Re: [Tarantool-patches] [PATCH luajit] FFI: Drop finalizer table rehash after GC cycle.

[Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Hi, Sergey!


thanks for the patch! LGTM with minor comments below

On 02.09.2024 15:54, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Reported by Sergey Kaplun.
>
> (cherry picked from commit fb22d0f80f291827a4004e16bc589b54bcc4a3c7)
>
> The raising of the OOM error when rehashing the finalizer table (when we
> can't allocate a new hash part) leads to crashes in either
> `lj_trace_exit()` or `lj_trace_unwind()` due to unprotected error
> raising, which either has no DWARF eh_frame or loses the context of the

I would add a link to a page about eh_frame, for example [1]

Feel free to ignore.

1. 
https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html

> JIT compiler.
>
> This patch drops rehashing of the finalizer table to avoid these

I would replace "finalizer" with "cdata finalizer".

And I would say about drawbacks of this. Otherwise, it looks like

rehashing was not needed from the beginning.

> crashes.
>
> Sergey Kaplun:
> * added the description and the test for the problem
>
> Part of tarantool/tarantool#10199
> Resolves tarantool/tarantool#10290
Usually "Closes" or "Fixes". Feel free to ignore.
> ---
>
> Branch:https://github.com/tarantool/luajit/tree/skaplun/lj-1247-fin-tab-rehashing-on-trace
> Related Issues:
> *https://github.com/tarantool/tarantool/issues/10290
> *https://github.com/LuaJIT/LuaJIT/issues/1247
> *https://github.com/tarantool/tarantool/issues/10199
>
>   src/lj_gc.c                                   |   7 -
>   src/lj_obj.h                                  |   2 +-
>   test/tarantool-tests/CMakeLists.txt           |   1 +
>   ...j-1247-fin-tab-rehashing-on-trace.test.lua | 127 ++++++++++++++++++
>   .../CMakeLists.txt                            |   1 +
>   .../lj_1247_allocinject.c                     |  49 +++++++
>   6 files changed, 179 insertions(+), 8 deletions(-)
>   create mode 100644 test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace.test.lua
>   create mode 100644 test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace/CMakeLists.txt
>   create mode 100644 test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace/lj_1247_allocinject.c
>
> diff --git a/src/lj_gc.c b/src/lj_gc.c
> index 4c222f21..a2fc93a0 100644
> --- a/src/lj_gc.c
> +++ b/src/lj_gc.c
> @@ -548,7 +548,6 @@ static void gc_finalize(lua_State *L)
>       setcdataV(L, &tmp, gco2cd(o));
>       tv = lj_tab_set(L, tabref(g->gcroot[GCROOT_FFI_FIN]), &tmp);
>       if (!tvisnil(tv)) {
> -      g->gc.nocdatafin = 0;
>         copyTV(L, &tmp, tv);
>         setnilV(tv);  /* Clear entry in finalizer table. */
>         gc_call_finalizer(g, L, &tmp, o);
> @@ -694,9 +693,6 @@ static size_t gc_onestep(lua_State *L)
>   	lj_str_resize(L, g->strmask >> 1);  /* Shrink string table. */
>         if (gcref(g->gc.mmudata)) {  /* Need any finalizations? */
>   	g->gc.state = GCSfinalize;
> -#if LJ_HASFFI
> -	g->gc.nocdatafin = 1;
> -#endif
>         } else {  /* Otherwise skip this phase to help the JIT. */
>   	g->gc.state = GCSpause;  /* End of GC cycle. */
>   	g->gc.debt = 0;
> @@ -713,9 +709,6 @@ static size_t gc_onestep(lua_State *L)
>   	g->gc.estimate -= GCFINALIZECOST;
>         return GCFINALIZECOST;
>       }
> -#if LJ_HASFFI
> -    if (!g->gc.nocdatafin) lj_tab_rehash(L, tabref(g->gcroot[GCROOT_FFI_FIN]));
> -#endif
>       g->gc.state = GCSpause;  /* End of GC cycle. */
