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Subject: Re: [PATCH v3] staging: vc04_services: Fix RCU dereference check
To:     Padmanabha Srinivasaiah <treasure4paddy@gmail.com>,
        linux-rpi-kernel@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org,
        linux-staging@lists.linux.dev, linux-kernel@vger.kernel.org
Cc:     gregkh@linuxfoundation.org, nsaenz@kernel.org,
        Gaston Gonzalez <gascoar@gmail.com>,
        Ojaswin Mujoo <ojaswin98@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Phil Elwell <phil@raspberrypi.com>,
        bcm-kernel-feedback-list@broadcom.com
References: <20211231185611.GA4463@pswork>
 <20211231195406.5479-1-treasure4paddy@gmail.com>
From:   Stefan Wahren <stefan.wahren@i2se.com>
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Message-ID: <7b0ce9d5-a278-3892-8d4a-905ca15e1edf@i2se.com>
Date:   Sun, 23 Jan 2022 18:32:15 +0100
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Am 31.12.21 um 20:54 schrieb Padmanabha Srinivasaiah:
> In service_callback path RCU dereferenced pointer struct vchiq_service
> need to be accessed inside rcu read-critical section.
>
> Also userdata/user_service part of vchiq_service is accessed around
> different synchronization mechanism, getting an extra reference to a
> pointer keeps sematics simpler and avoids prolonged graceperiod.
>
> Accessing vchiq_service with rcu_read_[lock/unlock] fixes below issue.
>
> [   32.201659] =============================
> [   32.201664] WARNING: suspicious RCU usage
> [   32.201670] 5.15.11-rt24-v8+ #3 Not tainted
> [   32.201680] -----------------------------
> [   32.201685] drivers/staging/vc04_services/interface/vchiq_arm/vchiq_core.h:529 suspicious rcu_dereference_check() usage!
> [   32.201695]
> [   32.201695] other info that might help us debug this:
> [   32.201695]
> [   32.201700]
> [   32.201700] rcu_scheduler_active = 2, debug_locks = 1
> [   32.201708] no locks held by vchiq-slot/0/98.
> [   32.201715]
> [   32.201715] stack backtrace:
> [   32.201723] CPU: 1 PID: 98 Comm: vchiq-slot/0 Not tainted 5.15.11-rt24-v8+ #3
> [   32.201733] Hardware name: Raspberry Pi 4 Model B Rev 1.4 (DT)
> [   32.201739] Call trace:
> [   32.201742]  dump_backtrace+0x0/0x1b8
> [   32.201772]  show_stack+0x20/0x30
> [   32.201784]  dump_stack_lvl+0x8c/0xb8
> [   32.201799]  dump_stack+0x18/0x34
> [   32.201808]  lockdep_rcu_suspicious+0xe4/0xf8
> [   32.201817]  service_callback+0x124/0x400
> [   32.201830]  slot_handler_func+0xf60/0x1e20
> [   32.201839]  kthread+0x19c/0x1a8
> [   32.201849]  ret_from_fork+0x10/0x20
>
> Signed-off-by: Padmanabha Srinivasaiah <treasure4paddy@gmail.com>

Tested-by: Stefan Wahren <stefan.wahren@i2se.com>

Thanks