recv() hangs until SIGCHLD ? - Kernel

This is a discussion on recv() hangs until SIGCHLD ? - Kernel ; Hi, We've been tracking a bug in our server application for some time now, and now that we could isolate it we're stuck without a meaningful explanation. Hope somehow would be able to give use some answers. We run a ...

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Thread: recv() hangs until SIGCHLD ?

  1. recv() hangs until SIGCHLD ?

    Hi,

    We've been tracking a bug in our server application for some time now,
    and now that we could isolate it we're stuck without a meaningful
    explanation. Hope somehow would be able to give use some answers.

    We run a multithread application which is using pthreads and sockets. A
    thread uses accept() then dispatch the socket to one of the workers
    threads that process it. Sockets are then not used simultaneously by
    several threads.

    In some rare cases, one (or several) threads are hanging in recv(). Both
    lsof and ls /proc//fd show that the socket used is in ESTABLISHED
    mode but when checking on the host on which it's connected (a mysql DB)
    we can't find the corresponding client socket (as it's been closed
    already on the other side).

    We are using the Boehm GC which uses the signals SIGXCPU and SIGPWR to
    pause+restart the threads when running a GC cycle. We are correctly
    handling EINTR in send() and recv() by restarting the call in case they
    get interrupted this way.

    However, when attaching GDB to our locked thread it seems that even when
    the GC runs, recv() does not exit (the breakpoint after it is not
    reached). If we send SIGCHLD to the hanging thread with GDB, recv() does
    exit and the thread is correctly unlocked. If we don't, it will hang
    forever.

    Additional details : recv() is using MSG_NOSIGNAL and we have enabled
    TCP_NODELAY on the socket by using setsockopt. Some other
    not-multithreaded apps are using the same Databases and this behavior
    does not occur for them.

    Any idea how we can stop this from happening or what additional things
    we can check to get more informations on what's occurring ?

    Thanks a lot,
    Nicolas

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  2. Re: recv() hangs until SIGCHLD ?

    On Fri, 10 Oct 2008 15:30:01 +0200
    Nicolas Cannasse wrote:

    > Hi,
    >
    > We've been tracking a bug in our server application for some time now,
    > and now that we could isolate it we're stuck without a meaningful
    > explanation. Hope somehow would be able to give use some answers.
    >
    > We run a multithread application which is using pthreads and sockets. A
    > thread uses accept() then dispatch the socket to one of the workers
    > threads that process it. Sockets are then not used simultaneously by
    > several threads.
    >
    > In some rare cases, one (or several) threads are hanging in recv(). Both
    > lsof and ls /proc//fd show that the socket used is in ESTABLISHED
    > mode but when checking on the host on which it's connected (a mysql DB)
    > we can't find the corresponding client socket (as it's been closed
    > already on the other side).
    >
    > We are using the Boehm GC which uses the signals SIGXCPU and SIGPWR to
    > pause+restart the threads when running a GC cycle. We are correctly
    > handling EINTR in send() and recv() by restarting the call in case they
    > get interrupted this way.
    >
    > However, when attaching GDB to our locked thread it seems that even when
    > the GC runs, recv() does not exit (the breakpoint after it is not
    > reached). If we send SIGCHLD to the hanging thread with GDB, recv() does
    > exit and the thread is correctly unlocked. If we don't, it will hang
    > forever.
    >
    > Additional details : recv() is using MSG_NOSIGNAL and we have enabled
    > TCP_NODELAY on the socket by using setsockopt. Some other
    > not-multithreaded apps are using the same Databases and this behavior
    > does not occur for them.
    >
    > Any idea how we can stop this from happening or what additional things
    > we can check to get more informations on what's occurring ?
    >
    > Thanks a lot,
    > Nicolas


    Look at Receive queue length with ss or netstat for the hung thread. It will
    show if there is anything that thread could read.

