ruby-changes:61647
From: Aaron <ko1@a...>
Date: Wed, 10 Jun 2020 05:53:45 +0900 (JST)
Subject: [ruby-changes:61647] 62ce8f96cd (master): Revert "Combine sweeping and moving"
https://git.ruby-lang.org/ruby.git/commit/?id=62ce8f96cd From 62ce8f96cd1bf8df50ac7aa1a6f5aa616f24b944 Mon Sep 17 00:00:00 2001 From: Aaron Patterson <tenderlove@r...> Date: Tue, 9 Jun 2020 13:46:29 -0700 Subject: Revert "Combine sweeping and moving" This reverts commit 02b216e5a70235f42f537e895d6f1afd05d8916a. This reverts commit 9b8825b6f94696c9659f93f5da9bf02644625f67. I found that combining sweep and move is not safe. I don't think that we can do compaction concurrently with _anything_ unless there is a read barrier installed. Here is a simple example. A class object is freed, and during it's free step, it tries to remove itself from its parent's subclass list. However, during the sweep step, the parent class was moved and the "currently being freed" class didn't have references updated yet. So we get a segv like this: ``` (lldb) bt * thread #1, name = 'ruby', stop reason = signal SIGSEGV * frame #0: 0x0000560763e344cb ruby`rb_st_lookup at st.c:320:43 frame #1: 0x0000560763e344cb ruby`rb_st_lookup(tab=0x2f7469672f6e6f72, key=3809, value=0x0000560765bf2270) at st.c:1010 frame #2: 0x0000560763e8f16a ruby`rb_search_class_path at variable.c:99:9 frame #3: 0x0000560763e8f141 ruby`rb_search_class_path at variable.c:145 frame #4: 0x0000560763e8f141 ruby`rb_search_class_path(klass=94589785585880) at variable.c:191 frame #5: 0x0000560763ec744e ruby`rb_vm_bugreport at vm_dump.c:996:17 frame #6: 0x0000560763f5b958 ruby`rb_bug_for_fatal_signal at error.c:675:5 frame #7: 0x0000560763e27dad ruby`sigsegv(sig=<unavailable>, info=<unavailable>, ctx=<unavailable>) at signal.c:955:5 frame #8: 0x00007f8b891d33c0 libpthread.so.0`___lldb_unnamed_symbol1$$libpthread.so.0 + 1 frame #9: 0x0000560763efa8bb ruby`rb_class_remove_from_super_subclasses(klass=94589790314280) at class.c:93:56 frame #10: 0x0000560763d10cb7 ruby`gc_sweep_step at gc.c:2674:2 frame #11: 0x0000560763d1187b ruby`gc_sweep at gc.c:4540:2 frame #12: 0x0000560763d101f0 ruby`gc_start at gc.c:6797:6 frame #13: 0x0000560763d15153 ruby`rb_gc_compact at gc.c:7479:12 frame #14: 0x0000560763eb4eb8 ruby`vm_exec_core at vm_insnhelper.c:5183:13 frame #15: 0x0000560763ea9bae ruby`rb_vm_exec at vm.c:1953:22 frame #16: 0x0000560763eac08d ruby`rb_yield at vm.c:1132:9 frame #17: 0x0000560763edb4f2 ruby`rb_ary_collect at array.c:3186:9 frame #18: 0x0000560763e9ee15 ruby`vm_call_cfunc_with_frame at vm_insnhelper.c:2575:12 frame #19: 0x0000560763eb2e66 ruby`vm_exec_core at vm_insnhelper.c:4177:11 frame #20: 0x0000560763ea9bae ruby`rb_vm_exec at vm.c:1953:22 frame #21: 0x0000560763eac08d ruby`rb_yield at vm.c:1132:9 frame #22: 0x0000560763edb4f2 ruby`rb_ary_collect at array.c:3186:9 frame #23: 0x0000560763e9ee15 ruby`vm_call_cfunc_with_frame at vm_insnhelper.c:2575:12 frame #24: 0x0000560763eb2e66 ruby`vm_exec_core at vm_insnhelper.c:4177:11 frame #25: 0x0000560763ea9bae ruby`rb_vm_exec at vm.c:1953:22 frame #26: 0x0000560763ceee01 ruby`rb_ec_exec_node(ec=0x0000560765afa530, n=0x0000560765b088e0) at eval.c:296:2 frame #27: 0x0000560763cf3b7b ruby`ruby_run_node(n=0x0000560765b088e0) at eval.c:354:12 frame #28: 0x0000560763cee4a3 ruby`main(argc=<unavailable>, argv=<unavailable>) at main.c:50:9 frame #29: 0x00007f8b88e560b3 libc.so.6`__libc_start_main + 243 frame #30: 0x0000560763cee4ee ruby`_start + 46 (lldb) f 9 frame #9: 0x0000560763efa8bb ruby`rb_class_remove_from_super_subclasses(klass=94589790314280) at class.c:93:56 90 91 *RCLASS_EXT(klass)->parent_subclasses = entry->next; 92 if (entry->next) { -> 93 RCLASS_EXT(entry->next->klass)->parent_subclasses = RCLASS_EXT(klass)->parent_subclasses; 94 } 95 xfree(entry); 96 } (lldb) command script import -r misc/lldb_cruby.py lldb scripts for ruby has been installed. (lldb) rp entry->next->klass (struct RMoved) $1 = (flags = 30, destination = 94589792806680, next = 94589784369160) (lldb) ``` diff --git a/NEWS.md b/NEWS.md index ea69498..6856a29 100644 --- a/NEWS.md +++ b/NEWS.md @@ -108,14 +108,6 @@ Outstanding ones only. https://github.com/ruby/ruby/blob/trunk/NEWS.md#L108 * Symbol#to_proc now returns a lambda Proc. [[Feature #16260]] -* GC - - * Modified method - - * GC.start now takes an option `compact: true` to compact the heap. - For example `GC.start(compact: true)` will have the same effect as - `GC.compact`. - ## Stdlib updates Outstanding ones only. diff --git a/gc.c b/gc.c index 547f9b2..3b2e7cd 100644 --- a/gc.c +++ b/gc.c @@ -480,7 +480,6 @@ typedef enum { https://github.com/ruby/ruby/blob/trunk/gc.c#L480 GPR_FLAG_HAVE_FINALIZE = 0x4000, GPR_FLAG_IMMEDIATE_MARK = 0x8000, GPR_FLAG_FULL_MARK = 0x10000, - GPR_FLAG_COMPACT = 0x20000, GPR_DEFAULT_REASON = (GPR_FLAG_FULL_MARK | GPR_FLAG_IMMEDIATE_MARK | @@ -635,8 +634,6 @@ typedef struct rb_heap_struct { https://github.com/ruby/ruby/blob/trunk/gc.c#L634 struct heap_page *using_page; struct list_head pages; struct heap_page *sweeping_page; /* iterator for .pages */ - struct heap_page *compact_cursor; - VALUE moved_list; #if GC_ENABLE_INCREMENTAL_MARK struct heap_page *pooled_pages; #endif @@ -670,7 +667,6 @@ typedef struct rb_objspace { https://github.com/ruby/ruby/blob/trunk/gc.c#L667 unsigned int gc_stressful: 1; unsigned int has_hook: 1; unsigned int during_minor_gc : 1; - unsigned int compact : 1; #if GC_ENABLE_INCREMENTAL_MARK unsigned int during_incremental_marking : 1; #endif @@ -4182,71 +4178,17 @@ gc_setup_mark_bits(struct heap_page *page) https://github.com/ruby/ruby/blob/trunk/gc.c#L4178 memcpy(&page->mark_bits[0], &page->uncollectible_bits[0], HEAP_PAGE_BITMAP_SIZE); } -static int gc_is_moveable_obj(rb_objspace_t *objspace, VALUE obj); -static VALUE gc_move(rb_objspace_t *objspace, VALUE scan, VALUE free, VALUE moved_list); - -static short -try_move(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_page, VALUE vp) -{ - struct heap_page * cursor = heap->compact_cursor; - - while(1) { - bits_t *mark_bits = cursor->mark_bits; - RVALUE * p = cursor->start; - RVALUE * offset = p - NUM_IN_PAGE(p); - - /* Find an object to move and move it. Movable objects must be - * marked, so we iterate using the marking bitmap */ - for (size_t i = 0; i < HEAP_PAGE_BITMAP_LIMIT; i++) { - bits_t bits = mark_bits[i]; - - if (bits) { - p = offset + i * BITS_BITLENGTH; - - do { - if (bits & 1) { - if (gc_is_moveable_obj(objspace, (VALUE)p)) { - heap->moved_list = gc_move(objspace, (VALUE)p, vp, heap->moved_list); - return 1; - } - } - p++; - bits >>= 1; - } while (bits); - } - } - - struct heap_page * next; - - next = list_prev(&heap->pages, cursor, page_node); - - // Cursors have met, lets quit - if (next == sweep_page) { - break; - } else { - heap->compact_cursor = next; - cursor = next; - } - } - - return 0; -} - static inline int gc_page_sweep(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_page) { int i; - int empty_slots = 0, freed_slots = 0, final_slots = 0, moved_slots = 0; + int empty_slots = 0, freed_slots = 0, final_slots = 0; RVALUE *p, *pend,*offset; bits_t *bits, bitset; gc_report(2, objspace, "page_sweep: start.\n"); sweep_page->flags.before_sweep = FALSE; - if (heap->compact_cursor && sweep_page == heap->compact_cursor) { - /* The compaction cursor and sweep page met, so we need to quit compacting */ - heap->compact_cursor = NULL; - } p = sweep_page->start; pend = p + sweep_page->total_slots; offset = p - NUM_IN_PAGE(p); @@ -4278,51 +4220,20 @@ gc_page_sweep(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_ https://github.com/ruby/ruby/blob/trunk/gc.c#L4220 } else { (void)VALGRIND_MAKE_MEM_UNDEFINED((void*)p, sizeof(RVALUE)); - - if (heap->compact_cursor) { - /* If try_move couldn't compact anything, it means - * the cursors have met and there are no objects left that - * /can/ be compacted, so we need to quit. */ - if (try_move(objspace, heap, sweep_page, vp)) { - moved_slots++; - } else { - heap->compact_cursor = NULL; - heap_page_add_freeobj(objspace, sweep_page, vp); - } - } else { - heap_page_add_freeobj(objspace, sweep_page, vp); - } - + heap_page_add_freeobj(objspace, sweep_page, vp); gc_report(3, objspace, "page_sweep: %s is added to freelist\n", obj_info(vp)); - freed_slots++; + freed_slots++; + asan_poison_object(vp); } break; } /* minor cases */ - case T_MOVED: - if (!objspace->flags.during_compacting) { - /* When compaction is combined with sweeping, some of the swept pages - * will have T_MOVED objects on them. These need to be kept alive - * until references are finished updating. Once references are finished - * updating, `gc_unlink_moved_list` will clear th (... truncated) -- ML: ruby-changes@q... Info: http://www.atdot.net/~ko1/quickml/