locks-mt.jl
# This file is a part of Julia. License is MIT: https://julialang.org/license
import .Base: unsafe_convert, lock, trylock, unlock, islocked, wait, notify, AbstractLock
export SpinLock
# Important Note: these low-level primitives defined here
# are typically not for general usage
##########################################
# Atomic Locks
##########################################
"""
SpinLock()
Create a non-reentrant, test-and-test-and-set spin lock.
Recursive use will result in a deadlock.
This kind of lock should only be used around code that takes little time
to execute and does not block (e.g. perform I/O).
In general, [`ReentrantLock`](@ref) should be used instead.
Each [`lock`](@ref) must be matched with an [`unlock`](@ref).
Test-and-test-and-set spin locks are quickest up to about 30ish
contending threads. If you have more contention than that, different
synchronization approaches should be considered.
"""
mutable struct SpinLock <: AbstractLock
owned::Int
SpinLock() = new(0)
end
import Base.Sys.WORD_SIZE
@eval _xchg!(x::SpinLock, v::Int) =
llvmcall($"""
%ptr = inttoptr i$WORD_SIZE %0 to i$WORD_SIZE*
%rv = atomicrmw xchg i$WORD_SIZE* %ptr, i$WORD_SIZE %1 acq_rel
ret i$WORD_SIZE %rv
""", Int, Tuple{Ptr{Int}, Int}, unsafe_convert(Ptr{Int}, pointer_from_objref(x)), v)
@eval _get(x::SpinLock) =
llvmcall($"""
%ptr = inttoptr i$WORD_SIZE %0 to i$WORD_SIZE*
%rv = load atomic i$WORD_SIZE, i$WORD_SIZE* %ptr monotonic, align $(gc_alignment(Int))
ret i$WORD_SIZE %rv
""", Int, Tuple{Ptr{Int}}, unsafe_convert(Ptr{Int}, pointer_from_objref(x)))
@eval _set!(x::SpinLock, v::Int) =
llvmcall($"""
%ptr = inttoptr i$WORD_SIZE %0 to i$WORD_SIZE*
store atomic i$WORD_SIZE %1, i$WORD_SIZE* %ptr release, align $(gc_alignment(Int))
ret void
""", Cvoid, Tuple{Ptr{Int}, Int}, unsafe_convert(Ptr{Int}, pointer_from_objref(x)), v)
# Note: this cannot assert that the lock is held by the correct thread, because we do not
# track which thread locked it. Users beware.
Base.assert_havelock(l::SpinLock) = islocked(l) ? nothing : Base.concurrency_violation()
function lock(l::SpinLock)
while true
if _get(l) == 0
GC.disable_finalizers()
p = _xchg!(l, 1)
if p == 0
return
end
GC.enable_finalizers()
end
ccall(:jl_cpu_pause, Cvoid, ())
# Temporary solution before we have gc transition support in codegen.
ccall(:jl_gc_safepoint, Cvoid, ())
end
end
function trylock(l::SpinLock)
if _get(l) == 0
GC.disable_finalizers()
p = _xchg!(l, 1)
if p == 0
return true
end
GC.enable_finalizers()
end
return false
end
function unlock(l::SpinLock)
_get(l) == 0 && error("unlock count must match lock count")
_set!(l, 0)
GC.enable_finalizers()
ccall(:jl_cpu_wake, Cvoid, ())
return
end
function islocked(l::SpinLock)
return _get(l) != 0
end