Re: [PATCH 1/8] futex: Check value after qemu_futex_wait()

[Akihiko Odaki]

On 2024/12/28 20:11, Phil Dennis-Jordan wrote:
> This is somewhat orthogonal to the issue being addressed here, but:
> While reading the man page to make sense of this patch, I noticed the 
> following:
>
>  > If the futex value does not match val, then the call fails
>  >              immediately with the error EAGAIN.
>
> And qemu_futex_wait does not seem to handle that case. In fact it seems 
> like it would take the default: abort(); code path?

It's handled as EWOULDBLOCK. The man page says:
> Note: on Linux, the symbolic names EAGAIN and EWOULDBLOCK (both of
> which appear in different parts of the kernel futex code) have the
> same value.

> If I've got this right, I'm surprised there aren't spurious abort()s 
> happening, but I suppose QemuEvent and qemu_futex_* are used fairly 
> sparingly and in low-contention areas.

QemuLockCnt, which relies on qemu_futex_*, is used in more contended 
areas so it will cause trouble if qemu_futex_* is broken.