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From: Yichen Zhao <zhaoyichen@google.com>
To: Marcel Holtmann <marcel@holtmann.org>
Cc: Yichen Zhao <zhaoyichen@google.com>,
	"Gustavo F. Padovan" <gustavo@padovan.org>,
	Johan Hedberg <johan.hedberg@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	linux-bluetooth@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Bluetooth: Fix l2cap_sock_teardown_cb race condition with bt_accept_dequeue
Date: Fri, 13 May 2016 14:00:30 -0700	[thread overview]
Message-ID: <1463173230-16159-1-git-send-email-zhaoyichen@google.com> (raw)
In-Reply-To: <45EE47FA-D319-4091-941A-C4005E32B572@holtmann.org>

Hi Marcel,

> so I am not big fan of the conditional locking in case of parent is set or not. Do you have a test case that reproduces the mentioned race. It would love to have that in tools/l2cap-tester or similar.

So far I could only reproduce the bug by repeatedly performing RFCOMM connections and resets. I'll try to implement something in rfcomm-tester or l2cap-tester.

Since this is a race condition, I'm not confident that I can help you reproduce the bug reliably on a different test setup. I'd appreciate it very much if you can offer any tips on triggering a race condition faster in a test case.

> Maybe the code needs some restructuring to avoid the conditional locking.

I agree that my patch is not very elegant, and I'd love any way to improve it.
I have some ideas, but I'm not familiar enough with kernel development to know whether other solutions are safe to implement, such as:

* Removing bt_accept_unlink from l2cap_teardown_cb, and relying on bt_accept_dequeue to unlink the socket when it's enumerated. Is it safe to leave a zapped sock in accept_q?
* Perform "unlock sock; lock parent; lock sock" before calling bt_accept_unlink in teardown_cb. This is still conditional locking, but around a smaller block of code. Is it safe to unlock a zapped sock?
* Use RCU for handling accept_q. Is this appropriate?

Please let me know what you think.

Regards,

Yichen Zhao

      reply	other threads:[~2016-05-13 21:02 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-05-10  1:15 [PATCH] Bluetooth: Fix l2cap_sock_teardown_cb race condition with bt_accept_dequeue Yichen Zhao
2016-05-13 14:54 ` Marcel Holtmann
2016-05-13 21:00   ` Yichen Zhao [this message]

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