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Add benchmark - #4
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borrow_dyn_multi_thread* tests occasionally hanged. A wait_to_lend() could register a waker that replaced an active LendGuard's waker, potentially preventing that LendGuard's thread from ever waking up. Fix that up by checking mutex.state in wait_to_lend() and return early if another lend()/wait_to_lend() is in progress. The borrow_dyn_multi_thread* tests need to be adjusted too as instead of deadlocking they would panic now. To properly have multiple concurrent lend-ers it's necessary to synchronize the lending externally, i.e. with a normal async mutex.
```
warning: use of deprecated module `core::intrinsics`: import this function via `std::mem` instead
--> src/mpmc.rs:174:39
|
174 | Ok(unsafe { core::intrinsics::transmute(slot) })
| ^^^^^^^^^
|
= note: `#[warn(deprecated)]` on by default
warning: use of deprecated module `core::intrinsics`: import this function via `std::mem` instead
--> src/mpmc.rs:235:41
|
235 | Some(unsafe { core::intrinsics::transmute(slot) })
| ^^^^^^^^^
warning: `borrow_mutex` (lib) generated 2 warnings (1 duplicate)
```
The uncontended benchmark was actually affected by the extra CAS+store, making uncontended wait_for_lend() marginally slower than smol::lock::Mutex. Use a separate AtomicWaker for lend() and wait_to_lend(), then don't check for multiple parallel wait_to_lend(). This means it's possible for some wait_to_lend() to overwrite another wait-er's waker, leading to that wait-er not being awoken. It's a misbehavior due to misuse, but too bad. In uncontended case, BorrowMutex is now again marginally faster than smol::lock::Mutex.
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Add benchmark and fixup the bugs it revealed.
While here, fixup no-std builds and add CI for it.