Ali Ghanbari (Jun 25 2024 at 12:10): Ali Ghanbari (Jun 25 2024 at 12:10):I'm trying to make a thread_join function in WebAssembly by setting a mutex to 1 in the main thread, and then launching a thread. the thread unlocks its mutex before exiting. the main thread can get notified with the threads exit by trying to acquire the mutex and releasing it.
This is my code:
(module
(type $main_type (func))
(type $simple_type (func (param i32)))
(type $thread_spawn_type (func (param i32) (result i32)))
(type $thread_start_type (func (param i32 i32)))
(import "wasi" "thread-spawn" (func $thread_spawn (type $thread_spawn_type)))
(func $try_lock_mutex (type $thread_spawn_type) (param $mutex_address i32) (result i32)
local.get $mutex_address
i32.const 0 ;; expected
i32.const 1 ;; locked
i32.atomic.rmw.cmpxchg
i32.eqz
)
(func $lock_mutex (type $simple_type) (param $mutex_address i32)
block $done
loop $retry
;; Try to lock the mutex. $tryLockMutex returns 1 if the mutex
;; was locked, and 0 otherwise.
local.get $mutex_address
call $try_lock_mutex
br_if $done
;; Wait for the other agent to finish with mutex.
local.get $mutex_address ;; mutex address
i32.const 1 ;; expected value (1 => locked)
i64.const -1 ;; infinite timeout
memory.atomic.wait32
drop
br $retry
end
end
)
(func $unlock_mutex (type $simple_type) (param $mutex_address i32)
;; mutex must be locked before by caller
;; Unlock the mutex.
local.get $mutex_address ;; mutex address
i32.const 0 ;; 0 => unlocked
i32.atomic.store
;; Notify one agent that is waiting on this lock.
local.get $mutex_address ;; mutex address
i32.const 1 ;; notify 1 waiter
memory.atomic.notify
drop
)
(func $wait_mutex_lock (type $simple_type) (param $mutex_address i32)
local.get $mutex_address
call $lock_mutex
local.get $mutex_address
call $unlock_mutex
)
(func $wasi_thread_start (type $thread_start_type) (param i32 i32)
local.get 1
call $unlock_mutex
)
(func $main (type $main_type)
i32.const 0
call $lock_mutex
i32.const 0
call $thread_spawn
drop
i32.const 0
call $wait_mutex_lock
)
(memory (;0;) 4 4 shared)
(export "wasi_thread_start" (func $wasi_thread_start))
(export "_start" (func $main))
)
I convert it to wasm using wat2wasm like this:
wat2wasm.exe mutex.wat --enable-threads -o mutex.wasm
Then run it using wasmtime with:
wasmtime -W all-proposals=y -S threads .\mutex.wasm
The mutex code is from the wasm-threads proposal.
The problem is that the mutex never gets unlocked by the launched thread and the main thread gets stuck in an infinite waiting state.
Marcin Kolny (Jun 25 2024 at 12:18):just tried the code on WAMR and it worked as expected, perhaps a wasmtime-specific problem ?
Ali Ghanbari (Jun 25 2024 at 15:29):no I tried it with wasmer & WAMR and it doesn't work. but I just found what is wrong with the code above. you need to import & export memory + explecitly specify atomic operation memory index for it to work.
Ali Ghanbari (Jun 25 2024 at 15:31):Here is new working code:
(module
(type $main_type (func))
(type $simple_type (func (param i32)))
(type $thread_spawn_type (func (param i32) (result i32)))
(type $thread_start_type (func (param i32 i32)))
(import "env" "memory" (memory (;0;) 4 4 shared))
(import "wasi" "thread-spawn" (func $thread_spawn (type $thread_spawn_type)))
(func $try_lock_mutex (type $thread_spawn_type) (param $mutex_address i32) (result i32)
local.get $mutex_address
i32.const 0 ;; expected
i32.const 1 ;; locked
i32.atomic.rmw.cmpxchg 0
i32.eqz
)
(func $lock_mutex (type $simple_type) (param $mutex_address i32)
block $done
loop $retry
;; Try to lock the mutex. $tryLockMutex returns 1 if the mutex
;; was locked, and 0 otherwise.
local.get $mutex_address
call $try_lock_mutex
br_if $done
;; Wait for the other agent to finish with mutex.
local.get $mutex_address ;; mutex address
i32.const 1 ;; expected value (1 => locked)
i64.const -1 ;; infinite timeout
memory.atomic.wait32 0
drop
br $retry
end
end
)
(func $unlock_mutex (type $simple_type) (param $mutex_address i32)
;; mutex must be locked before by caller
;; Unlock the mutex.
local.get $mutex_address ;; mutex address
i32.const 0 ;; 0 => unlocked
i32.atomic.store 0
;; Notify one agent that is waiting on this lock.
local.get $mutex_address ;; mutex address
i32.const 1 ;; notify 1 waiter
memory.atomic.notify 0
drop
)
(func $wait_mutex_lock (type $simple_type) (param $mutex_address i32)
local.get $mutex_address
call $lock_mutex
local.get $mutex_address
call $unlock_mutex
)
(func $wasi_thread_start (type $thread_start_type) (param i32 i32)
local.get 1
call $unlock_mutex
)
(func $main (type $main_type)
i32.const 0
call $lock_mutex
i32.const 0
call $thread_spawn
drop
i32.const 0
call $wait_mutex_lock
)
(export "memory" (memory 0))
(export "wasi_thread_start" (func $wasi_thread_start))
(export "_start" (func $main))
)
and tested with:
wat2wasm.exe mutex.wat --enable-multi-memory --enable-threads -o mutex.wasm
wasmtime -W all-proposals=y -S threads .\mutex.wasm
Ali Ghanbari has marked this topic as resolved.
Last updated: Dec 23 2024 at 12:05 UTC