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SubjectRe: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggered
On Tue, 29 Jan 2019, Sebastian Sewior wrote:

> On 2019-01-29 16:10:58 [+0100], Heiko Carstens wrote:
> > Finally... the trace output is quite large with 26 MB... Therefore an
> > xz compressed attachment. Hope that's ok.
> >
> > The kernel used was linux-next 20190129 + your patch.
> | ld64.so.1-10237 [006] .... 14232.031726: sys_futex(uaddr: 3ff88e80618, op: 7, val: 3ff00000007, utime: 3ff88e7f910, uaddr2: 3ff88e7f910, val3: 3ffc167e8d7)
> FUTEX_UNLOCK_PI | SHARED
>
> | ld64.so.1-10237 [006] .... 14232.031726: sys_futex -> 0x0
> …
> | ld64.so.1-10237 [006] .... 14232.051751: sched_process_exit: comm=ld64.so.1 pid=10237 prio=120
> …
> | ld64.so.1-10148 [006] .... 14232.061826: sys_futex(uaddr: 3ff88e80618, op: 6, val: 1, utime: 0, uaddr2: 2, val3: 0)
> FUTEX_LOCK_PI | SHARED
>
> | ld64.so.1-10148 [006] .... 14232.061826: sys_futex -> 0xfffffffffffffffd
>
> So there got to be another task that acquired the lock in userland and
> left since the last in kernel-user unlocked it. This might bring more

Well, that would mean that this very task did not have a valid robust list,
which is very unlikely according to the test case.

We might actually stick a trace point into the robust list code as well.

> light to it:
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 599da35c2768..aaa782a8a115 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -1209,6 +1209,9 @@ static int handle_exit_race(u32 __user *uaddr, u32 uval,
> * corrupted or the user space value in *uaddr is simply bogus.
> * Give up and tell user space.
> */
> + trace_printk("uval2 vs uval %08x vs %08x (%d)\n", uval2, uval,
> + tsk ? tsk->pid : -1);
> + __WARN();
> return -ESRCH;
> }
>
> @@ -1233,8 +1236,10 @@ static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,
> if (!pid)
> return -EAGAIN;
> p = find_get_task_by_vpid(pid);
> - if (!p)
> + if (!p) {
> + trace_printk("Missing pid %d\n", pid);
> return handle_exit_race(uaddr, uval, NULL);
> + }
>
> if (unlikely(p->flags & PF_KTHREAD)) {
> put_task_struct(p);

Yep, that should give us some more clue.

> I am not sure, but isn't this the "known" issue where the kernel drops
> ESRCH in a valid case and glibc upstream does not recognize it because
> it is not a valid /POSIX-defined error code? (I *think* same is true for
> -ENOMEM) If it is, the following C snippet is a small tc:

That testcase is not using robust futexes, but yes it's demonstrating the
glibc does not handle all documented error codes. But I don't think it has
anything to do with the problem at hand. Famous last words....

Thanks,

tglx
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