[LinearAlgebra] Cache lbt_get_config() result (JuliaLang#44383)

* [LinearAlgebra] Cache `lbt_get_config()` result

In the event that users want to call `lbt_get_config()` multiple times
(e.g. for runtime checks of which BLAS vendor is providing a symbol),
let's cache the value and clear it only when someone calls something
that would cause the config itself to change.

* Use double-checked locking to cache LBT config (JuliaLang#44387)

* Use double-checked locking to cache LBT config

* Use Any field for lock-free load and store

* Revert "Use Any field for lock-free load and store"

This reverts commit c6acae5.

The compiler already handled it:
JuliaLang#44387 (comment)

* (rerun)

* Add lock as a field of ConfigCache

Co-authored-by: Takafumi Arakaki <[email protected]>

stdlib/LinearAlgebra/src/lbt.jl

@@ -171,9 +171,32 @@ function Base.show(io::IO, mime::MIME{Symbol("text/plain")}, lbt::LBTConfig)
  end
 end
 
+mutable struct ConfigCache
+ @atomic config::Union{Nothing,LBTConfig}
+ lock::ReentrantLock
+end
+
+# In the event that users want to call `lbt_get_config()` multiple times (e.g. for
+# runtime checks of which BLAS vendor is providing a symbol), let's cache the value
+# and clear it only when someone calls something that would cause it to change.
+const _CACHED_CONFIG = ConfigCache(nothing, ReentrantLock())
+
 function lbt_get_config()
- config_ptr = ccall((:lbt_get_config, libblastrampoline), Ptr{lbt_config_t}, ())
- return LBTConfig(unsafe_load(config_ptr))
+ config = @atomic :acquire _CACHED_CONFIG.config
+ config === nothing || return config
+ return lock(_CACHED_CONFIG.lock) do
+ local config = @atomic :monotonic _CACHED_CONFIG.config
+ config === nothing || return config
+ config_ptr = ccall((:lbt_get_config, libblastrampoline), Ptr{lbt_config_t}, ())
+ @atomic :release _CACHED_CONFIG.config = LBTConfig(unsafe_load(config_ptr))
+ end
+end
+
+function _clear_config_with(f)
+ lock(_CACHED_CONFIG.lock) do
+ @atomic :release _CACHED_CONFIG.config = nothing
+ f()
+ end
 end
 
 function lbt_get_num_threads()
@@ -185,11 +208,15 @@ function lbt_set_num_threads(nthreads)
 end
 
 function lbt_forward(path; clear::Bool = false, verbose::Bool = false, suffix_hint::Union{String,Nothing} = nothing)
- ccall((:lbt_forward, libblastrampoline), Int32, (Cstring, Int32, Int32, Cstring), path, clear ? 1 : 0, verbose ? 1 : 0, something(suffix_hint, C_NULL))
+ _clear_config_with() do
+ return ccall((:lbt_forward, libblastrampoline), Int32, (Cstring, Int32, Int32, Cstring), path, clear ? 1 : 0, verbose ? 1 : 0, something(suffix_hint, C_NULL))
+ end
 end
 
 function lbt_set_default_func(addr)
- return ccall((:lbt_set_default_func, libblastrampoline), Cvoid, (Ptr{Cvoid},), addr)
+ _clear_config_with() do
+ return ccall((:lbt_set_default_func, libblastrampoline), Cvoid, (Ptr{Cvoid},), addr)
+ end
 end
 
 function lbt_get_default_func()
@@ -241,17 +268,19 @@ end
 function lbt_set_forward(symbol_name, addr, interface,
  complex_retstyle = LBT_COMPLEX_RETSTYLE_NORMAL,
  f2c = LBT_F2C_PLAIN; verbose::Bool = false)
- return ccall(
- (:lbt_set_forward, libblastrampoline),
- Int32,
- (Cstring, Ptr{Cvoid}, Int32, Int32, Int32, Int32),
- string(symbol_name),
- addr,
- Int32(interface),
- Int32(complex_retstyle),
- Int32(f2c),
- verbose ? Int32(1) : Int32(0),
- )
+ _clear_config_with() do
+ return ccall(
+ (:lbt_set_forward, libblastrampoline),
+ Int32,
+ (Cstring, Ptr{Cvoid}, Int32, Int32, Int32, Int32),
+ string(symbol_name),
+ addr,
+ Int32(interface),
+ Int32(complex_retstyle),
+ Int32(f2c),
+ verbose ? Int32(1) : Int32(0),
+ )
+ end
 end
 function lbt_set_forward(symbol_name, addr, interface::Symbol,
  complex_retstyle::Symbol = :normal,