Allow recycling fibers by GC if not referenced directly#6253
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`mrb_env_unshare()` calls `mrb_realloc_simple()` and follows `mrb_full_gc()` to avoid an infinite loop where `mrb_env_unshare()` is called again. This does not occur at this time, but may occur in subsequent patches.
The patch assumes that `struct REnv::cxt` only performs checks with the `OP_BREAK` and `OP_RETURN_BLK` instructions, and does not reference the entity. Therefore, by changing to a weak reference, it is possible to collect fibers that are no longer directly referenced while in the suspended state. However, we need to detach the living env objects that remain in the call stack of the fiber. So, in effect, it involves a revert of following commits. - commit a3365d8 - commit 57ffa1c Examples of the effects of change are shown below. Note that it was built with `rake MRUBY_CONFIG=host-debug`. ```ruby f = Fiber.new { (x, y, z) = "X", "Y", "Z"; Fiber.yield -> { [x, y, z] } } g = f.resume GC.start p ObjectSpace.memsize_of_all # => 59532 g.call # => ["X", "Y", "Z"] f = nil GC.start ObjectSpace.memsize_of_all # BEFORE => 59532 # AFTER => 58044 g.call # => ["X", "Y", "Z"] ```
matz
reviewed
Apr 23, 2024
| struct mrb_context *c = ((struct RFiber*)obj)->cxt; | ||
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| if (c != mrb->root_c) { | ||
| if (c && c != mrb->root_c) { |
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Since mrb->root_c is never NULL, I don't think the c && check is necessary.
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Thanks for your review.
I checked and it seems that omitting the NULL check on the c variable causes a crash due to a "null pointer dereference".
% lldb18 -- bin/mruby -e 'Fiber.allocate; GC.start'
(lldb) target create "bin/mruby"
Current executable set to '/var/tmp/mruby/bin/mruby' (x86_64).
(lldb) settings set -- target.run-args "-e" "Fiber.allocate; GC.start"
(lldb) r
Process 90188 launched: '/var/tmp/mruby/bin/mruby' (x86_64)
Process 90188 stopped
* thread #1, name = 'mruby', stop reason = signal SIGSEGV: address not mapped to object (fault address: 0x30)
frame #0: 0x0000000000413264 mruby`obj_free(mrb=0x000008d4df809000, obj=0x000008d4df82ad70, end=false) at gc.c:776:24
773 struct mrb_context *c = ((struct RFiber*)obj)->cxt;
774
775 if (c != mrb->root_c) {
-> 776 if (!end && c->status != MRB_FIBER_TERMINATED) {
777 mrb_callinfo *ci = c->ci;
778 mrb_callinfo *ce = c->cibase;
779mrb_free_context() is "NULL safe", so there is no problem.
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Well, thank you for the confirmation. My bad.
dearblue
commented
Apr 24, 2024
Comment on lines
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| if (MRB_ENV_ONSTACK_P(e) && e->cxt && e->cxt->fib) { | ||
| mrb_gc_mark(mrb, (struct RBasic*)e->cxt->fib); | ||
| } |
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I was checking again and found a period when this section did not exist.
% git log -L627,640:src/gc.c 22518b5b789293e7ddad7a94def44039937d8e8cThere is no mention of a commit message, but it corresponds to a revert of commit b6598e0.
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The patch assumes that
struct REnv::cxtonly performs checks with theOP_BREAKandOP_RETURN_BLKinstructions, and does not reference the entity.Therefore, by changing to a weak reference, it is possible to collect fibers that are no longer directly referenced while in the suspended state.
However, we need to detach the living env objects that remain in the call stack of the fiber.
So, in effect, it involves a revert of following commits.
Examples of the effects of change are shown below.
Note that it was built with
rake MRUBY_CONFIG=host-debug.In addition, it also takes the necessary precautions to prevent the possibility of infinite loops with this change.