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Add
update method to CLS variables (theseus-os#1034)
Not sure if `update` is the "correct" name. The closest analogue I
could find in `std` is [`Cell::update`], but that has a very different
API.
This function is necessary for some upcoming work on the scheduler.
Essentially, there's a global list of schedulers, and then also a
CPU-local which contains a reference to the current CPUs scheduler.
```rust
static SCHEDULERS: Mutex<Vec<(CpuId, &'static ConcurrentScheduler)>> = Mutex::new(Vec::new());
#[cls::cpu_local]
static SCHEDULER: Option<&'static ConcurrentScheduler> = None;
type ConcurrentScheduler = Mutex<Box<dyn Scheduler>>;
```
So `SCHEDULER` would contain a reference that is also an entry in
`SCHEDULERS`.
Now, when updating the policy we need to remove the scheduler from
`SCHEDULERS` and update `SCHEDULER` in a special way to prevent race
conditions:
```rust
let mut locked = SCHEDULERS.lock();
let guard = hold_preemption();
let old_scheduler = SCHEDULER.replace(new_scheduler);
// Do some stuff with `locked` and `old_scheduler`.
drop(guard);
```
But we should really be abstracting away from preemption:
```rust
let mut locked = SCHEDULERS.lock();
SCHEDULER.update(|old_scheduler| {
// Do some stuff with `locked` and `old_scheduler`.
*old_scheduler = new_scheduler;
});
```
Both snippets of code are functionally equivalent, but the latter one
is at a higher abstraction level with fewer footguns.
[`Cell::update`]: https://doc.rust-lang.org/std/cell/struct.Cell.html#method.update
Signed-off-by: Klimenty Tsoutsman <[email protected]> 9405569- Loading branch information
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