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 Tue, 22 Dec 2020 20:48:43 +0300 (MSK)
Date: Tue, 22 Dec 2020 20:47:57 +0300
From: Sergey Kaplun <skaplun@tarantool.org>
Message-ID: <20201222174757.GA9101@root>
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In-Reply-To: <60416D4E-B738-4E6D-A9DB-7D3E678E12C1@tarantool.org>
Subject: Re: [Tarantool-patches] [PATCH] lua: avoid panic if HOOK_GC is not
	active
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To: Sergey Ostanevich <sergos@tarantool.org>
Cc: Michael Filonenko <filonenko.mikhail@gmail.com>, tarantool-patches@dev.tarantool.org

Hi, Sergos!

Thanks for the review!

On 22.12.20, Sergey Ostanevich wrote:
> Hi!
> 
> Thanks for the patch!
> 
> Sergos
> 
> > On 22 Dec 2020, at 16:08, Sergey Kaplun <skaplun@tarantool.org> wrote:
> > 
> > Panic at fiber.yield() occures inside any active hook.
> > 
> > This is the regression from 96dbc49d097a96af5273cce2b5663db5917f4ea9
>   ^^^^^^^^^^ It is a     ^^^ caused by

Fixed. Thanks!

> 
> > ('lua: prohibit fiber yield when GC hook is active').

<snipped>

> > --- a/src/lua/utils.c
> > +++ b/src/lua/utils.c
> > @@ -1375,7 +1375,7 @@ void cord_on_yield(void)
> > 	 * earlier. As a result fiber switch is prohibited when
> > 	 * GC hook is active and the platform is forced to stop.
> > 	 */
> > -	if (unlikely(g->hookmask & (HOOK_ACTIVE|HOOK_GC))) {
> > +	if (unlikely(g->hookmask & HOOK_GC)) {
> 
> It can happens the GC hook is not active? Should we have a second test for HOOK_ACTIVE?

No, it can't. HOOK_GC always is set with HOOK_ACTIVE inside
`gc_call_finalizer()` (see <lj_gc.c>). Old hook is restored after
finalizer's call.

> 
> > 		struct lua_State *L = fiber()->storage.lua.stack;

<snipped>

> 

I've updated commit message and comments in code considering your and
Igor comments. Branch is force-pushed.

New commit message is the follows:
| lua: avoid panic if HOOK_GC is not an active hook
|
| Platform panic occurs when fiber.yield() is used within any active
| (i.e. being executed now) hook.
|
| It is a regression caused by 96dbc49d097a96af5273cce2b5663db5917f4ea9
| ('lua: prohibit fiber yield when GC hook is active').
|
| This patch fixes false positive panic in cases when VM is not running
| a GC hook.
|
| Relates to #4518
| Closes #5649
|
| Reported-by: Michael Filonenko <filonenko.mikhail@gmail.com>

-- 
Best regards,
Sergey Kaplun