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From:   Qian Cai <cai@lca.pw>
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Subject: Kmemleak infrastructure improvement for task_struct leaks and
 call_rcu() 
Message-Id: <45D2D811-C3B0-442B-9744-415B4AC5CCDB@lca.pw>
Date:   Wed, 6 May 2020 12:22:37 -0400
Cc:     Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>
To:     Catalin Marinas <catalin.marinas@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
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=3D=3D task struck leaks =3D=3D
There are leaks from task struct from time to time where someone forgot =
to call put_task_struct() somewhere leading to leaks. For example,

=
https://lore.kernel.org/lkml/C1CCBDAC-A453-4FF2-908F-0B6E356223D1@lca.pw/

It was such a pain to debug this kind of leaks at the moment, as all we =
could do was to audit the code by checking all new put_task_struct()  =
and get_task_struct() call sites which is error-prone because there =
could be other new call sites like get_pid_task() which would also need =
to be balanced with put_task_struct() as well.

What do you think about adding some aux call traces for kmemleak in =
general? For example, if the tracking object is a task struct, it would =
save call traces for the first and last call of both get_task_struct() =
and put_task_struct(). Then, it could be expanded to track other =
refcount-based leaks in the future.

=3D=3D call_rcu() leaks =3D=3D
Another issue that might be relevant is that it seems sometimes, =
kmemleak will give a lot of false positives (hundreds) because the =
memory was supposed to be freed by call_rcu()  (for example, in =
dst_release()) but for some reasons, it takes a long time probably =
waiting for grace periods or some kind of RCU self-stall, but the memory =
had already became an orphan. I am not sure how we are going to resolve =
this properly until we have to figure out why call_rcu() is taking so =
long to finish?

Another solution is to add aux call traces for both skb_dst_drop() and =
skb_dst_set() for this case, but that there are many places to free =
memory via call_rcu() like inode free etc.=