[Tarantool-patches] [PATCH luajit] core: fix cdata decrementing

[Sergey Kaplun]

Hi, Igor!

Thanks for the review!

Added your tag:
| Reviewed-by: Igor Munkin <imun at tarantool.org>

On 27.02.21, Igor Munkin wrote:
> Sergey,
> 
> Thanks for the patch, nice catch! I guess we missed this bug since the
> tests do not cover the case for GCcdata object with a custom finalizer.
> But these are frequently used in Tarantool though. The changes LGTM,
> except several nits below.
> 
> On 16.02.21, Sergey Kaplun wrote:
> > When cdata has custom finalizer (and so LJ_GC_CDATA_FIN flag) it is not
> > collected immediately, when lj_cdata_free() is called. Instead, it is
> > resurrected and marked finalized, so it is collected at the next GC
> 
> Minor: AFAIR, the payload is released (or finalized in LuaJIT terms) at
> the current GC cycle. Hence, the GCobject is marked finalized at this
> cycle too, but it is processed once more at the next one to be released.

Yes, as you said below: it is made white (is resurrected) in sweep phase
and finalized additional payload in GCfinalize phase. The cdata
object is collected at the next GC cycle.

I haven't change commit message since it means the same. Please
correct me if I'm missing something. You may update this commit
message in way what you want during cherry picking.

> 
> > cycle. The reason of the bug is that gc_cdatanum is decremented when
> > cdata is resurrected too (i.e. twice).
> > 
> > This patch excludes cdata decrementing from resurrection branch and
> > adds corresponding tests.
> > 
> > Resolves tarantool/tarantool#5820
> > Follows up tarantool/tarantool#5187
> > ---
> > 
> > Branch: https://github.com/tarantool/luajit/tree/skaplun/gh-5820-improperly-cdata-counting
> > Test Branch: https://github.com/tarantool/tarantool/tree/skaplun/gh-5820-improperly-cdata-counting
> > Issue: https://github.com/tarantool/tarantool/issues/5820
> > 
> > ChangeLog entry for bumping LuaJIT:
> > ===================================================================
> > ## bugfix/LuaJIT
> > 
> > * Fix double cdata decrementing in platform metrics when finalizer is set (gh-5820).
> > ===================================================================
> > 
> >  src/lj_cdata.c                                |  3 +-
> >  test/misclib-getmetrics-capi.test.lua         | 15 +++++++++-
> >  test/misclib-getmetrics-capi/testgetmetrics.c | 28 +++++++++++++++++++
> >  test/misclib-getmetrics-lapi.test.lua         | 15 +++++++++-
> >  4 files changed, 58 insertions(+), 3 deletions(-)
> 
> <snipped>
> 
> > diff --git a/test/misclib-getmetrics-capi.test.lua b/test/misclib-getmetrics-capi.test.lua
> > index 1ad6958..e088c48 100755
> > --- a/test/misclib-getmetrics-capi.test.lua
> > +++ b/test/misclib-getmetrics-capi.test.lua
> 
> <snipped>
> 
> > @@ -62,6 +62,19 @@ test:ok(testgetmetrics.objcount(function(iterations)
> >      jit.opt.start(unpack(jit_opt_default))
> >  end))
> >  
> > +test:ok(testgetmetrics.objcount_cdata_decrement(function()
> > +    -- cdata decrement test.
> 
> Minor: The metric name is 'gc_cdatanum', not 'cdata'.
> 
> > +    -- See https://github.com/tarantool/tarantool/issues/5820.
> > +    local ffi = require("ffi")
> > +    local function nop() end
> > +    ffi.gc(ffi.cast("void *", 0), nop)
> > +    -- Does not collect cdata, but removes LJ_GC_CDATA_FIN flag
> > +    -- and resurrects object.
> 
> Typo: s/resurrects object/resurrects the object/.
> 
> Minor: Strictly saying, the order is a bit different: GC resurrects the
> object first at GCSsweep phase, and removes LJ_GC_CDATA_FIN right before
> invoking the finalizer at GCSfinalize phase.

