netdev - Re: [PATCH] can: j1939: prevent deadlock by changing j1939_socks

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Date: Thu, 13 Jul 2023 22:58:42 -0700
From: Astra Joan <astrajoan@...oo.com>
To: stephen@...workplumber.org
Cc: Astra Joan <astrajoan@...oo.com>,
 davem@...emloft.net,
 Dmitry Vyukov <dvyukov@...gle.com>,
 edumazet@...gle.com,
 ivan.orlov0322@...il.com,
 kernel@...gutronix.de,
 kuba@...nel.org,
 linux-can@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 linux@...pel-privat.de,
 mkl@...gutronix.de,
 netdev@...r.kernel.org,
 o.rempel@...gutronix.de,
 pabeni@...hat.com,
 robin@...tonic.nl,
 skhan@...uxfoundation.org,
 socketcan@...tkopp.net,
 syzbot+1591462f226d9cbf0564@...kaller.appspotmail.com,
 syzkaller-bugs@...glegroups.com,
 syzkaller@...glegroups.com
Subject: Re: [PATCH] can: j1939: prevent deadlock by changing j1939_socks_lock
 to rwlock

Hi Stephen,

Thank you for your reply!

Changing the lock sequence may be difficult here since the function

- j1939_sk_errqueue

has may call sites. However, this function attempts to hold the

- j1939_socks_lock

whose hierchy is pretty high in the given code logic. On the other hand,
this function only reads from the

- j1939_socks_list

which the above lock protects against. Therefore, it seems appropriate to
lock the above list inside the function for read access only.

The RCU approach makes sense here, but I probably need to use RCU in
conjunction with other spinlocks and rwlocks in the codebase. Would that
be okay? If not, should I be looking into replacing all the locks with
RCU? I'm actually a mentee for the bug fixing mentorship this summer.
So please bear with me if some of these questions seem a bit naive :D
Thank you so much for your time!

Best regards,
Ziqi