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Docs: Use non-SeqCst in module example of atomics #119138

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Jan 19, 2024
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Docs: Use non-SeqCst in module example of atomics #119138

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13 changes: 9 additions & 4 deletions library/core/src/sync/atomic.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -138,7 +138,7 @@
//!
//! In general, *all* atomic accesses on read-only memory are Undefined Behavior. For instance, attempting
//! to do a `compare_exchange` that will definitely fail (making it conceptually a read-only
//! operation) can still cause a page fault if the underlying memory page is mapped read-only. Since
//! operation) can still cause a segmentation fault if the underlying memory page is mapped read-only. Since
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AFAIK, page faults are about valid memory that is not mapped to physical pages and absolutely legal (but slow) while text describes illegal memory access which is called segmentation fault. I am not an expert though and may be not correct.

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I think page fault is the more precise term here it's more lower level. The OS will then either do something useful with the page fault and continue normally, or it will crash the process with a SIGSEGV if it did something bad (which would be the case here) (I'm not entirely sure whether this is accurate but I think it is?). I'm not sure what the best way to describe this is here, it seems fine to change it to segmentation fault, that is probably more useful to users.

//! atomic `load`s might be implemented using compare-exchange operations, even a `load` can fault
//! on read-only memory.
//!
Expand Down Expand Up @@ -181,12 +181,13 @@
//! let spinlock = Arc::new(AtomicUsize::new(1));
//!
//! let spinlock_clone = Arc::clone(&spinlock);
//!
//! let thread = thread::spawn(move|| {
//! spinlock_clone.store(0, Ordering::SeqCst);
//! spinlock_clone.store(0, Ordering::Release);
//! });
//!
//! // Wait for the other thread to release the lock
//! while spinlock.load(Ordering::SeqCst) != 0 {
//! while spinlock.load(Ordering::Acquire) != 0 {
//! hint::spin_loop();
//! }
//!
Expand All @@ -203,7 +204,11 @@
//!
//! static GLOBAL_THREAD_COUNT: AtomicUsize = AtomicUsize::new(0);
//!
//! let old_thread_count = GLOBAL_THREAD_COUNT.fetch_add(1, Ordering::SeqCst);
//! // Note that Relaxed ordering doesn't synchronize anything
//! // except the global thread counter itself.
//! let old_thread_count = GLOBAL_THREAD_COUNT.fetch_add(1, Ordering::Relaxed);
//! // Note that this number may not be true at the moment of printing
//! // because some other thread may have changed static value already.
//! println!("live threads: {}", old_thread_count + 1);
//! ```

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