GNU/Weeb Mailing List <[email protected]>
 help / color / mirror / Atom feed
From: Alviro Iskandar Setiawan <[email protected]>
To: Ammar Faizi <[email protected]>
Cc: Alviro Iskandar Setiawan <[email protected]>,
	Irvan Malik Azantha <[email protected]>,
	GNU/Weeb Mailing List <[email protected]>
Subject: [RFC PATCH v1 2/3] core/thread: Fix undefined behavior in the C++ mutex implementation
Date: Sat, 11 Mar 2023 11:28:09 +0000	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

The current C++ mutex implementation is undefined behavior, specifically
in the cond_wait() function because std::unique_lock<std::mutex> is
constructed with std::defer_lock while the same thread has acquired the
lock. Also, right after that defer_lock, std::condition_variable calls
wait() with a unique lock, not in a locked state.

In such a situation, the correct construction is using std::adopt_lock.
However, using std::adopt_lock leads to another issue. The issue is the
lock will be released upon return in the cond_wait(). To solve the
problem, introduce a new helper function, __cond_wait() which will
release the lock with std::adopt_lock and then call it from cond_wait().
The cond_wait() then acquires the lock again before it returns. The
result is that we correctly fulfill the __must_hold() semantic while
conforming to the C++ mutex implementation.

Signed-off-by: Alviro Iskandar Setiawan <[email protected]>
 core/ | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/core/ b/core/
index 2ff81e5..54e06e7 100644
--- a/core/
+++ b/core/
@@ -181,13 +181,22 @@ int cond_init(struct cond_struct **c)
 	return 0;
-int cond_wait(struct cond_struct **c, struct mutex_struct **m)
+static int __cond_wait(struct cond_struct *c, struct mutex_struct *m)
-	std::unique_lock<std::mutex> lock((*m)->mutex, std::defer_lock);
-	(*c)->cond.wait(lock);
+	std::unique_lock<std::mutex> lock(m->mutex, std::adopt_lock);
+	c->cond.wait(lock);
 	return 0;
+int cond_wait(struct cond_struct **c, struct mutex_struct **m)
+	int ret;
+	ret = __cond_wait(*c, *m);
+	mutex_lock(m);
+	return ret;
 int cond_signal(struct cond_struct **c)
Alviro Iskandar Setiawan

  parent reply	other threads:[~2023-03-11 11:28 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-03-11 11:28 [RFC PATCH v1 0/3] Fix undefined behavior in the C++ mutex implementation Alviro Iskandar Setiawan
2023-03-11 11:28 ` [RFC PATCH v1 1/3] MAINTAINERS: Add myself as the thread maintainer Alviro Iskandar Setiawan
2023-03-11 11:28 ` Alviro Iskandar Setiawan [this message]
2023-03-11 11:28 ` [RFC PATCH v1 3/3] configure: Introduce `--cpp-thread` option Alviro Iskandar Setiawan
2023-03-12 19:40 ` [RFC PATCH v1 0/3] Fix undefined behavior in the C++ mutex implementation Ammar Faizi

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:

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

  git send-email \ \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox