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

[Caspar Zhang <caspar@casparzhang.com>]

On 12/14/2011 07:05 PM, Jan Stancek wrote:
> 
> 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 <jstancek@redhat.com>

looks good.

Reviewed-by: Caspar Zhang <caspar@casparzhang.com>

> ---
>   testcases/kernel/ipc/pipeio/pipeio.c |   37 
> ++++++++++++++++++++++++++++++++-
>   1 files changed, 35 insertions(+), 2 deletions(-)
> 

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