>       g->gc.debt = 0;
>       return 0;
> diff --git a/src/lj_obj.h b/src/lj_obj.h
> index 06ea0cd0..ff22e5f8 100644
> --- a/src/lj_obj.h
> +++ b/src/lj_obj.h
> @@ -611,7 +611,7 @@ typedef struct GCState {
>     GCSize threshold;	/* Memory threshold. */
>     uint8_t currentwhite;	/* Current white color. */
>     uint8_t state;	/* GC state. */
> -  uint8_t nocdatafin;	/* No cdata finalizer called. */
> +  uint8_t unused0;
>   #if LJ_64
>     uint8_t lightudnum;	/* Number of lightuserdata segments - 1. */
>   #else
> diff --git a/test/tarantool-tests/CMakeLists.txt b/test/tarantool-tests/CMakeLists.txt
> index e3750bf3..e5d5a470 100644
> --- a/test/tarantool-tests/CMakeLists.txt
> +++ b/test/tarantool-tests/CMakeLists.txt
> @@ -37,6 +37,7 @@ add_subdirectory(lj-flush-on-trace)
>   add_subdirectory(lj-1004-oom-error-frame)
>   add_subdirectory(lj-1066-fix-cur_L-after-coroutine-resume)
>   add_subdirectory(lj-1166-error-stitch)
> +add_subdirectory(lj-1247-fin-tab-rehashing-on-trace)
>   
>   # The part of the memory profiler toolchain is located in tools
>   # directory, jit, profiler, and bytecode toolchains are located
> diff --git a/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace.test.lua b/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace.test.lua
> new file mode 100644
> index 00000000..308043a2
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace.test.lua
> @@ -0,0 +1,127 @@
> +local tap = require('tap')
> +
> +-- The test file to demonstrate the incorrect JIT behaviour during
> +-- OOM on the finalizer table rehashing in the context of the JIT
> +-- trace.
> +-- See also:
> +-- *https://github.com/LuaJIT/LuaJIT/issues/1247,
> +-- *https://github.com/tarantool/tarantool/issues/10290.
> +
> +local test = tap.test('lj-1247-fin-tab-rehashing-on-trace'):skipcond({
> +  ['Broken unwiding in tarantool_panic_handler'] = _TARANTOOL and
> +                                                   (jit.os == 'OSX'),
> +  ['Disabled on MacOS due to #8652'] = jit.os == 'OSX',
> +  ['Test requires JIT enabled'] = not jit.status(),
> +})
> +
> +-- XXX: The original issue has 2 ways to crash:
> +-- 1) in `lj_trace_unwind()`
> +-- 2) in `lj_trace_exit()`
> +-- But, since we have an additional GC pressure due to requiring a
> +-- `tap` module, the second case needs an impossibly big
> +-- `gcstepmul` value to reproduce the issue. So, since the root
> +-- issue is the same and now rehashing of finalizer table is
> +-- omitted, we test only the first case.
> +test:plan(2)
> +
> +local allocinject = require('lj_1247_allocinject')
> +
> +local ffi = require('ffi')
> +ffi.cdef[[
> +  struct test {int a;};
> +]]
> +
> +local N_GC_STEPS = 100
> +local N_GC_FINALIZERS = 100
> +
> +local function empty() end
> +
> +-- Create a chunk like the following:
> +--[[
> +  local tostring = tostring
> +  local r = ...
> +  for _ = 1, 4 do
> +    r[1] = tostring(1)
> +    -- ...
> +    r[N_GCSTEPS] = tostring(N_GC_STEPS)
> +  end
> +--]]
> +local function create_chunk(n_steps)
> +  local chunk = 'local tostring = tostring\n'
> +  chunk = chunk .. ('local r = ...\n')
> +  chunk = chunk .. 'for _ = 1, 4 do\n'
> +  for i = 1, n_steps do
> +    chunk = chunk .. ('  r[%d] = tostring(%d)\n'):format(i, i)
> +  end
> +  chunk = chunk .. 'end\n'
> +  chunk = chunk .. 'return r\n'
> +  return chunk
> +end
> +
> +local function add_more_garbage(size)
> +  return ffi.new('char[?]', size)
> +end
> +
> +-- Helper to skip the atomic phase.
> +local function skip_atomic()
