Re: [PATCH v4 00/11] Improve futex usage

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2025/05/26 23:48, Peter Xu wrote:
> On Mon, May 26, 2025 at 02:29:10PM +0900, Akihiko Odaki wrote:
>> Akihiko Odaki (11):
>>        futex: Check value after qemu_futex_wait()
>>        futex: Support Windows
>>        qemu-thread: Remove qatomic_read() in qemu_event_set()
>>        qemu-thread: Replace __linux__ with CONFIG_LINUX
>>        qemu-thread: Avoid futex abstraction for non-Linux
>>        qemu-thread: Use futex for QemuEvent on Windows
>>        qemu-thread: Use futex if available for QemuLockCnt
>>        migration: Replace QemuSemaphore with QemuEvent
>>        migration/colo: Replace QemuSemaphore with QemuEvent
>>        migration/postcopy: Replace QemuSemaphore with QemuEvent
> 
> In case it makes things easier.. I queued the three migration patches;
> AFAIU they look like standalone to go even without prior patches, meanwhile
> it shouldn't be an issue if they're queued in two pulls.
> 
> I am still not sure whether patch 1 is needed at all, but I'll leave that
> to others to decide.

The migration patches shouldn't be applied before patches "futex: Check 
value after qemu_futex_wait()" and "qemu-thread: Avoid futex abstraction 
for non-Linux" as they fix QemuEvent. Merging migration patches earlier 
can trigger bugs just like one we faced with hw/display/apple-gfx*

Regarding patch 1 ("futex: Check value after qemu_futex_wait()"), I can 
argue that we should have it to comply the generic "upstream-first" 
principle; the upstream (man page) says to make a loop, so the 
downstream (QEMU) should follow that until the upstream says otherwise. 
But I think it's a good idea to spend efforts to understand the 
reasoning behind the man page since it's so important that it affects 
not only QEMU but also any userspace program that uses libpthread (i.e., 
practically everything).

Regards,
Akihiko Odaki

* 
https://lore.kernel.org/qemu-devel/a8b4472c-8741-4f80-87e5-b406c56d1acd@daynix.com/