From: Brandon Williams <bmwill@google.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: git@vger.kernel.org, sbeller@google.com, pclouds@gmail.com
Subject: Re: [PATCH v2 25/27] attr: store attribute stack in attr_check structure
Date: Wed, 25 Jan 2017 15:19:36 -0800 [thread overview]
Message-ID: <20170125231936.GC83343@google.com> (raw)
In-Reply-To: <20170125215426.GB83343@google.com>
On 01/25, Brandon Williams wrote:
> On 01/25, Junio C Hamano wrote:
> > Brandon Williams <bmwill@google.com> writes:
> >
> > >> In my mind, the value of having a constant check_attr is primarily
> > >> that it gives us a stable pointer to serve as a hashmap key,
> > >> i.e. the identifier for each call site, in a later iteration.
> > >
> > > We didn't really discuss this notion of having the pointer be a key into
> > > a hashmap, what sort of information are you envisioning being stored in
> > > this sort of hashmap?
> >
> > The "entries relevant to this attr_check() call, that is specific to
> > the <check_attr instance, the thread> tuple" (aka "what used to be
> > called the global attr_stack") we discussed would be the primary
> > example. A thread is likely be looping in a caller that has many
> > paths inside a directory, calling a function that has a call to
> > attr_check() for each path. Having something that can use to
> > identify the check_attr instance in a stable way, even when the
> > inner function is called and returns many times, would allow us to
> > populate the "attr stack" just once for the thread when it enters a
> > directory for the first time (remember, another thread may be
> > executing the same codepath, checking for paths in a different
> > directory) and keep using it. There may be other mechanisms you can
> > come up with, so I wouldn't say it is the only valid way, but it is
> > a way. That is why I said:
> >
> > >> But all of the above comes from my intuition, and I'll very much
> > >> welcome to be proven wrong with an alternative design, or better
> > >> yet, a working code based on an alternative design ;-).
> >
> > near the end of my message.
> >
> > > One issue I can see with this is that the
> > > functions which have a static attr_check struct would still not be thread
> > > safe if the initialization of the structure isn't surrounded by a mutex
> > > itself. ie
> >
> > Yes, that goes without saying. That is why I suggested Stefan to do
> > not this:
> >
> > > static struct attr_check *check;
> > >
> > > if (!check)
> > > init(check);
> > >
> > > would need to be:
> > >
> > > lock()
> > > if (!check)
> > > init(check);
> > > unlock();
> >
> > but this:
> >
> > static struct attr_check *check;
> > init(&check);
> >
> > and hide the lock/unlock gymnastics inside the API. I thought that
> > already was in what you inherited from him and started your work
> > on top of?
>
> I essentially built off of the series you had while using Stefan's
> patches as inspiration, but I don't believe the kind of mechanism you
> are describing existed in Stefan's series. His series had a single lock
> for the entire system, only allowing a single caller to be in it at any
> given time. This definitely isn't ideal, hence why I picked it up.
>
> Implementation aside I want to try and nail down what the purpose of
> this refactor is. There are roughly two notions of being "thread-safe".
>
> 1. The first is that the subsystem itself is thread safe, that is
> multiple threads can be executing inside the subsystem without stepping
> on each others work.
>
> 2. The second is that the object itself is thread safe or that multiple
> threads can use the same object.
>
> I thought that the main purpose of this was to achieve (1) since
> currently that is not the case.
Ok, so I discovered a very good reason why we should do as Stefan
originally did and split the question and answer (beyond the reasoning
for using the reference as a hashkey).
One motivation behind making this API thread-safe is that we can use it
in pathspec code to match against attributes. This means that a
pathspec structure will contain an attr_check member describing the
attributes that a pathspec item is interested in. Then the pathspec
structure is passed to match_pathspec() as a const pointer. To me, when
passing something as 'const' I expect none of the members should change
at all. The struct should remain exactly in the same form as before I
invoked the function.
Requiring the attr_check structure to be modified in the process of a
check_attrs() call would violate this "contract" when calling
match_pathspec() as the attr_check structure would have modified state.
