threads: avoid deadlock from recursive lock acquire

Finalizers can't safely acquire many essential locks (such as the
iolock, to cleanup libuv objects) if they are run inside another lock.
Therefore, inhibit all finalizers on the thread until all locks are
released (previously, this was only true for our internal locks).

NEWS.md

@@ -72,6 +72,8 @@ Command-line option changes
 Multi-threading changes
 -----------------------
 
+* Locks now automatically inhibit finalizers from running, to avoid deadlock ([#TBD]).
+* New function `Base.Threads.foreach(f, channel::Channel)` for multithreaded `Channel` consumption ([#34543]).
 
 Build system changes
 --------------------
@@ -85,7 +87,6 @@ New library functions
 ---------------------
 
 * New function `Base.kron!` and corresponding overloads for various matrix types for performing Kronecker product in-place ([#31069]).
-* New function `Base.Threads.foreach(f, channel::Channel)` for multithreaded `Channel` consumption ([#34543]).
 * New function `Base.readeach(io, T)` for iteratively performing `read(io, T)` ([#36150]).
 * `Iterators.map` is added. It provides another syntax `Iterators.map(f, iterators...)`
   for writing `(f(args...) for args in zip(iterators...))`, i.e. a lazy `map` ([#34352]).

base/gcutils.jl

@@ -106,6 +106,16 @@ Control whether garbage collection is enabled using a boolean argument (`true` f
 """
 enable(on::Bool) = ccall(:jl_gc_enable, Int32, (Int32,), on) != 0
 
+"""
+    GC.enable_finalizers(on::Bool)
+
+Increment or decrement the counter that controls the running of finalizers on
+the current Task. Finalizers will only run when the counter is at zero. (Set
+`true` for enabling, `false` for disabling). They may still run concurrently on
+another Task or thread.
+"""
+enable_finalizers(on::Bool) = ccall(:jl_gc_enable_finalizers, Cvoid, (Ptr{Cvoid}, Int32,), C_NULL, on)
+
 """
     GC.@preserve x1 x2 ... xn expr
 

base/lock.jl

@@ -6,9 +6,24 @@ const ThreadSynchronizer = GenericCondition{Threads.SpinLock}
 """
     ReentrantLock()
 
-Creates a re-entrant lock for synchronizing [`Task`](@ref)s.
-The same task can acquire the lock as many times as required.
-Each [`lock`](@ref) must be matched with an [`unlock`](@ref).
+Creates a re-entrant lock for synchronizing [`Task`](@ref)s. The same task can
+acquire the lock as many times as required. Each [`lock`](@ref) must be matched
+with an [`unlock`](@ref).
+
+Calling 'lock' will also inhibit running of finalizers on that thread until the
+corresponding 'unlock'. Use of the standard lock pattern illustrated below
+should naturally be supported, but beware of inverting the try/lock order or
+missing the try block entirely (e.g. attempting to return with the lock still
+held):
+
+```
+lock(l)
+try
+    <atomic work>
+finally
+    unlock(l)
+end
+```
 """
 mutable struct ReentrantLock <: AbstractLock
     locked_by::Union{Task, Nothing}
@@ -50,6 +65,7 @@ function trylock(rl::ReentrantLock)
     if rl.reentrancy_cnt == 0
         rl.locked_by = t
         rl.reentrancy_cnt = 1
+        GC.enable_finalizers(false)
         got = true
     else
         got = false
@@ -77,6 +93,7 @@ function lock(rl::ReentrantLock)
             if rl.reentrancy_cnt == 0
                 rl.locked_by = t
                 rl.reentrancy_cnt = 1
+                GC.enable_finalizers(false)
                 break
             end
             try
@@ -118,6 +135,7 @@ function unlock(rl::ReentrantLock)
                 rethrow()
             end
         end
+        GC.enable_finalizers(true)
         unlock(rl.cond_wait)
     end
     return
@@ -139,6 +157,7 @@ function unlockall(rl::ReentrantLock)
             rethrow()
         end
     end
+    GC.enable_finalizers(true)
     unlock(rl.cond_wait)
     return n
 end