    If there is data and the thread didn't wake up then that is a libc or kernel problem;
    but if there is no data, then look for cases where earlier interrupted io actually
    consumed the data already or blame the sending process not the receiver.
    Also are the sockets blocking or non-blocking?
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  3. Re: recv() hangs until SIGCHLD ?

    >> We run a multithread application which is using pthreads and sockets. A
    >> thread uses accept() then dispatch the socket to one of the workers
    >> threads that process it. Sockets are then not used simultaneously by
    >> several threads.
    >>
    >> In some rare cases, one (or several) threads are hanging in recv(). Both
    >> lsof and ls /proc//fd show that the socket used is in ESTABLISHED
    >> mode but when checking on the host on which it's connected (a mysql DB)
    >> we can't find the corresponding client socket (as it's been closed
    >> already on the other side).
    >>
    >> We are using the Boehm GC which uses the signals SIGXCPU and SIGPWR to
    >> pause+restart the threads when running a GC cycle. We are correctly
    >> handling EINTR in send() and recv() by restarting the call in case they
    >> get interrupted this way.
    >>
    >> However, when attaching GDB to our locked thread it seems that even when
    >> the GC runs, recv() does not exit (the breakpoint after it is not
    >> reached). If we send SIGCHLD to the hanging thread with GDB, recv() does
    >> exit and the thread is correctly unlocked. If we don't, it will hang
    >> forever.
    >>
    >> Additional details : recv() is using MSG_NOSIGNAL and we have enabled
    >> TCP_NODELAY on the socket by using setsockopt. Some other
    >> not-multithreaded apps are using the same Databases and this behavior
    >> does not occur for them.
    >>
    >> Any idea how we can stop this from happening or what additional things
    >> we can check to get more informations on what's occurring ?
    >>
    >> Thanks a lot,
    >> Nicolas

    >
    > Look at Receive queue length with ss or netstat for the hung thread. It will
    > show if there is anything that thread could read.
    >
    > If there is data and the thread didn't wake up then that is a libc or kernel problem;
    > but if there is no data, then look for cases where earlier interrupted io actually
    > consumed the data already or blame the sending process not the receiver.
    > Also are the sockets blocking or non-blocking?


    The sockets are non-blocking.

    Checking with netstat and ss I can confirm that both Send and Recv
    queues are empty, which makes the recv() behavior consistent.

    However since this problem does not occur without threads, we can be
    sure that the blame is still on the receiver.

    In a practical case, we have a thread blocked in recv() for more than 12
    hours, which is way beyond the timeout of the sender connection. The
    socket has already been closed by the sender so recv() should at least
    be noticed and returns 0.

    Is it safe to assume that when either send() or recv() get interrupted
    by a signal and returns EINTR, no actual data has been either sent or
    consumed ? And if it's not, is there any other way around this ?

    Best,
    Nicolas

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  4. RE: recv() hangs until SIGCHLD ?


    Nicolas Cannasse wrote:

    > The sockets are non-blocking.


    Ouch, that's a serious bug. Non-blocking operations shouldn't block!

    > Checking with netstat and ss I can confirm that both Send and Recv
    > queues are empty, which makes the recv() behavior consistent.
    >
    > However since this problem does not occur without threads, we can be
    > sure that the blame is still on the receiver.
    >
    > In a practical case, we have a thread blocked in recv() for more than 12
    > hours, which is way beyond the timeout of the sender connection. The
    > socket has already been closed by the sender so recv() should at least
    > be noticed and returns 0.


    Can you clarify what you mean by "the socket has already been closed by the
    sender"? You mean the other end of the TCP connection shut it down? By "the
    socket", you don't mean the socket you called 'recv' on, right? You mean the
    socket on the other end that's connected to it?

    > Is it safe to assume that when either send() or recv() get interrupted
    > by a signal and returns EINTR, no actual data has been either sent or
    > consumed ? And if it's not, is there any other way around this ?


    EINTR can only be return if 'send' or 'recv' have not sent or received
    anything. Otherwise the connection would be left in an indeterminate state.