The compiler wouldn't catch this as the "const" modifier isn't passed on
to struct members.
--
Brandon Williams
next prev parent reply other threads:[~2017-01-25 23:19 UTC|newest]
Thread overview: 111+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-12 23:53 [PATCH 00/27] Revamp the attribute system; another round Brandon Williams
2017-01-12 23:53 ` [PATCH 01/27] commit.c: use strchrnul() to scan for one line Brandon Williams
2017-01-12 23:53 ` [PATCH 02/27] attr.c: " Brandon Williams
2017-01-12 23:53 ` [PATCH 03/27] attr.c: update a stale comment on "struct match_attr" Brandon Williams
2017-01-12 23:53 ` [PATCH 04/27] attr.c: explain the lack of attr-name syntax check in parse_attr() Brandon Williams
2017-01-12 23:53 ` [PATCH 05/27] attr.c: complete a sentence in a comment Brandon Williams
2017-01-12 23:53 ` [PATCH 06/27] attr.c: mark where #if DEBUG ends more clearly Brandon Williams
2017-01-12 23:53 ` [PATCH 07/27] attr.c: simplify macroexpand_one() Brandon Williams
2017-01-12 23:53 ` [PATCH 08/27] attr.c: tighten constness around "git_attr" structure Brandon Williams
2017-01-12 23:53 ` [PATCH 09/27] attr.c: plug small leak in parse_attr_line() Brandon Williams
2017-01-12 23:53 ` [PATCH 10/27] attr: support quoting pathname patterns in C style Brandon Williams
2017-01-12 23:53 ` [PATCH 11/27] attr.c: add push_stack() helper Brandon Williams
2017-01-12 23:53 ` [PATCH 12/27] Documentation: fix a typo Brandon Williams
2017-01-12 23:53 ` [PATCH 13/27] attr.c: outline the future plans by heavily commenting Brandon Williams
2017-01-12 23:53 ` [PATCH 14/27] attr: rename function and struct related to checking attributes Brandon Williams
2017-01-12 23:53 ` [PATCH 15/27] attr: (re)introduce git_check_attr() and struct attr_check Brandon Williams
2017-01-12 23:53 ` [PATCH 16/27] attr: convert git_all_attrs() to use "struct attr_check" Brandon Williams
2017-01-12 23:53 ` [PATCH 17/27] attr: convert git_check_attrs() callers to use the new API Brandon Williams
2017-01-12 23:53 ` [PATCH 18/27] attr: retire git_check_attrs() API Brandon Williams
2017-01-12 23:53 ` [PATCH 19/27] attr: pass struct attr_check to collect_some_attrs Brandon Williams
2017-01-12 23:53 ` [PATCH 20/27] attr: change validity check for attribute names to use positive logic Brandon Williams
2017-01-12 23:53 ` [PATCH 21/27] attr: use hashmap for attribute dictionary Brandon Williams
2017-01-18 20:20 ` Stefan Beller
2017-01-18 20:23 ` Brandon Williams
2017-01-12 23:53 ` [PATCH 22/27] attr: eliminate global check_all_attr array Brandon Williams
2017-01-12 23:53 ` [PATCH 23/27] attr: remove maybe-real, maybe-macro from git_attr Brandon Williams
2017-01-12 23:53 ` [PATCH 24/27] attr: tighten const correctness with git_attr and match_attr Brandon Williams
2017-01-12 23:53 ` [PATCH 25/27] attr: store attribute stacks in hashmap Brandon Williams
2017-01-13 21:20 ` Junio C Hamano
2017-01-18 20:34 ` Brandon Williams
2017-01-23 18:08 ` Brandon Williams
2017-01-18 20:39 ` Stefan Beller
2017-01-18 20:45 ` Stefan Beller
2017-01-18 20:50 ` Brandon Williams
2017-01-12 23:53 ` [PATCH 26/27] attr: push the bare repo check into read_attr() Brandon Williams
2017-01-12 23:53 ` [PATCH 27/27] attr: reformat git_attr_set_direction() function Brandon Williams