base/locks-mt.jl

@@ -61,10 +61,12 @@ Base.assert_havelock(l::SpinLock) = islocked(l) ? nothing : Base.concurrency_vio
 function lock(l::SpinLock)
     while true
         if _get(l) == 0
+            GC.enable_finalizers(false)
             p = _xchg!(l, 1)
             if p == 0
                 return
             end
+            GC.enable_finalizers(true)
         end
         ccall(:jl_cpu_pause, Cvoid, ())
         # Temporary solution before we have gc transition support in codegen.
@@ -74,13 +76,20 @@ end
 
 function trylock(l::SpinLock)
     if _get(l) == 0
-        return _xchg!(l, 1) == 0
+        GC.enable_finalizers(false)
+        p = _xchg!(l, 1)
+        if p == 0
+            return true
+        end
+        GC.enable_finalizers(true)
     end
     return false
 end
 
 function unlock(l::SpinLock)
+    _get(l) == 0 && error("unlock count must match lock count")
     _set!(l, 0)
+    GC.enable_finalizers(true)
     ccall(:jl_cpu_wake, Cvoid, ())
     return
 end

src/gc.c

@@ -280,7 +280,7 @@ static void run_finalizer(jl_ptls_t ptls, jl_value_t *o, jl_value_t *ff)
         jl_printf((JL_STREAM*)STDERR_FILENO, "error in running finalizer: ");
         jl_static_show((JL_STREAM*)STDERR_FILENO, jl_current_exception());
         jl_printf((JL_STREAM*)STDERR_FILENO, "\n");
-        jlbacktrace(); // writen to STDERR_FILENO
+        jlbacktrace(); // written to STDERR_FILENO
     }
 }
 
@@ -392,12 +392,36 @@ static void run_finalizers(jl_ptls_t ptls)
     arraylist_free(&copied_list);
 }
 
+JL_DLLEXPORT int jl_gc_get_finalizers_inhibited(jl_ptls_t ptls)
+{
+    if (ptls == NULL)
+        ptls = jl_get_ptls_states();
+    return ptls->finalizers_inhibited;
+}
+
 JL_DLLEXPORT void jl_gc_enable_finalizers(jl_ptls_t ptls, int on)
 {
+    if (ptls == NULL)
+        ptls = jl_get_ptls_states();
     int old_val = ptls->finalizers_inhibited;
     int new_val = old_val + (on ? -1 : 1);
+    if (new_val < 0) {
+        JL_TRY {
+            jl_error(""); // get a backtrace
+        }
+        JL_CATCH {
+            jl_printf((JL_STREAM*)STDERR_FILENO, "WARNING: GC finalizers already enabled on this thread.\n");
+            // Only print the backtrace once, to avoid spamming the logs
+            static int backtrace_printed = 0;
+            if (backtrace_printed == 0) {
+                backtrace_printed = 1;
+                jlbacktrace(); // written to STDERR_FILENO
+            }
+        }
+        return;
+    }
     ptls->finalizers_inhibited = new_val;
-    if (!new_val && old_val && !ptls->in_finalizer) {
+    if (!new_val && old_val && !ptls->in_finalizer && ptls->locks.len == 0) {
         ptls->in_finalizer = 1;
         run_finalizers(ptls);
         ptls->in_finalizer = 0;
@@ -1581,7 +1605,7 @@ STATIC_INLINE uintptr_t gc_read_stack(void *_addr, uintptr_t offset,
 JL_NORETURN NOINLINE void gc_assert_datatype_fail(jl_ptls_t ptls, jl_datatype_t *vt,
                                                   jl_gc_mark_sp_t sp)
 {
-    jl_printf(JL_STDOUT, "GC error (probable corruption) :\n");
+    jl_safe_printf("GC error (probable corruption) :\n");
     gc_debug_print_status();
     jl_(vt);
     gc_debug_critical_error();
@@ -3192,7 +3216,7 @@ JL_DLLEXPORT void jl_gc_collect(jl_gc_collection_t collection)
     // Only disable finalizers on current thread
     // Doing this on all threads is racy (it's impossible to check
     // or wait for finalizers on other threads without dead lock).
-    if (!ptls->finalizers_inhibited) {
+    if (!ptls->finalizers_inhibited && ptls->locks.len == 0) {
         int8_t was_in_finalizer = ptls->in_finalizer;
         ptls->in_finalizer = 1;
         run_finalizers(ptls);