> +  local first_gc_called = false
> +  local function mark_fin() first_gc_called = true end
> +  jit.off(mark_fin)
> +  debug.getmetatable(newproxy(true)).__gc = mark_fin
> +
> +  -- Skip the atomic phase.
> +  jit.off()
> +  while not first_gc_called do collectgarbage('step') end
> +  jit.on()
> +end
> +
> +local function crash_on_trace_unwind_gc_setup()
> +  skip_atomic()
> +  collectgarbage('setstepmul', 1000)
> +  add_more_garbage(1024 * 1024)
> +end
> +
> +local f = assert(loadstring(create_chunk(N_GC_STEPS)))
> +
> +-- Create a really long trace.
> +jit.flush()
> +jit.opt.start('hotloop=2', 'maxirconst=5000', 'maxrecord=10000', 'maxsnap=1000',
> +              '-fold')
> +
> +-- luacheck: no unused
> +local gc_anchor = {}
> +local function anchor_finalizer(i)
> +  gc_anchor[i] = ffi.gc(ffi.new('struct test', i), empty)
> +end
> +
> +for i = 1, N_GC_FINALIZERS do
> +  anchor_finalizer(i)
> +end
> +
> +-- Record the trace first.
> +f({})
> +
> +-- The table for anchoring cdata objects.
> +local res_tab = {}
> +
> +collectgarbage()
> +collectgarbage()
> +collectgarbage('setpause', 0)
> +collectgarbage('setstepmul', 1)
> +
> +gc_anchor = nil
> +
> +crash_on_trace_unwind_gc_setup()
> +
> +-- OOM on every allocation (i.e., on finalizer table rehashing
> +-- too).
> +allocinject.enable()
> +
> +local r, err = pcall(f, res_tab)
> +
> +allocinject.disable()
> +
> +test:ok(not r, 'correct status')
> +test:like(err, 'not enough memory', 'correct error message')
> +
> +test:done(true)
> diff --git a/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace/CMakeLists.txt b/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace/CMakeLists.txt
> new file mode 100644
> index 00000000..c3742e45
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace/CMakeLists.txt
> @@ -0,0 +1 @@
> +BuildTestCLib(lj_1247_allocinject lj_1247_allocinject.c)
> diff --git a/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace/lj_1247_allocinject.c b/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace/lj_1247_allocinject.c
> new file mode 100644
> index 00000000..81aea60b
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1247-fin-tab-rehashing-on-trace/lj_1247_allocinject.c
> @@ -0,0 +1,49 @@
> +#include "lua.h"
> +#include "lauxlib.h"
> +
> +#undef NDEBUG
> +#include <assert.h>
> +
> +static lua_Alloc old_allocf = NULL;
> +static void *old_alloc_state = NULL;
> +
> +/* Function to be used instead of the default allocator. */
> +static void *allocf_with_injection(void *ud, void *ptr, size_t osize,
> +				   size_t nsize)
> +{
> +	/* Always OOM on allocation (not on realloc). */
> +	if (ptr == NULL)
> +		return NULL;
> +	else
> +		return old_allocf(ud, ptr, osize, nsize);
> +}
> +
> +static int enable(lua_State *L)
> +{
> +	assert(old_allocf == NULL);
> +	old_allocf = lua_getallocf(L, &old_alloc_state);
> +	lua_setallocf(L, allocf_with_injection, old_alloc_state);
> +	return 0;
> +}
> +
> +static int disable(lua_State *L)
> +{
> +	assert(old_allocf != NULL);
> +	assert(old_allocf != allocf_with_injection);
> +	lua_setallocf(L, old_allocf, old_alloc_state);
> +	old_allocf = NULL;
> +	old_alloc_state = NULL;
> +	return 0;
> +}
> +
> +static const struct luaL_Reg allocinject[] = {
> +	{"enable", enable},
> +	{"disable", disable},
> +	{NULL, NULL}
> +};
> +
> +LUA_API int luaopen_lj_1247_allocinject(lua_State *L)
> +{
> +	luaL_register(L, "lj_1247_allocinject", allocinject);
> +	return 1;
> +}

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