2017-01-15 23:47 ` [PATCH 00/27] Revamp the attribute system; another round Junio C Hamano
2017-01-16 8:10 ` Jeff King
2017-01-23 20:34 ` [PATCH v2 " Brandon Williams
2017-01-23 20:34 ` [PATCH v2 01/27] commit.c: use strchrnul() to scan for one line Brandon Williams
2017-01-23 20:35 ` [PATCH v2 02/27] attr.c: " Brandon Williams
2017-01-23 20:35 ` [PATCH v2 03/27] attr.c: update a stale comment on "struct match_attr" Brandon Williams
2017-01-23 20:35 ` [PATCH v2 04/27] attr.c: explain the lack of attr-name syntax check in parse_attr() Brandon Williams
2017-01-23 20:35 ` [PATCH v2 05/27] attr.c: complete a sentence in a comment Brandon Williams
2017-01-23 20:35 ` [PATCH v2 06/27] attr.c: mark where #if DEBUG ends more clearly Brandon Williams
2017-01-23 20:35 ` [PATCH v2 07/27] attr.c: simplify macroexpand_one() Brandon Williams
2017-01-23 20:35 ` [PATCH v2 08/27] attr.c: tighten constness around "git_attr" structure Brandon Williams
2017-01-23 20:35 ` [PATCH v2 09/27] attr.c: plug small leak in parse_attr_line() Brandon Williams
2017-01-23 20:35 ` [PATCH v2 10/27] attr: support quoting pathname patterns in C style Brandon Williams
2017-01-23 20:35 ` [PATCH v2 11/27] attr.c: add push_stack() helper Brandon Williams
2017-01-23 20:35 ` [PATCH v2 12/27] Documentation: fix a typo Brandon Williams
2017-01-23 20:35 ` [PATCH v2 13/27] attr.c: outline the future plans by heavily commenting Brandon Williams
2017-01-23 20:35 ` [PATCH v2 14/27] attr: rename function and struct related to checking attributes Brandon Williams
2017-01-23 20:35 ` [PATCH v2 15/27] attr: (re)introduce git_check_attr() and struct attr_check Brandon Williams
2017-01-23 20:35 ` [PATCH v2 16/27] attr: convert git_all_attrs() to use "struct attr_check" Brandon Williams
2017-01-23 20:35 ` [PATCH v2 17/27] attr: convert git_check_attrs() callers to use the new API Brandon Williams
2017-01-23 20:35 ` [PATCH v2 18/27] attr: retire git_check_attrs() API Brandon Williams
2017-01-23 20:35 ` [PATCH v2 19/27] attr: pass struct attr_check to collect_some_attrs Brandon Williams
2017-01-23 20:35 ` [PATCH v2 20/27] attr: change validity check for attribute names to use positive logic Brandon Williams
2017-01-23 20:35 ` [PATCH v2 21/27] attr: use hashmap for attribute dictionary Brandon Williams
2017-01-23 20:35 ` [PATCH v2 22/27] attr: eliminate global check_all_attr array Brandon Williams
2017-01-23 21:11 ` Junio C Hamano
2017-01-23 20:35 ` [PATCH v2 23/27] attr: remove maybe-real, maybe-macro from git_attr Brandon Williams
2017-01-23 20:35 ` [PATCH v2 24/27] attr: tighten const correctness with git_attr and match_attr Brandon Williams
2017-01-23 20:35 ` [PATCH v2 25/27] attr: store attribute stack in attr_check structure Brandon Williams
2017-01-23 21:42 ` Junio C Hamano
2017-01-23 22:06 ` Brandon Williams
2017-01-24 1:11 ` Brandon Williams
2017-01-24 2:28 ` Junio C Hamano
2017-01-25 19:57 ` Brandon Williams
2017-01-25 20:10 ` Stefan Beller
2017-01-25 20:14 ` Junio C Hamano
2017-01-25 21:54 ` Brandon Williams
2017-01-25 23:19 ` Brandon Williams [this message]
2017-01-23 20:35 ` [PATCH v2 26/27] attr: push the bare repo check into read_attr() Brandon Williams
