From: Matheus Tavares Bernardino <matheus.bernardino@usp.br>
To: Jeff King <peff@peff.net>
Cc: "Jonathan Tan" <jonathantanmy@google.com>,
git <git@vger.kernel.org>,
"Christian Couder" <christian.couder@gmail.com>,
"Оля Тележная" <olyatelezhnaya@gmail.com>,
"Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>,
"Junio C Hamano" <gitster@pobox.com>,
"Jonathan Nieder" <jrnieder@gmail.com>,
"Stefan Beller" <stefanbeller@gmail.com>
Subject: Re: [PATCH v2 05/11] object-store: allow threaded access to object reading
Date: Thu, 19 Dec 2019 19:27:42 -0300 [thread overview]
Message-ID: <CAHd-oW5qT5LmUd6GTL=O+-yXPmq5Uy9gk3ohL_2r+_K+6UJS3Q@mail.gmail.com> (raw)
In-Reply-To: <20191115041215.GB21654@sigill.intra.peff.net>
Hi, Peff and Jonathan
Sorry for the delay in re-rolling this series, it was a very busy end
of semester. But I finally completed my degree and had time to get
back to it :)
I tried the rwlock approach, but there were some subtle difficulties.
For example, we should hold the lock in write mode when free()-ing the
window, and thus, the lock couldn't be declared inside the struct
pack_window.
Also, it seemed that protecting the window reading at git_inflate()
wouldn't be enough: suppose a thread has just read from a window in
git_inflate() (holding the rwlock) and is now waiting for the
obj_read_mutex to continue its object reading operation. If another
thread (that already has the obj_read_mutex) acquires the rwlock in
sequence, it could free() the said window. It might not sound like a
problem since the first thread has already finished reading from it.
But since a pointer to the window would still be in the first thread's
stack as a window cursor, it could be later passed down to use_pack()
leading to a segfault. I couldn't come up with a solution for this
yet.
However, re-inspecting the code, it seemed to me that we might already
have a thread-safe mechanism. The window disposal operations (at
close_pack_windows() and unuse_one_window()) are only performed if
window.inuse_cnt == 0. So as a thread which reads from the window will
also previously increment its inuse_cnt, wouldn't the reading
operation be already protected?
Another concern would be close_pack_fd(), which can close packs even
with in-use windows. However, as the mmap docs[1] says: "closing the
file descriptor does not unmap the region".
Finally, we also considered reprepare_packed_git() as a possible
conflicting operation. But the function called by it to handle
packfile opening, prepare_pack(), won't reopen already available
packs. Therefore, IIUC, it will leave the opened windows intact.
So, aren't perhaps the window readings performed outside the
obj_read_mutex critical section already thread-safe?
Thanks,
Matheus
[1]: https://linux.die.net/man/2/mmap
next prev parent reply other threads:[~2019-12-19 22:27 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
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 [this message]
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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