[LTP] [PATCH] pipeio: prevent race between SIGCHLD and open()

[Jan S. <jst...@re...>]

This test occasionally hangs on some machines. The hang has been
observed on some single CPU ones.

pipeio code is using signal(2), setting by default SA_RESTART
flag, which is also the case for SIGCHLD.

If last child manages to exit while parent is still at open(),
parent gets SIGCHLD and open() is restarted. At this point test
hangs.

Here's strace output from parent point of view:

=== snip ===
brk(0)                                  = 0x11bb000
brk(0x11dd000)                          = 0x11dd000
getpid()                                = 18826
stat("tpipe.18826", 0x7fff89e1d410)     = -1 ENOENT (No such file or
directory)
mknod("tpipe.18826", S_IFIFO|0777)      = 0
rt_sigaction(SIGCHLD, {0x400a54, [CHLD], SA_RESTORER|SA_RESTART,
   0x354aa32a20}, {SIG_DFL, [], 0}, 8) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|
   CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f53b9ddd9d0) = 18827
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 3), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0)
   = 0x7f53b9de5000
open("tpipe.18826", O_RDONLY
)           = ? ERESTARTSYS (To be restarted)
--- SIGCHLD (Child exited) @ 0 (0) ---
wait4(-1, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 18827
rt_sigreturn(0xffffffffffffffff)        = 2
open("tpipe.18826", O_RDONLY
=== /snip ===

This patch is introducing semaphore, which prevents children from
exiting until parent completes open(). It also adds timed wait,
so parent waits for children to exit before it deletes pipe and
semaphore.

Signed-off-by: Jan Stancek <jst...@re...>
---
  testcases/kernel/ipc/pipeio/pipeio.c |   37 
++++++++++++++++++++++++++++++++-
  1 files changed, 35 insertions(+), 2 deletions(-)