Re: Exiting with locks still held (was Re: [PATCH] kmemleak: Fix scheduling-while-atomic bug)

[Pekka Enberg <penberg@cs.helsinki.fi>]

Hi Catalin,

On Thu, Jul 2, 2009 at 3:48 PM, Catalin Marinas<catalin.marinas@arm.com> wrote:
> It could be but I can't figure out a solution. If there is only one task
> opening and closing the kmemleak file, everything is fine. In
> combination with shell piping I think I get the kmemleak file descriptor
> released from a different task than the one that opened it.
>
> For example, the badly written code below opens kmemleak and acquires
> the scan_mutex in the parent task but releases it in the child (it needs
> a few tries to trigger it). With waitpid() in parent everything is fine.
>
> #include <stdio.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <sys/wait.h>
>
> int main(void)
> {
>        int fd = open("/sys/kernel/debug/kmemleak", O_RDONLY);
>
>        printf("fd = %d\n", fd);
>        if (fd < 0)
>                return 2;
>
>        if (!fork()) {
>                /* child */
>                sleep(2);
>                close(fd);
>                printf("fd closed in child\n");
>        }
>
>        return 0;
> }
>
> Running this gives (the ### lines are printed in the
> kmemleak_open/release functions):
>
> # ./cat-kmemleak
> ### kmemleak_open current->pid = 1409
> fd = 3
> =====================================
> [ BUG: lock held at task exit time! ]
> -------------------------------------
> cat-kmemleak/1409 is exiting with locks still held!
> 1 lock held by cat-kmemleak/1409:
>  #0:  (scan_mutex){+.+.+.}, at: [<c00662b1>] kmemleak_open+0x31/0x68
>
> stack backtrace:
> [<c0024025>] (unwind_backtrace+0x1/0x80) from [<c01cddd7>] (dump_stack+0xb/0xc)
> [<c01cddd7>] (dump_stack+0xb/0xc) from [<c0043d2d>] (debug_check_no_locks_held+0x49/0x64)
> [<c0043d2d>] (debug_check_no_locks_held+0x49/0x64) from [<c0031423>] (do_exit+0x3fb/0x43c)
> [<c0031423>] (do_exit+0x3fb/0x43c) from [<c00314c5>] (do_group_exit+0x61/0x80)
> [<c00314c5>] (do_group_exit+0x61/0x80) from [<c00314f3>] (sys_exit_group+0xf/0x14)
> [<c00314f3>] (sys_exit_group+0xf/0x14) from [<c001fc41>] (ret_fast_syscall+0x1/0x40)
>
> ### kmemleak_release current->pid = 1410
> fd closed in child
>
> Any suggestions? Thanks.

Well, you are not supposed to hold on to locks when returning from a
system call ("sys_open") anyway. You can probably do the exclusion
with a kmemcheck specific flag?

                        Pekka