git@vger.kernel.org list mirror (unofficial, one of many)
 help / color / mirror / code / Atom feed
From: Matheus Tavares <matheus.bernardino@usp.br>
To: git@vger.kernel.org
Cc: sandals@crustytoothpaste.net, j6t@kdbg.org,
	jonathantanmy@google.com, peff@peff.net,
	Johannes.Schindelin@gmx.de, Fredrik Kuivinen <frekui@gmail.com>
Subject: [PATCH 2/2] hex: make hash_to_hex_algop() and friends thread-safe
Date: Thu, 25 Jun 2020 17:32:57 -0300	[thread overview]
Message-ID: <0104cd9c763aee220a2df357834c79b10695ee35.1593115455.git.matheus.bernardino@usp.br> (raw)
In-Reply-To: <cover.1593115455.git.matheus.bernardino@usp.br>

hash_to_hex_algop() returns a static buffer, relieving callers from the
responsibility of freeing memory after use. But the current
implementation uses the same static data for all threads and, thus, is
not thread-safe. We could avoid using this function and its wrappers
in threaded code, but they are sometimes too deep in the call stack to
be noticed or even avoided.

For example, we can take a look at the number of oid_to_hex() calls,
which calls hash_to_hex_algop():

$ git grep 'oid_to_hex(' | wc -l
818

Although these functions don't seem to be causing problems out there for
now (at least not reported), making them thread-safe makes the codebase
more robust against race conditions. We can easily do that replicating
the static buffer in each thread's local storage.

Original-patch-by: Fredrik Kuivinen <frekui@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
---
 hex.c | 45 +++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 41 insertions(+), 4 deletions(-)

diff --git a/hex.c b/hex.c
index da51e64929..1094ed25bd 100644
--- a/hex.c
+++ b/hex.c
@@ -136,12 +136,49 @@ char *oid_to_hex_r(char *buffer, const struct object_id *oid)
 	return hash_to_hex_algop_r(buffer, oid->hash, the_hash_algo);
 }
 
+struct hexbuf_array {
+	int idx;
+	char bufs[4][GIT_MAX_HEXSZ + 1];
+};
+
+#ifdef HAVE_THREADS
+static pthread_key_t hexbuf_array_key;
+static pthread_once_t hexbuf_array_once = PTHREAD_ONCE_INIT;
+
+void init_hexbuf_array_key(void)
+{
+	if (pthread_key_create(&hexbuf_array_key, free))
+		die(_("failed to initialize threads' key for hash to hex conversion"));
+}
+
+#else
+static struct hexbuf_array default_hexbuf_array;
+#endif
+
 char *hash_to_hex_algop(const unsigned char *hash, const struct git_hash_algo *algop)
 {
-	static int bufno;
-	static char hexbuffer[4][GIT_MAX_HEXSZ + 1];
-	bufno = (bufno + 1) % ARRAY_SIZE(hexbuffer);
-	return hash_to_hex_algop_r(hexbuffer[bufno], hash, algop);
+	struct hexbuf_array *ha;
+
+#ifdef HAVE_THREADS
+	void *value;
+
+	if (pthread_once(&hexbuf_array_once, init_hexbuf_array_key))
+		die(_("failed to initialize threads' key for hash to hex conversion"));
+
+	value = pthread_key_getspecific(&hexbuf_array_key);
+	if (value) {
+		ha = (struct hexbuf_array *) value;
+	} else {
+		ha = xmalloc(sizeof(*ha));
+		if (pthread_key_setspecific(&hexbuf_array_key, (void *)ha))
+			die(_("failed to set thread buffer for hash to hex conversion"));
+	}
+#else
+	ha = &default_hexbuf_array;
+#endif
+
+	ha->idx = (ha->idx + 1) % ARRAY_SIZE(ha->bufs);
+	return hash_to_hex_algop_r(ha->bufs[ha->idx], hash, algop);
 }
 
 char *hash_to_hex(const unsigned char *hash)
-- 
2.26.2


  parent reply	other threads:[~2020-06-25 20:35 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-24 22:29 [RFC] Thread safety in some low-level functions Matheus Tavares Bernardino
2020-06-24 22:52 ` Matheus Tavares Bernardino
2020-06-25  1:38   ` brian m. carlson
2020-06-25 20:32     ` [PATCH 0/2] Make oid_to_hex() thread-safe Matheus Tavares
2020-06-25 20:32       ` [PATCH 1/2] compat/win32/pthread: add pthread_once() Matheus Tavares
2020-06-26  5:45         ` Christian Couder
2020-06-26 14:13           ` Matheus Tavares Bernardino
2020-06-25 20:32       ` Matheus Tavares [this message]
2020-06-25 23:07         ` [PATCH 2/2] hex: make hash_to_hex_algop() and friends thread-safe brian m. carlson
2020-06-25 23:54           ` Matheus Tavares Bernardino
2020-06-26  0:00             ` Matheus Tavares Bernardino
2020-06-26  6:02         ` Christian Couder
2020-06-26  8:22       ` [PATCH 0/2] Make oid_to_hex() thread-safe Christian Couder
2020-06-26 16:22         ` Matheus Tavares Bernardino
2020-06-26 21:54       ` [PATCH v2 " Matheus Tavares
2020-06-26 21:54         ` [PATCH v2 1/2] compat/win32/pthread: add pthread_once() Matheus Tavares
2020-06-26 21:54         ` [PATCH v2 2/2] hex: make hash_to_hex_algop() and friends thread-safe Matheus Tavares
2020-06-29 15:11           ` Johannes Schindelin
2020-06-30 20:37             ` Matheus Tavares Bernardino
2020-07-01 11:32               ` Johannes Schindelin
2020-07-16 11:29           ` Johannes Schindelin
2020-07-18  3:09             ` Matheus Tavares Bernardino
2020-08-10 14:15               ` Johannes Schindelin
2020-07-18  3:52             ` Matheus Tavares
2020-07-26 17:41               ` René Scharfe

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=0104cd9c763aee220a2df357834c79b10695ee35.1593115455.git.matheus.bernardino@usp.br \
    --to=matheus.bernardino@usp.br \
    --cc=Johannes.Schindelin@gmx.de \
    --cc=frekui@gmail.com \
    --cc=git@vger.kernel.org \
    --cc=j6t@kdbg.org \
    --cc=jonathantanmy@google.com \
    --cc=peff@peff.net \
    --cc=sandals@crustytoothpaste.net \
    --subject='Re: [PATCH 2/2] hex: make hash_to_hex_algop() and friends thread-safe' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Code repositories for project(s) associated with this inbox:

	https://80x24.org/mirrors/git.git

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).