Fixed, branch is force pushed. See the itterative patch below.
===================================================================
diff --git a/test/misclib-getmetrics-capi.test.lua b/test/misclib-getmetrics-capi.test.lua
index e088c48..5c0e659 100755
--- a/test/misclib-getmetrics-capi.test.lua
+++ b/test/misclib-getmetrics-capi.test.lua
@@ -63,15 +63,15 @@ test:ok(testgetmetrics.objcount(function(iterations)
 end))
 
 test:ok(testgetmetrics.objcount_cdata_decrement(function()
-    -- cdata decrement test.
+    -- gc_cdatanum decrement test.
     -- See https://github.com/tarantool/tarantool/issues/5820.
     local ffi = require("ffi")
     local function nop() end
     ffi.gc(ffi.cast("void *", 0), nop)
-    -- Does not collect cdata, but removes LJ_GC_CDATA_FIN flag
-    -- and resurrects object.
+    -- Does not collect the cdata, but resurrects the object and
+    -- removes LJ_GC_CDATA_FIN flag.
     collectgarbage()
-    -- Collects cdata.
+    -- Collects the cdata.
     collectgarbage()
 end))
 
diff --git a/test/misclib-getmetrics-lapi.test.lua b/test/misclib-getmetrics-lapi.test.lua
index 59bcea6..0d054d3 100755
--- a/test/misclib-getmetrics-lapi.test.lua
+++ b/test/misclib-getmetrics-lapi.test.lua
@@ -231,15 +231,15 @@ test:test("objcount", function(subtest)
     subtest:is(new_metrics.gc_cdatanum, old_metrics.gc_cdatanum,
                "cdatanum don't change")
 
-    -- cdata decrement test.
+    -- gc_cdatanum decrement test.
     -- See https://github.com/tarantool/tarantool/issues/5820.
     local function nop() end
     local cdatanum_old = misc.getmetrics().gc_cdatanum
     ffi.gc(ffi.cast("void *", 0), nop)
-    -- Does not collect cdata, but removes LJ_GC_CDATA_FIN flag
-    -- and resurrects object.
+    -- Does not collect the cdata, but resurrects the object and
+    -- removes LJ_GC_CDATA_FIN flag.
     collectgarbage()
-    -- Collects cdata.
+    -- Collects the cdata.
     collectgarbage()
     subtest:is(misc.getmetrics().gc_cdatanum, cdatanum_old,
                "cdatanum is decremented correctly")
===================================================================

> > @@ -231,6 +231,19 @@ test:test("objcount", function(subtest)
> >      subtest:is(new_metrics.gc_cdatanum, old_metrics.gc_cdatanum,
> >                 "cdatanum don't change")
> >  
> > +    -- cdata decrement test.
> > +    -- See https://github.com/tarantool/tarantool/issues/5820.
> > +    local function nop() end
> > +    local cdatanum_old = misc.getmetrics().gc_cdatanum
> > +    ffi.gc(ffi.cast("void *", 0), nop)
> > +    -- Does not collect cdata, but removes LJ_GC_CDATA_FIN flag
> > +    -- and resurrects object.
> > +    collectgarbage()
> 
> Side note: It's worth to check whether Tarantool internals do not spoil
> the counters the same way they do after making LuaJIT testing machinery
> self-sufficient. There is <collectgarbage> call at the beginning of the
> test, but I haven't tested your patch against my series.

I've checked it at the new branch version:
https://github.com/tarantool/tarantool/tree/skaplun/gh-5820-improperly-cdata-counting
Looks OK.

> 
> > +    -- Collects cdata.
> > +    collectgarbage()
> > +    subtest:is(misc.getmetrics().gc_cdatanum, cdatanum_old,
> > +               "cdatanum is decremented correctly")
> 
> Minor: The metric name is 'gc_cdatanum', not 'cdatanum'.
> 
> > +
> >      -- Restore default jit settings.
> >      jit.opt.start(unpack(jit_opt_default))
> >  end)
> > -- 
> > 2.28.0
> > 
> 
> -- 
> Best regards,
> IM

-- 
Best regards,
Sergey Kaplun