From: Junio C Hamano <gitster@pobox•com>
To: Michael Haggerty <mhagger@alum•mit.edu>
Cc: "Stefan Beller" <sbeller@google•com>,
"Ronnie Sahlberg" <ronniesahlberg@gmail•com>,
"Jonathan Nieder" <jrnieder@gmail•com>,
"Nguyễn Thái Ngọc" <pclouds@gmail•com>,
"git@vger•kernel.org" <git@vger•kernel.org>,
"Carlos Martín Nieto" <cmn@elego•de>
Subject: Re: [PATCH 8/8] reflog_expire(): lock symbolic refs themselves, not their referent
Date: Fri, 13 Feb 2015 10:05:25 -0800 [thread overview]
Message-ID: <xmqqoaoxoffe.fsf@gitster.dls.corp.google.com> (raw)
In-Reply-To: <54DE259C.4030001@alum.mit.edu> (Michael Haggerty's message of "Fri, 13 Feb 2015 17:26:04 +0100")
Michael Haggerty <mhagger@alum•mit.edu> writes:
> I also realized that Git's current policy is probably not tenable if one
> process is re-seating a symref at the same time as another process is
> expiring its reflog. The "git reflog expire HEAD" might grab
> "refs/heads/master.lock" then start rewriting "logs/HEAD". Meanwhile,
> "git checkout foo" would grab "HEAD.lock" (not being blocked by the
> "expire" process), then rewrite it to "ref: refs/heads/foo", then grab
> "refs/heads/foo.lock" before updating "logs/HEAD". So both processes
> could be writing "logs/HEAD" at the same time.
> ...
> Switching to holding only "HEAD.lock" while updating "logs/HEAD" is the
> right solution,...
We convinced ourselves that not locking the symref is wrong, but
have we actually convinced us that not locking the underlying ref,
as long as we have a lock on the symref, is safe?
To protect you, the holder of a lock on refs/remotes/origin/HEAD
that happens to point at refs/remotes/origin/next, from somebody who
is updating the underlying refs/remotes/origin/next directly without
going through the symbolic ref (e.g. receive-pack), wouldn't the
other party need to find any and all symbolic refs that point at the
underlying ref and take locks on them?
As dereferencing a symbolic ref in the forward direction is much
cheaper than in the reverse, and because you need to dereference it
anyway, I wonder if we want the endgame to be "hold locks on both",
not "just hold a lock on the symlink".
next prev parent reply other threads:[~2015-02-13 18:05 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-09 9:12 [PATCH 0/8] Fix some problems with reflog expiration Michael Haggerty
2015-02-09 9:12 ` [PATCH 1/8] write_ref_sha1(): remove check for lock == NULL Michael Haggerty
2015-02-10 22:52 ` Stefan Beller
2015-02-11 0:06 ` Jeff King
2015-02-09 9:12 ` [PATCH 2/8] write_ref_sha1(): Move write elision test to callers Michael Haggerty
2015-02-12 19:58 ` Junio C Hamano
2015-02-09 9:12 ` [PATCH 3/8] lock_ref_sha1_basic(): do not set force_write for missing references Michael Haggerty
2015-02-10 23:24 ` Stefan Beller
2015-02-11 0:05 ` Jeff King
2015-02-11 0:07 ` Stefan Beller
2015-02-12 12:09 ` Michael Haggerty
2015-02-09 9:12 ` [PATCH 4/8] reflog: fix documentation Michael Haggerty
2015-02-10 23:25 ` Stefan Beller
2015-02-09 9:12 ` [PATCH 5/8] reflog: rearrange the manpage Michael Haggerty
2015-02-10 23:42 ` Stefan Beller
2015-02-12 15:17 ` Michael Haggerty
2015-02-12 20:09 ` Junio C Hamano
2015-02-09 9:12 ` [PATCH 6/8] reflog_expire(): ignore --updateref for symbolic references Michael Haggerty
2015-02-11 0:44 ` Stefan Beller
2015-02-12 16:08 ` Michael Haggerty
2015-02-12 17:04 ` Stefan Beller
2015-02-12 20:16 ` Junio C Hamano
2015-02-12 21:54 ` Jeff King
2015-02-13 14:34 ` Michael Haggerty
2015-02-09 9:12 ` [PATCH 7/8] reflog_expire(): never update a reference to null_sha1 Michael Haggerty
2015-02-09 20:55 ` Eric Sunshine
2015-02-12 11:51 ` Michael Haggerty
2015-02-09 9:12 ` [PATCH 8/8] reflog_expire(): lock symbolic refs themselves, not their referent Michael Haggerty
2015-02-11 0:49 ` Stefan Beller
2015-02-11 22:49 ` Junio C Hamano
2015-02-11 23:25 ` Stefan Beller
2015-02-12 16:52 ` Michael Haggerty
2015-02-12 18:04 ` Stefan Beller
2015-02-13 16:26 ` Michael Haggerty
2015-02-13 17:16 ` Stefan Beller
2015-02-13 18:05 ` Junio C Hamano [this message]
2015-02-13 18:21 ` Stefan Beller
2015-02-13 18:26 ` Junio C Hamano
2015-02-13 18:32 ` Stefan Beller
2015-02-13 19:12 ` Junio C Hamano
2015-02-13 20:11 ` Michael Haggerty
2015-02-13 21:53 ` Junio C Hamano
2015-02-14 5:58 ` Michael Haggerty
2015-02-09 18:57 ` [PATCH 0/8] Fix some problems with reflog expiration Stefan Beller
2015-02-10 23:12 ` [PATCH] refs.c: get rid of force_write flag Stefan Beller
2015-02-12 15:35 ` Michael Haggerty
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