src/julia.h

@@ -1730,7 +1730,6 @@ typedef struct _jl_handler_t {
     int8_t gc_state;
     size_t locks_len;
     sig_atomic_t defer_signal;
-    int finalizers_inhibited;
     jl_timing_block_t *timing_stack;
     size_t world_age;
 } jl_handler_t;
@@ -1753,8 +1752,6 @@ typedef struct _jl_task_t {
     int16_t tid;
     // multiqueue priority
     int16_t prio;
-    // current world age
-    size_t world_age;
     // saved exception stack
     jl_excstack_t *excstack;
     // current exception handler

src/locks.h

@@ -84,11 +84,9 @@ static inline void jl_lock_frame_pop(void)
 
 static inline void jl_mutex_lock(jl_mutex_t *lock)
 {
-    jl_ptls_t ptls = jl_get_ptls_states();
     JL_SIGATOMIC_BEGIN();
     jl_mutex_wait(lock, 1);
     jl_lock_frame_push(lock);
-    jl_gc_enable_finalizers(ptls, 0);
 }
 
 static inline int jl_mutex_trylock_nogc(jl_mutex_t *lock)
@@ -111,10 +109,8 @@ static inline int jl_mutex_trylock(jl_mutex_t *lock)
 {
     int got = jl_mutex_trylock_nogc(lock);
     if (got) {
-        jl_ptls_t ptls = jl_get_ptls_states();
         JL_SIGATOMIC_BEGIN();
         jl_lock_frame_push(lock);
-        jl_gc_enable_finalizers(ptls, 0);
     }
     return got;
 }
@@ -134,9 +130,12 @@ static inline void jl_mutex_unlock(jl_mutex_t *lock)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
     jl_mutex_unlock_nogc(lock);
-    jl_gc_enable_finalizers(ptls, 1);
     jl_lock_frame_pop();
     JL_SIGATOMIC_END();
+    if (ptls->locks.len == 0 && ptls->finalizers_inhibited == 0) {
+        ptls->finalizers_inhibited = 1;
+        jl_gc_enable_finalizers(ptls, 1); // call run_finalizers (may GC)
+    }
 }
 
 static inline void jl_mutex_init(jl_mutex_t *lock) JL_NOTSAFEPOINT

src/rtutils.c

@@ -218,7 +218,6 @@ JL_DLLEXPORT void jl_enter_handler(jl_handler_t *eh)
     eh->gc_state = ptls->gc_state;
     eh->locks_len = ptls->locks.len;
     eh->defer_signal = ptls->defer_signal;
-    eh->finalizers_inhibited = ptls->finalizers_inhibited;
     eh->world_age = ptls->world_age;
     current_task->eh = eh;
 #ifdef ENABLE_TIMINGS
@@ -249,14 +248,14 @@ JL_DLLEXPORT void jl_eh_restore_state(jl_handler_t *eh)
     current_task->eh = eh->prev;
     ptls->pgcstack = eh->gcstack;
     small_arraylist_t *locks = &ptls->locks;
-    if (locks->len > eh->locks_len) {
-        for (size_t i = locks->len;i > eh->locks_len;i--)
+    int unlocks = locks->len > eh->locks_len;
+    if (unlocks) {
+        for (size_t i = locks->len; i > eh->locks_len; i--)
             jl_mutex_unlock_nogc((jl_mutex_t*)locks->items[i - 1]);
         locks->len = eh->locks_len;
     }
     ptls->world_age = eh->world_age;
     ptls->defer_signal = eh->defer_signal;
-    ptls->finalizers_inhibited = eh->finalizers_inhibited;
     if (old_gc_state != eh->gc_state) {
         jl_atomic_store_release(&ptls->gc_state, eh->gc_state);
         if (old_gc_state) {
@@ -266,6 +265,11 @@ JL_DLLEXPORT void jl_eh_restore_state(jl_handler_t *eh)
     if (old_defer_signal && !eh->defer_signal) {
         jl_sigint_safepoint(ptls);
     }
+    if (unlocks && eh->locks_len == 0 && ptls->finalizers_inhibited == 0) {
+        // call run_finalizers
+        ptls->finalizers_inhibited = 1;
+        jl_gc_enable_finalizers(ptls, 1);
+    }
 }
 
