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We can't grab the VM Lock in free functions

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This is due to the way MMTK frees objects, which is on another native thread.
Due to this, there's no `ec` so we can't grab the VM Lock.

This was causing issues in release builds of MMTK on CI like:

```
  /home/runner/work/ruby/ruby/build/ruby(sigsegv+0x46) [0x557905117ef6] ../src/signal.c:948
  /lib/x86_64-linux-gnu/libc.so.6(0x7f555f845330) [0x7f555f845330]
  /home/runner/work/ruby/ruby/build/ruby(rb_ec_thread_ptr+0x0) [0x5579051d59d5] ../src/vm_core.h:2087
  /home/runner/work/ruby/ruby/build/ruby(rb_ec_ractor_ptr) ../src/vm_core.h:2036
  /home/runner/work/ruby/ruby/build/ruby(rb_current_execution_context) ../src/vm_core.h:2105
  /home/runner/work/ruby/ruby/build/ruby(rb_current_ractor_raw) ../src/vm_core.h:2104
  /home/runner/work/ruby/ruby/build/ruby(rb_current_ractor) ../src/vm_core.h:2112
  /home/runner/work/ruby/ruby/build/ruby(rb_current_ractor) ../src/vm_core.h:2110
  /home/runner/work/ruby/ruby/build/ruby(vm_locked) ../src/vm_sync.c:15
  /home/runner/work/ruby/ruby/build/ruby(rb_vm_lock_enter_body) ../src/vm_sync.c:141
  /home/runner/work/ruby/ruby/build/ruby(rb_vm_lock_enter+0xa) [0x557905390a5a] ../src/vm_sync.h:76
  /home/runner/work/ruby/ruby/build/ruby(fiber_pool_stack_release) ../src/cont.c:777
  /home/runner/work/ruby/ruby/build/ruby(fiber_stack_release+0xe) [0x557905392075] ../src/cont.c:919
  /home/runner/work/ruby/ruby/build/ruby(cont_free) ../src/cont.c:1087
  /home/runner/work/ruby/ruby/build/ruby(fiber_free) ../src/cont.c:1180
```

This would have ran into an assertion error in a debug build but we don't run debug builds of MMTK on Github's CI.

Co-authored-by: john.hawthorn@shopify.com
This commit is contained in:
Luke Gruber 2025-10-10 14:47:46 -04:00 committed by John Hawthorn
parent 31a1a39ace
commit 27ff586152
Notes: git 2025-10-15 17:50:06 +00:00 
Merged: https://github.com/ruby/ruby/pull/14811
3 changed files with 45 additions and 31 deletions
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71
cont.c
View File

