Re: [PATCH 3/4] http.c: avoid danging pointer to local variable `finished`

From: Junio C Hamano <gitster@pobox.com>
Date: May 24, 2022, 20:06 UTC
Message-ID: <xmqq35gyhf11.fsf@gitster.g>
In-Reply-To: <220524.86fskzxsvq.gmgdl@evledraar.gmail.com>

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

Show 13 quoted lines

> On Tue, May 24 2022, Johannes Schindelin via GitGitGadget wrote:
>
>> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>> [...]
>> Let's drop that local variable and introduce a new flag in the slot that
>> is used to indicate that even while the slot is no longer in use, it is
>> still reserved until further notice. It is the responsibility of
>> `run_active_slot()` to clear that flag once it is done with that slot.
>>
>> Initial-patch-by: Junio C Hamano <gitster@pobox.com>
>
> Don't you mean by me?
> I.e. https://lore.kernel.org/git/patch-1.1-1cec367e805-20220126T212921Z-avarab@gmail.com/

Most likely, but this version is so distant from the "clear slot->finished before leaving run_active_slot()" Dscho and I were recently discussing, that I do not think it can be said to have been derived from that one. This is completely a different patch that makes different changes.

The "clear slot->finished", by the way, is what I think is the right thing to do, especially that the objective is to squelch the false positive warning from a new compiler. If there is a way to annotate the line for the compiler to tell it not to warn about it, that would have been even better.

> This seems to be derived from that, or perhaps you just came up with
> something similar independently. Junio then came up with the smaller
> https://lore.kernel.org/git/xmqq8rv2nggn.fsf@gitster.g/

I actually do not think so. Yours is revert of the existing fix the compiler is confused about, and I have a feeling that if the original fix is still relevant, the problem the original fix wanted to address will resurface as a regression.

If I am reading the patch correctly, Dscho's is to avoid [*] reusing a slot while any run_active_slot() is still waiting for its completion. The approach would solve the problem the original fix wanted to solve in a different way. Personally I do not think such a surgery is necessary only to squelch false positives from a new warning compiler, though.

[Footnote]

 * I said "is to avoid", not "avoids", because I haven't studied the
   patch with sufficient degree of carefulness to say for sure, even
   though I can see that is the intent.