Use UnsafeAtomics for real FAA (#2)

Author: tkf

Project.toml

@@ -3,5 +3,9 @@ uuid = "78a604cf-3861-458a-9dc0-49592e8d97d7"
 authors = ["Takafumi Arakaki <[email protected]> and contributors"]
 version = "0.1.0-DEV"
 
+[deps]
+UnsafeAtomics = "013be700-e6cd-48c3-b4a1-df204f14c38f"
+
 [compat]
+UnsafeAtomics = "0.1"
 julia = "1.7"

src/LeftRight.jl

@@ -8,6 +8,8 @@ function guarding end
 
 module Internal
 
+using UnsafeAtomics: UnsafeAtomics, acq_rel, seq_cst
+
 using ..LeftRight: LeftRight
 
 include("utils.jl")

src/core.jl

@@ -12,16 +12,31 @@ end
 
 ReadIndicator() = ReadIndicator(0, pad7(), nothing)
 
+const OFFSET_STATE =
+    fieldoffset(ReadIndicator, findfirst(==(:state), fieldnames(ReadIndicator)))
+
 function arrive!(ind::ReadIndicator)
+    #=
     @atomic ind.state += UInt(1)  # [^seq_cst_state_leftright]
+    =#
+    ptr = Ptr{UInt}(pointer_from_objref(ind) + OFFSET_STATE)
+    GC.@preserve ind begin
+        UnsafeAtomics.modify!(ptr, +, UInt(1), seq_cst)
+    end
     return ind
 end
 
 function depart!(ind::ReadIndicator)
+    #=
     state = @atomic(
         :acquire_release,  # [^acq_rel_ind_state] [^acq_rel_ind_waiter]
         ind.state -= UInt(1)
     )
+    =#
+    ptr = Ptr{UInt}(pointer_from_objref(ind) + OFFSET_STATE)
+    GC.@preserve ind begin
+        _old, state = UnsafeAtomics.modify!(ptr, -, UInt(1), acq_rel)
+    end
     if state == HAS_WAITER_MASK  # no readers, one waiter
         # Choosing at most one reader task that wakes up the waiter using CAS [^cas_waker].
         _, ok = @atomicreplace(