@ -774,44 +774,40 @@ static void
fiber_pool_stack_release(struct fiber_pool_stack * stack)
{
struct fiber_pool * pool = stack->pool;
RB_VM_LOCK_ENTER();
{
struct fiber_pool_vacancy * vacancy = fiber_pool_vacancy_pointer(stack->base, stack->size);
struct fiber_pool_vacancy * vacancy = fiber_pool_vacancy_pointer(stack->base, stack->size);
if (DEBUG) fprintf(stderr, "fiber_pool_stack_release: %p used=%"PRIuSIZE"\n", stack->base, stack->pool->used);
if (DEBUG) fprintf(stderr, "fiber_pool_stack_release: %p used=%"PRIuSIZE"\n", stack->base, stack->pool->used);
// Copy the stack details into the vacancy area:
vacancy->stack = *stack;
// After this point, be careful about updating/using state in stack, since it's copied to the vacancy area.
// Copy the stack details into the vacancy area:
vacancy->stack = *stack;
// After this point, be careful about updating/using state in stack, since it's copied to the vacancy area.
// Reset the stack pointers and reserve space for the vacancy data:
fiber_pool_vacancy_reset(vacancy);
// Reset the stack pointers and reserve space for the vacancy data:
fiber_pool_vacancy_reset(vacancy);
// Push the vacancy into the vancancies list:
pool->vacancies = fiber_pool_vacancy_push(vacancy, pool->vacancies);
pool->used -= 1;
// Push the vacancy into the vancancies list:
pool->vacancies = fiber_pool_vacancy_push(vacancy, pool->vacancies);
pool->used -= 1;
#ifdef FIBER_POOL_ALLOCATION_FREE
struct fiber_pool_allocation * allocation = stack->allocation;
struct fiber_pool_allocation * allocation = stack->allocation;
allocation->used -= 1;
allocation->used -= 1;
// Release address space and/or dirty memory:
if (allocation->used == 0) {
fiber_pool_allocation_free(allocation);
}
else if (stack->pool->free_stacks) {
fiber_pool_stack_free(&vacancy->stack);
}
#else
// This is entirely optional, but clears the dirty flag from the stack
// memory, so it won't get swapped to disk when there is memory pressure:
if (stack->pool->free_stacks) {
fiber_pool_stack_free(&vacancy->stack);
}
#endif
// Release address space and/or dirty memory:
if (allocation->used == 0) {
fiber_pool_allocation_free(allocation);
}
RB_VM_LOCK_LEAVE();
else if (stack->pool->free_stacks) {
fiber_pool_stack_free(&vacancy->stack);
}
#else
// This is entirely optional, but clears the dirty flag from the stack
// memory, so it won't get swapped to disk when there is memory pressure:
if (stack->pool->free_stacks) {
fiber_pool_stack_free(&vacancy->stack);
}
#endif
}
static inline void
@ -924,6 +920,17 @@ fiber_stack_release(rb_fiber_t * fiber)
rb_ec_clear_vm_stack(ec);
}
static void
fiber_stack_release_locked(rb_fiber_t *fiber)
{
if (!ruby_vm_during_cleanup) {
// We can't try to acquire the VM lock here because MMTK calls free in its own native thread which has no ec.
// This assertion will fail on MMTK but we currently don't have CI for debug releases of MMTK, so we can assert for now.
ASSERT_vm_locking_with_barrier();
}
fiber_stack_release(fiber);
}
static const char *
fiber_status_name(enum fiber_status s)
{
@ -1084,7 +1091,7 @@ cont_free(void *ptr)
else {
rb_fiber_t *fiber = (rb_fiber_t*)cont;
coroutine_destroy(&fiber->context);
fiber_stack_release(fiber);
fiber_stack_release_locked(fiber);
}
RUBY_FREE_UNLESS_NULL(cont->saved_vm_stack.ptr);
@ -2741,7 +2748,9 @@ fiber_switch(rb_fiber_t *fiber, int argc, const VALUE *argv, int kw_splat, rb_fi
// We cannot free the stack until the pthread is joined:
#ifndef COROUTINE_PTHREAD_CONTEXT
if (resuming_fiber && FIBER_TERMINATED_P(fiber)) {
fiber_stack_release(fiber);
RB_VM_LOCKING() {
fiber_stack_release(fiber);
}
}
#endif

3
vm.c
View File

@ -59,6 +59,8 @@ int ruby_assert_critical_section_entered = 0;
static void *native_main_thread_stack_top;
bool ruby_vm_during_cleanup = false;
VALUE rb_str_concat_literals(size_t, const VALUE*);
VALUE vm_exec(rb_execution_context_t *);
@ -3287,6 +3289,7 @@ int
ruby_vm_destruct(rb_vm_t *vm)
{
RUBY_FREE_ENTER("vm");
ruby_vm_during_cleanup = true;
if (vm) {
rb_thread_t *th = vm->ractor.main_thread;

2
vm_core.h
View File

@ -823,6 +823,8 @@ typedef struct rb_vm_struct {
} default_params;
} rb_vm_t;
extern bool ruby_vm_during_cleanup;
/* default values */
#define RUBY_VM_SIZE_ALIGN 4096