Re: [PATCH v2 0/3] futex: Create set_robust_list2

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Nov 02, 2024 at 10:58:42PM +0100, Florian Weimer wrote:

> QEMU hints towards further problems (in linux-user/syscall.c):
> 
>     case TARGET_NR_set_robust_list:
>     case TARGET_NR_get_robust_list:
>         /* The ABI for supporting robust futexes has userspace pass
>          * the kernel a pointer to a linked list which is updated by
>          * userspace after the syscall; the list is walked by the kernel
>          * when the thread exits. Since the linked list in QEMU guest
>          * memory isn't a valid linked list for the host and we have
>          * no way to reliably intercept the thread-death event, we can't
>          * support these. Silently return ENOSYS so that guest userspace
>          * falls back to a non-robust futex implementation (which should
>          * be OK except in the corner case of the guest crashing while
>          * holding a mutex that is shared with another process via
>          * shared memory).
>          */
>         return -TARGET_ENOSYS;

I don't think we can sanely fix that. Can't QEMU track the robust thing
itself and use waitpid() to discover the thread is gone and fudge things
from there?