Update portable-atomic to 1 (mvdnes#139)

Author: taiki-e

Cargo.toml

@@ -14,7 +14,7 @@ rust-version = "1.38"
 
 [dependencies]
 lock_api_crate = { package = "lock_api", version = "0.4", optional = true }
-portable-atomic = { version = "0.3", optional = true, default-features = false }
+portable-atomic = { version = "1", optional = true, default-features = false }
 
 [features]
 default = ["lock_api", "mutex", "spin_mutex", "rwlock", "once", "lazy", "barrier"]
@@ -52,9 +52,14 @@ lock_api = ["lock_api_crate"]
 # Enables std-only features such as yield-relaxing.
 std = []
 
-# Use the portable_atomic crate to support platforms without native atomic operations
-# cfg 'portable_atomic_unsafe_assume_single_core' must also be set by the final binary crate.
-# This cfg is unsafe and enabling it for multicore systems is unsound.
+# Use the portable_atomic crate to support platforms without native atomic operations.
+# The `portable_atomic_unsafe_assume_single_core` cfg or `critical-section` feature
+# of `portable-atomic` crate must also be set by the final binary crate.
+# When using the cfg, note that it is unsafe and enabling it for multicore systems is unsound.
+# When using the `critical-section` feature, you need to implement the critical-section
+# implementation that sound for your system by implementing an unsafe trait.
+# See the documentation for the `portable-atomic` crate for more information:
+# https://docs.rs/portable-atomic/latest/portable_atomic/#optional-cfg
 portable_atomic = ["portable-atomic"]
 
 [package.metadata.docs.rs]

README.md

@@ -94,15 +94,21 @@ The crate comes with a few feature flags that you may wish to use.
 
 - `portable_atomic` enables usage of the `portable-atomic` crate
   to support platforms without native atomic operations (Cortex-M0, etc.).
-  The `portable_atomic_unsafe_assume_single_core` cfg flag
-  must also be set by the final binary crate.
-  This can be done by adapting the following snippet to the `.cargo/config` file:
+  The `portable_atomic_unsafe_assume_single_core` cfg or `critical-section` feature
+  of `portable-atomic` crate must also be set by the final binary crate.
+
+  When using the cfg, this can be done by adapting the following snippet to the `.cargo/config` file:
   ```
   [target.<target>]
   rustflags = [ "--cfg", "portable_atomic_unsafe_assume_single_core" ]
   ```
   Note that this cfg is unsafe by nature, and enabling it for multicore systems is unsound.
 
+  When using the `critical-section` feature, you need to implement the critical-section
+  implementation that sound for your system by implementing an unsafe trait.
+  See [the documentation for the `portable-atomic` crate](https://docs.rs/portable-atomic/latest/portable_atomic/#optional-cfg)
+  for more information.
+
 ## Remarks
 
 It is often desirable to have a lock shared between threads. Wrapping the lock in an