Add vector clock to epoll ready lists #3932
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This test demonstrates the need to synchronize the clock of the thread waking up from an epoll_wait from the thread that issued the epoll awake event.
A couple of instructions were left over from an earlier rebase it would seem. They don't impact the logic but the ready_list type is about to change in the next commit. Rather than modify one of these lines in the commit that changes ready_list, only to have these lines removed later on, remove them now. They don't impact the tests results.
This adds a VClock to the epoll implementation's ready_list and has this VClock synced from the thread that updates an event in the ready_list and then has the VClocks of any threads being made runnable again, out of the calls to epoll_wait, synced from it.
A simplification that doesn't impact the epoll implementation's logic. It is not necessary to clone the ready_list before reading its `is_empty` state. This avoids the clone step but more importantly avoids the invisible drop step of the clone.
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@bors r+ |
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☀️ Test successful - checks-actions |
RalfJung
reviewed
Oct 5, 2024
| #[derive(Debug, Default)] | ||
| struct ReadyList { | ||
| mapping: RefCell<BTreeMap<(FdId, i32), EpollEventInstance>>, | ||
| clock: RefCell<VClock>, |
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Didn't we say in the issue that we wanted a clock per event?
That would avoid accidentally synchronizing with events we didn't even receive.
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@RalfJung Do you feel this synch is too broad?
/// Callback function after epoll_wait unblocks
fn blocking_epoll_callback<'tcx>(
epfd_value: i32,
weak_epfd: WeakFileDescriptionRef,
dest: &MPlaceTy<'tcx>,
events: &MPlaceTy<'tcx>,
ecx: &mut MiriInterpCx<'tcx>,
) -> InterpResult<'tcx> {
let Some(epfd) = weak_epfd.upgrade() else {
throw_unsup_format!("epoll FD {epfd_value} got closed while blocking.")
};
let epoll_file_description = epfd
.downcast::<Epoll>()
.ok_or_else(|| err_unsup_format!("non-epoll FD passed to `epoll_wait`"))?;
let ready_list = epoll_file_description.get_ready_list();
// Synchronize waking thread from the epoll ready list.
ecx.acquire_clock(&ready_list.clock.borrow());
let mut ready_list = ready_list.mapping.borrow_mut();
let mut num_of_events: i32 = 0;
let mut array_iter = ecx.project_array_fields(events)?;
while let Some(des) = array_iter.next(ecx)? {
if let Some(epoll_event_instance) = ready_list_next(ecx, &mut ready_list) {
ecx.write_int_fields_named(
&[
("events", epoll_event_instance.events.into()),
("u64", epoll_event_instance.data.into()),
],
&des.1,
)?;
num_of_events = num_of_events.strict_add(1);
} else {
break;
}
}
ecx.write_int(num_of_events, dest)?;
interp_ok(())
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That's what you are currently doing, right? Yes I think this syncs too much: #3944
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Replaces #3928
Fixes #3911