2017-01-23 20:35 ` [PATCH v2 27/27] attr: reformat git_attr_set_direction() function Brandon Williams
2017-01-28 2:01 ` [PATCH v3 00/27] Revamp the attribute system; another round Brandon Williams
2017-01-28 2:01 ` [PATCH v3 01/27] commit.c: use strchrnul() to scan for one line Brandon Williams
2017-01-28 2:01 ` [PATCH v3 02/27] attr.c: " Brandon Williams
2017-01-28 2:01 ` [PATCH v3 03/27] attr.c: update a stale comment on "struct match_attr" Brandon Williams
2017-01-28 2:01 ` [PATCH v3 04/27] attr.c: explain the lack of attr-name syntax check in parse_attr() Brandon Williams
2017-01-28 2:01 ` [PATCH v3 05/27] attr.c: complete a sentence in a comment Brandon Williams
2017-01-28 2:01 ` [PATCH v3 06/27] attr.c: mark where #if DEBUG ends more clearly Brandon Williams
2017-01-28 2:01 ` [PATCH v3 07/27] attr.c: simplify macroexpand_one() Brandon Williams
2017-01-28 2:01 ` [PATCH v3 08/27] attr.c: tighten constness around "git_attr" structure Brandon Williams
2017-01-28 2:01 ` [PATCH v3 09/27] attr.c: plug small leak in parse_attr_line() Brandon Williams
2017-01-28 2:01 ` [PATCH v3 10/27] attr: support quoting pathname patterns in C style Brandon Williams
2017-01-28 2:01 ` [PATCH v3 11/27] attr.c: add push_stack() helper Brandon Williams
2017-01-28 2:01 ` [PATCH v3 12/27] Documentation: fix a typo Brandon Williams
2017-01-28 2:01 ` [PATCH v3 13/27] attr.c: outline the future plans by heavily commenting Brandon Williams
2017-01-28 2:01 ` [PATCH v3 14/27] attr: rename function and struct related to checking attributes Brandon Williams
2017-01-28 2:01 ` [PATCH v3 15/27] attr: (re)introduce git_check_attr() and struct attr_check Brandon Williams
2017-01-30 18:05 ` Brandon Williams
2017-01-28 2:01 ` [PATCH v3 16/27] attr: convert git_all_attrs() to use "struct attr_check" Brandon Williams
2017-01-28 23:50 ` Stefan Beller
2017-01-29 2:44 ` Brandon Williams
2017-01-30 18:06 ` Brandon Williams
2017-01-28 2:01 ` [PATCH v3 17/27] attr: convert git_check_attrs() callers to use the new API Brandon Williams
2017-01-28 2:01 ` [PATCH v3 18/27] attr: retire git_check_attrs() API Brandon Williams
2017-01-28 2:01 ` [PATCH v3 19/27] attr: pass struct attr_check to collect_some_attrs Brandon Williams
2017-01-28 2:02 ` [PATCH v3 20/27] attr: change validity check for attribute names to use positive logic Brandon Williams
2017-01-28 2:02 ` [PATCH v3 21/27] attr: use hashmap for attribute dictionary Brandon Williams
2017-01-28 2:02 ` [PATCH v3 22/27] attr: eliminate global check_all_attr array Brandon Williams
2017-01-28 2:02 ` [PATCH v3 23/27] attr: remove maybe-real, maybe-macro from git_attr Brandon Williams
2017-01-28 2:02 ` [PATCH v3 24/27] attr: tighten const correctness with git_attr and match_attr Brandon Williams
2017-01-28 2:02 ` [PATCH v3 25/27] attr: store attribute stack in attr_check structure Brandon Williams
2017-01-28 2:02 ` [PATCH v3 26/27] attr: push the bare repo check into read_attr() Brandon Williams
2017-01-28 2:02 ` [PATCH v3 27/27] attr: reformat git_attr_set_direction() function Brandon Williams
2017-02-02 19:14 ` [PATCH v3 00/27] Revamp the attribute system; another round Junio C Hamano
2017-02-09 17:18 ` Brandon Williams
2017-02-09 19:31 ` Junio C Hamano
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