sched: clock wrap bug in 2.6.35-stable kills scheduling

On Mon, 2012-06-25 at 11:45 +0200, Peter Zijlstra wrote:
> On Sun, 2012-06-24 at 19:32 +0200, Thomas Lange wrote:
> > Bug was introduced in 2.6.35.12 and is still present in linux-2.6.35.y HEAD.
>
> Ok, so nobody cares about that.. what does something recent like 3.5-rc4
> do?
>
> If that's fixed, find the patch that fixes it. If not, we'll have a
> look.

If anything, I think something like the below ought to cure things.
fifo: Do not restart open() if it already found a partner
If a parent and child process open the two ends of a fifo, and the
child immediately exits, the parent may receive a SIGCHLD before its
open() returns.  In that case, we need to make sure that open() will
return successfully after the SIGCHLD handler returns, instead of
throwing EINTR or being restarted.  Otherwise, the restarted open()
would incorrectly wait for a second partner on the other end.

The following test demonstrates the EINTR that was wrongly thrown from
the parent.s open().  Change .sa_flags = 0 to .sa_flags = SA_RESTART
to see a deadlock instead, in which the restarted open() waits for a
second reader that will never come.

  #include <sys/stat.h>
  #include <sys/types.h>
  #include <sys/wait.h>
  #include <fcntl.h>
  #include <signal.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>

  #define CHECK(x) do {if ((x) == -1) {perror(#x); abort();}} while(0)
  void handler(int signum) {}
  int main()
  {
      struct sigaction act = {.sa_handler = handler, .sa_flags = 0};
      CHECK(sigaction(SIGCHLD, &act, NULL));
      CHECK(mknod("fifo", S_IFIFO | S_IRWXU, 0));
      for (;;) {
          int fd;
          pid_t pid;
          putc('.', stderr);
          CHECK(pid = fork());
          if (pid == 0) {
              CHECK(fd = open("fifo", O_RDONLY));
              _exit(0);
          }
          CHECK(fd = open("fifo", O_WRONLY));
          CHECK(close(fd));
          CHECK(waitpid(pid, NULL, 0));
      }
  }
This is what I suspect was causing the Git test suite to fail in
t9010-svn-fe.sh:
http://bugs.debian.org/678852

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
backported to Linux 2.6.39 from Linux 3.5+ by faux123
block: fix infinite loop in __getblk_slow
Hi,

This commit:

commit 080399a
Author: Jeff Moyer <jmoyer@redhat.com>
Date:   Fri May 11 16:34:10 2012 +0200

    block: don't mark buffers beyond end of disk as mapped

exposed a bug in __getblk_slow that causes mount to hang as it loops
infinitely waiting for a buffer that lies beyond the end of the disk to
become uptodate.  The problem was initially reported by Torsten Hilbrich
here: https://lkml.org/lkml/2012/6/18/54, and also reported
independently here:
http://www.sysresccd.org/forums/viewtopic.php?f=13&t=4511, and then
Richard W.M. Jones and Marcos Mello noted a few separate bugzillas also
associated with the same issue.

The main problem is here, in __getblk_slow:

        for (;;) {
                struct buffer_head * bh;
                int ret;
                bh = __find_get_block(bdev, block, size);
                if (bh)
                        return bh;
                ret = grow_buffers(bdev, block, size);
                if (ret < 0)
                        return NULL;
                if (ret == 0)
                        free_more_memory();
        }
__find_get_block does not find the block, since it will not be marked as
mapped, and so grow_buffers is called to fill in the buffers for the
associated page.  I believe the for (;;) loop is there primarily to
retry in the case of memory pressure keeping grow_buffers from
succeeding.  However, we also continue to loop for other cases, like the
block lying beond the end of the disk.  So, the fix I came up with is to
only loop when grow_buffers fails due to memory allocation issues
(return value of 0).

The attached patch was tested by myself, Torsten, and Rich, and was
found to resolve the problem in call cases.

Comments, as always, are appreciated.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Reported-and-Tested-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Cc: Stable <stable@vger.kernel.org>
backported from Linux 3.5+ to Linux 2.6.39 by faux123
defconfig: add my own defconfig