#[cfg(target_feature = "atomics")]
mod lock {
- use crate::arch::wasm32;
use crate::sync::atomic::{AtomicI32, Ordering::SeqCst};
static LOCKED: AtomicI32 = AtomicI32::new(0);
if LOCKED.swap(1, SeqCst) == 0 {
return DropLock
}
- unsafe {
- let r = wasm32::i32_atomic_wait(
- &LOCKED as *const AtomicI32 as *mut i32,
- 1, // expected value
- -1, // timeout
- );
- debug_assert!(r == 0 || r == 1);
- }
+ // Ok so here's where things get a little depressing. At this point
+ // in time we need to synchronously acquire a lock, but we're
+ // contending with some other thread. Typically we'd execute some
+ // form of `i32.atomic.wait` like so:
+ //
+ // unsafe {
+ // let r = core::arch::wasm32::i32_atomic_wait(
+ // &LOCKED as *const AtomicI32 as *mut i32,
+ // 1, // expected value
+ // -1, // timeout
+ // );
+ // debug_assert!(r == 0 || r == 1);
+ // }
+ //
+ // Unfortunately though in doing so we would cause issues for the
+ // main thread. The main thread in a web browser *cannot ever
+ // block*, no exceptions. This means that the main thread can't
+ // actually execute the `i32.atomic.wait` instruction.
+ //
+ // As a result if we want to work within the context of browsers we
+ // need to figure out some sort of allocation scheme for the main
+ // thread where when there's contention on the global malloc lock we
+ // do... something.
+ //
+ // Possible ideas include:
+ //
+ // 1. Attempt to acquire the global lock. If it fails, fall back to
+ // memory allocation via `memory.grow`. Later just ... somehow
+ // ... inject this raw page back into the main allocator as it
+ // gets sliced up over time. This strategy has the downside of
+ // forcing allocation of a page to happen whenever the main
+ // thread contents with other threads, which is unfortunate.
+ //
+ // 2. Maintain a form of "two level" allocator scheme where the main
+ // thread has its own allocator. Somehow this allocator would
+ // also be balanced with a global allocator, not only to have
+ // allocations cross between threads but also to ensure that the
+ // two allocators stay "balanced" in terms of free'd memory and
+ // such. This, however, seems significantly complicated.
+ //
+ // Out of a lack of other ideas, the current strategy implemented
+ // here is to simply spin. Typical spin loop algorithms have some
+ // form of "hint" here to the CPU that it's what we're doing to
+ // ensure that the CPU doesn't get too hot, but wasm doesn't have
+ // such an instruction.
+ //
+ // To be clear, spinning here is not a great solution.
+ // Another thread with the lock may take quite a long time to wake
+ // up. For example it could be in `memory.grow` or it could be
+ // evicted from the CPU for a timeslice like 10ms. For these periods
+ // of time our thread will "helpfully" sit here and eat CPU time
+ // until it itself is evicted or the lock holder finishes. This
+ // means we're just burning and wasting CPU time to no one's
+ // benefit.
+ //
+ // Spinning does have the nice properties, though, of being
+ // semantically correct, being fair to all threads for memory
+ // allocation, and being simple enough to implement.
+ //
+ // This will surely (hopefully) be replaced in the future with a
+ // real memory allocator that can handle the restriction of the main
+ // thread.
+ //
+ //
+ // FIXME: We can also possibly add an optimization here to detect
+ // when a thread is the main thread or not and block on all
+ // non-main-thread threads. Currently, however, we have no way
+ // of knowing which wasm thread is on the browser main thread, but
+ // if we could figure out we could at least somewhat mitigate the
+ // cost of this spinning.
}
}
fn drop(&mut self) {
let r = LOCKED.swap(0, SeqCst);
debug_assert_eq!(r, 1);
- unsafe {
- wasm32::atomic_notify(
- &LOCKED as *const AtomicI32 as *mut i32,
- 1, // only one thread
- );
- }
+
+ // Note that due to the above logic we don't actually need to wake
+ // anyone up, but if we did it'd likely look something like this:
+ //
+ // unsafe {
+ // core::arch::wasm32::atomic_notify(
+ // &LOCKED as *const AtomicI32 as *mut i32,
+ // 1, // only one thread
+ // );
+ // }
}
}
}