    DS


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  5. Re: recv() hangs until SIGCHLD ?

    On Sat, 11 Oct 2008 05:20:37 -0700
    "David Schwartz" wrote:

    >
    > Nicolas Cannasse wrote:
    >
    > > The sockets are non-blocking.

    >
    > Ouch, that's a serious bug. Non-blocking operations shouldn't block!
    >
    > > Checking with netstat and ss I can confirm that both Send and Recv
    > > queues are empty, which makes the recv() behavior consistent.
    > >
    > > However since this problem does not occur without threads, we can be
    > > sure that the blame is still on the receiver.
    > >
    > > In a practical case, we have a thread blocked in recv() for more than 12
    > > hours, which is way beyond the timeout of the sender connection. The
    > > socket has already been closed by the sender so recv() should at least
    > > be noticed and returns 0.

    >
    > Can you clarify what you mean by "the socket has already been closed by the
    > sender"? You mean the other end of the TCP connection shut it down? By "the
    > socket", you don't mean the socket you called 'recv' on, right? You mean the
    > socket on the other end that's connected to it?
    >
    > > Is it safe to assume that when either send() or recv() get interrupted
    > > by a signal and returns EINTR, no actual data has been either sent or
    > > consumed ? And if it's not, is there any other way around this ?

    >
    > EINTR can only be return if 'send' or 'recv' have not sent or received
    > anything. Otherwise the connection would be left in an indeterminate state.


    Does application correctly handle the case where recv() returns 0?
    This indicates the TCP connection is closed by the other end.
    It is incorrect to assume that a return of 0 in non-blocking mode
    is the same as -1. The only correct action after receiving 0 bytes
    (even in non-blocking mode), is to close the socket. If you attempt
    to do another receive, the result could be that the recv() waits for
    another event (more data or FIN), which can never happen since socket
    is closed.
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  6. Re: recv() hangs until SIGCHLD ?

    >> If there is data and the thread didn't wake up then that is a libc or
    >> kernel problem;
    >> but if there is no data, then look for cases where earlier interrupted
    >> io actually
    >> consumed the data already or blame the sending process not the receiver.
    >> Also are the sockets blocking or non-blocking?

    >
    > The sockets are non-blocking.


    Sorry, I made a spelling mistake here.

    I wanted to tell that the sockets ARE blocking (default behavior).

    > In a practical case, we have a thread blocked in recv() for more than 12
    > hours, which is way beyond the timeout of the sender connection. The
    > socket has already been closed by the sender so recv() should at least
    > be noticed and returns 0.


    To provide more informations :

    Doing a lsof on the receiver, we can see that it has several ESTABLISHED
    sockets connected to a given host/sender. Doing a lsof on the host does
    not give any socket connected to the receiver (since they have been
    closed due to a timeout).

    Also, the application correctly handles 0.
    The pseudo-code is the following :

    loop:
    ret = recv()
    if( ret == -1 ) {
    if( errno == EINTR ) goto loop;
    return -1;
    }
    return ret;

    Then, on the higher level, in case we get an error ( ret <= 0 ) then we
    close the socket.

    At first, we were using the libmysqlclient but since we had the bug with
    it we rewrote a mysql client so we can more easily check what's
    occurring. The same bug seems to occur with both implementations.

    Best,
    Nicolas
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  7. Re: recv() hangs until SIGCHLD ?

    Nicolas Cannasse a écrit :
    >>> If there is data and the thread didn't wake up then that is a libc or
    >>> kernel problem;
    >>> but if there is no data, then look for cases where earlier
    >>> interrupted io actually
    >>> consumed the data already or blame the sending process not the receiver.
    >>> Also are the sockets blocking or non-blocking?


    One other thing :

    We tried to use a poll(POLLIN) on the socket before entering the recv().
    The poll() does exit (and we are looping in case of EINTR result) but
    after that recv() blocks infinitely.

    Hope that helps,

    Nicolas

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