From: Jeff King <peff@peff.net>
To: Jonathan Tan <jonathantanmy@google.com>
Cc: matheus.bernardino@usp.br, git@vger.kernel.org,
christian.couder@gmail.com, olyatelezhnaya@gmail.com,
pclouds@gmail.com, gitster@pobox.com, jrnieder@gmail.com,
stefanbeller@gmail.com
Subject: Re: [PATCH v2 05/11] object-store: allow threaded access to object reading
Date: Wed, 13 Nov 2019 00:20:44 -0500 [thread overview]
Message-ID: <20191113052044.GB3547@sigill.intra.peff.net> (raw)
In-Reply-To: <20191112025418.254880-1-jonathantanmy@google.com>
On Mon, Nov 11, 2019 at 06:54:18PM -0800, Jonathan Tan wrote:
> > @@ -1580,7 +1585,9 @@ static void *unpack_compressed_entry(struct packed_git *p,
> > do {
> > in = use_pack(p, w_curs, curpos, &stream.avail_in);
> > stream.next_in = in;
> > + obj_read_unlock();
> > st = git_inflate(&stream, Z_FINISH);
> > + obj_read_lock();
> > if (!stream.avail_out)
> > break; /* the payload is larger than it should be */
> > curpos += stream.next_in - in;
>
> As I see it, the main purpose of this patch set is to move the mutex
> guarding object reading from builtin/grep.c (grep_read_mutex) to
> object-store.h (obj_read_mutex), so that we can add "holes" (non-mutex
> sections) such as the one quoted above, in order that zlib inflation can
> happen outside the mutex.
>
> My concern is that the presence of these "holes" make object reading
> non-thread-safe, defeating the purpose of obj_read_mutex. In particular,
> the section quoted above assumes that the window section returned by
> use_pack() is still valid throughout the inflation, but that window
> could have been invalidated by things like an excess of windows open,
> reprepare_packed_git(), etc.
Yeah, I don't think the code above is safe. The map window can be
modified by other threads.
> I thought of this for a while but couldn't think of a good solution. If
> we introduced a reference counting mechanism into Git, that would allow
> us to hold the window open outside the mutex, but things like
> reprepare_packed_git() would still be difficult.
I think you could put a reader-writer lock into each window. The code
here would take the reader lock, and multiple readers could use it at
the same time. Any time the window needs to be shifted, resized, or
discarded, that code would take the writer lock, waiting for (and then
blocking out) any readers.
A pthread_rwlock would work, but it would be the first use in Git. I
think we'd need to find an equivalent for compat/win32/pthread.h.
-Peff
next prev parent reply other threads:[~2019-11-13 5:20 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-10 20:27 [GSoC][PATCH 0/4] grep: re-enable threads when cached, w/ parallel inflation Matheus Tavares
2019-08-10 20:27 ` [GSoC][PATCH 1/4] object-store: add lock to read_object_file_extended() Matheus Tavares
2019-08-10 20:27 ` [GSoC][PATCH 2/4] grep: allow locks to be enabled individually Matheus Tavares
2019-08-10 20:27 ` [GSoC][PATCH 3/4] grep: disable grep_read_mutex when possible Matheus Tavares
2019-08-10 20:27 ` [GSoC][PATCH 4/4] grep: re-enable threads in some non-worktree cases Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 00/11] grep: improve threading and fix race conditions Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 01/11] grep: fix race conditions on userdiff calls Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 02/11] grep: fix race conditions at grep_submodule() Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 03/11] grep: fix racy calls in grep_objects() Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 04/11] replace-object: make replace operations thread-safe Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 05/11] object-store: allow threaded access to object reading Matheus Tavares
2019-11-12 2:54 ` Jonathan Tan
2019-11-13 5:20 ` Jeff King [this message]
2019-11-14 5:57 ` Matheus Tavares Bernardino
2019-11-14 6:01 ` Jeff King
2019-11-14 18:15 ` Jonathan Tan
2019-11-15 4:12 ` Jeff King
2019-12-19 22:27 ` Matheus Tavares Bernardino
2020-01-09 22:02 ` Matheus Tavares Bernardino
2020-01-10 19:07 ` Christian Couder
2019-09-30 1:50 ` [PATCH v2 06/11] grep: replace grep_read_mutex by internal obj read lock Matheus Tavares
2019-10-01 19:23 ` [PATCH] squash! " Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 07/11] submodule-config: add skip_if_read option to repo_read_gitmodules() Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 08/11] grep: allow submodule functions to run in parallel Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 09/11] grep: protect packed_git [re-]initialization Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 10/11] grep: re-enable threads in non-worktree case Matheus Tavares
2019-09-30 1:50 ` [PATCH v2 11/11] grep: move driver pre-load out of critical section Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 00/12] grep: improve threading and fix race conditions Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 01/12] grep: fix race conditions on userdiff calls Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 02/12] grep: fix race conditions at grep_submodule() Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 03/12] grep: fix racy calls in grep_objects() Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 04/12] replace-object: make replace operations thread-safe Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 05/12] object-store: allow threaded access to object reading Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 06/12] grep: replace grep_read_mutex by internal obj read lock Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 07/12] submodule-config: add skip_if_read option to repo_read_gitmodules() Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 08/12] grep: allow submodule functions to run in parallel Matheus Tavares
2020-01-29 11:26 ` SZEDER Gábor
2020-01-29 18:49 ` Junio C Hamano
2020-01-29 18:57 ` Junio C Hamano
2020-01-29 20:42 ` Matheus Tavares Bernardino
2020-01-30 13:28 ` Philippe Blain
2020-01-16 2:39 ` [PATCH v3 09/12] grep: protect packed_git [re-]initialization Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 10/12] grep: re-enable threads in non-worktree case Matheus Tavares
2020-01-16 2:39 ` [PATCH v3 11/12] grep: move driver pre-load out of critical section Matheus Tavares
2020-01-16 2:40 ` [PATCH v3 12/12] grep: use no. of cores as the default no. of threads Matheus Tavares
2020-01-16 13:11 ` Victor Leschuk
2020-01-16 14:47 ` [PATCH] " Matheus Tavares
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