Document the semantics of copyto! and add pagelock! (#555)

Co-authored-by: Tim Besard <tim.besard@gmail.com>
Co-authored-by: Ludovic Räss <61313342+luraess@users.noreply.github.com>

src/KernelAbstractions.jl

@@ -110,13 +110,53 @@ macro Const end
 """
     copyto!(::Backend, dest::AbstractArray, src::AbstractArray)
 
-Perform a `copyto!` operation that execution ordered with respect to the backend.
+Perform an asynchronous `copyto!` operation that is execution ordered with respect to the back-end.
+
+For most users, `Base.copyto!` should suffice, performance a simple, synchronous copy.
+Only when you know you need asynchronicity w.r.t. the host, you should consider using
+this asynchronous version, which requires additional lifetime guarantees as documented below.
+
+!!! warning
+
+    Because of the asynchronous nature of this operation, the user is required to guarantee that the lifetime
+    of the source extends past the *completion* of the copy operation as to avoid a use-after-free. It is not
+    sufficient to simply use `GC.@preserve` around the call to `copyto!`, because that only extends the
+    lifetime past the operation getting queued. Instead, it may be required to `synchronize()`,
+    or otherwise guarantee that the source will still be around when the copy is executed:
+
+    ```julia
+    arr = zeros(64)
+    GC.@preserve arr begin
+        copyto!(backend, arr, ...)
+        # other operations
+        synchronize(backend)
+    end
+    ```
 
 !!! note
-    Backend implementations **must** implement this function.
+
+    On some back-ends it may be necessary to first call [`pagelock!`](@ref) on host memory
+    to enable fully asynchronous behavior w.r.t to the host.
+
+!!! note
+    Backends **must** implement this function.
 """
 function copyto! end
 
+"""
+    pagelock!(::Backend, dest::AbstractArray)
+
+Pagelock (pin) a host memory buffer for a backend device. This may be necessary for [`copyto!`](@ref)
+to perform asynchronously w.r.t to the host/
+
+This function should return `nothing`; or `missing` if not implemented.
+
+
+!!! note
+    Backends **may** implement this function.
+"""
+function pagelock! end
+
 """
     synchronize(::Backend)
 
@@ -562,6 +602,10 @@ function functional(::Backend)
     return missing
 end
 
+function pagelock!(::Backend, x)
+    return missing
+end
+
 include("nditeration.jl")
 using .NDIteration
 import .NDIteration: get

src/cpu.jl

@@ -33,6 +33,7 @@ function copyto!(backend::CPU, A, B)
 end
 
 functional(::CPU) = true
+pagelock!(::CPU, x) = nothing
 
 function (obj::Kernel{CPU})(args...; ndrange = nothing, workgroupsize = nothing)
     ndrange, workgroupsize, iterspace, dynamic = launch_config(obj, ndrange, workgroupsize)