socket. It calls sys_revcmsg on that
> socket. That function goes all the way to tcp_recvmsg function.
>
> Function tcp_recvmsg.c (defined in net/ipv4/tcp.c) blocks it self on
> function sk_wait_data if socket recv queue is empty. I wanted to know
> how socked is checked in time. Soo i Looked up this sk_wait_data
> function (http://lxr.free-electrons.com/source/net/core/sock.c#L1933):
>
> 1933 int sk_wait_data(struct sock *sk, long *timeo)
> 1934 {
> 1935         int rc;
> 1936         DEFINE_WAIT(wait);
> 1937
> 1938         prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
> 1939         set_bit(SOCK_ASYNC_WAITDATA, &sk->sk_socket->flags);
> 1940         rc = sk_wait_event(sk, timeo,
> !skb_queue_empty(&sk->sk_receive_queue));
> 1941         clear_bit(SOCK_ASYNC_WAITDATA, &sk->sk_socket->flags);
> 1942         finish_wait(sk_sleep(sk), &wait);
> 1943         return rc;
> 1944 }
>
> This function blocks it self on the sk_wait_event macro. Here is it
> definition (http://lxr.free-electrons.com/source/include/net/sock.h#L893):
>
> 893 #define sk_wait_event(__sk, __timeo, __condition)                       \
> 894         ({      int __rc;                                               \
> 895                 release_sock(__sk);                                     \
> 896                 __rc = __condition;                                     \
> 897                 if (!__rc) {                                            \
> 898                         *(__timeo) = schedule_timeout(*(__timeo));      \
> 899                 }                                                       \
> 900                 lock_sock(__sk);                                        \
> 901                 __rc = __condition;                                     \
> 902                 __rc;                                                   \
> 903         })
>
> This macro is blocked in the schedule_timeout all the time. I know
> this because I added printk functions with some marks easly to target
> the line blocking the socket. It hangs on this schedule_timeout and it
> magicaly released when some data apear in the revc queue.
>
> How does the kernel know when to "unblock" the process? Function name:
> sk_wait_data and macro name: sk_wait_event tells me that there should
> be some queue checning from time to time...
>
> Can someone explain this to me?

Perhaps you should see sk_data_ready callback's default implementation for this.

>
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-- 
        ---P.K.S