 JL_DLLEXPORT void jl_pop_handler(int n)

src/task.c

@@ -393,9 +393,8 @@ static void ctx_switch(jl_ptls_t ptls)
     }
 
     // set up global state for new task
-    lastt->world_age = ptls->world_age;
     ptls->pgcstack = t->gcstack;
-    ptls->world_age = t->world_age;
+    ptls->world_age = 0;
     t->gcstack = NULL;
 #ifdef MIGRATE_TASKS
     ptls->previous_task = lastt;
@@ -510,8 +509,14 @@ JL_DLLEXPORT void jl_switch(void)
     else if (t->tid != ptls->tid) {
         jl_error("cannot switch to task running on another thread");
     }
+
+    // Store old values on the stack and reset
     sig_atomic_t defer_signal = ptls->defer_signal;
     int8_t gc_state = jl_gc_unsafe_enter(ptls);
+    size_t world_age = ptls->world_age;
+    int finalizers_inhibited = ptls->finalizers_inhibited;
+    ptls->world_age = 0;
+    ptls->finalizers_inhibited = 0;
 
 #ifdef ENABLE_TIMINGS
     jl_timing_block_t *blk = ct->timing_stack;
@@ -533,7 +538,12 @@ JL_DLLEXPORT void jl_switch(void)
     assert(ptls == refetch_ptls());
 #endif
 
-    ct = ptls->current_task;
+    // Pop old values back off the stack
+    assert(ct == ptls->current_task &&
+           0 == ptls->world_age &&
+           0 == ptls->finalizers_inhibited);
+    ptls->world_age = world_age;
+    ptls->finalizers_inhibited = finalizers_inhibited;
 
 #ifdef ENABLE_TIMINGS
     assert(blk == ct->timing_stack);
@@ -796,6 +806,7 @@ STATIC_OR_JS void NOINLINE JL_NORETURN start_task(void)
     jl_ptls_t ptls = jl_get_ptls_states();
     jl_task_t *t = ptls->current_task;
     jl_value_t *res;
+    assert(ptls->finalizers_inhibited == 0);
 
 #ifdef MIGRATE_TASKS
     jl_task_t *pt = ptls->previous_task;

stdlib/Distributed/src/messages.jl

@@ -135,6 +135,7 @@ function flush_gc_msgs(w::Worker)
     end
 
     # del_msgs gets populated by finalizers, so be very careful here about ordering of allocations
+    # XXX: threading requires this to be atomic
     new_array = Any[]
     msgs = w.del_msgs
     w.del_msgs = new_array
@@ -166,7 +167,7 @@ function send_msg_(w::Worker, header, msg, now::Bool)
         wait(w.initialized)
     end
     io = w.w_stream
-    lock(io.lock)
+    lock(io)
     try
         reset_state(w.w_serializer)
         serialize_hdr_raw(io, header)
@@ -179,7 +180,7 @@ function send_msg_(w::Worker, header, msg, now::Bool)
             flush(io)
         end
     finally
-        unlock(io.lock)
+        unlock(io)
     end
 end