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    SubjectRe: SMP-ix86-threads-fork: Linux 2.4.x kernel problem identified [phantom read()]
    From
    Date
    Hi,

    > So it's perfectly possible to observe the behavior you are seeing
    > if __libc_read() returns -1. Because then sz_read will acquire the
    > value -1, and the guard expresssion sz_read < sizeof(request) will yield
    > zero, terminating the loop.

    Argh! That indeed seems to be the case, _no_ apparent kernel problem
    is involved, sorry for the false alarm. read() 'just' returns -1 with
    errno becoming EINTR. One should never code up a loop like that in
    the last minute (I only found the effect yesterday)..

    > Could you recode the test patch to eliminate these suspicions and re-test?

    The analysis was nevertheless correct, LinuxThreads gets out of synch
    with disastrous effects. I was puzzled how read() could return -1,
    however it may make sense with a __pthread_sig_cancel pending in the
    manager thread due to a child exiting (?). But then, why wasn't this
    possibility discovered before?

    Below is a corrected patch for glibc-2.2.4. I've run fork-malloc with
    this for a couple of hours.

    Thanks,
    Wolfram.

    2001-09-11 Wolfram Gloger <wg@malloc.de>

    * manager.c (__pthread_manager): When reading from pipe. account
    for possible error return from read().

    --- linuxthreads/manager.c.orig Mon Jul 23 19:54:13 2001
    +++ linuxthreads/manager.c Tue Sep 11 00:47:48 2001
    @@ -150,8 +150,19 @@
    }
    /* Read and execute request */
    if (n == 1 && (ufd.revents & POLLIN)) {
    - n = __libc_read(reqfd, (char *)&request, sizeof(request));
    - ASSERT(n == sizeof(request));
    + int sz_read = 0;
    +
    + while (sz_read < sizeof(request)) {
    + n = __libc_read(reqfd, (char *)&request + sz_read,
    + sizeof(request) - sz_read);
    + if (n < 0) {
    +#ifdef DEBUG
    + char d[64];
    + sprintf(d, "*** read err %d\n", errno);
    +#endif
    + } else
    + sz_read += n;
    + }
    switch(request.req_kind) {
    case REQ_CREATE:
    request.req_thread->p_retcode =
    -
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