linux-kernel - Re: [PATCH] staging: vc04_services: Fix RCU dereference check

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Message-ID: <a30849fa-dd7c-f661-0b27-a04c6531cfb8@i2se.com>
Date:   Wed, 29 Dec 2021 20:13:40 +0100
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Padmanabha Srinivasaiah <treasure4paddy@...il.com>,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc:     gregkh@...uxfoundation.org, nsaenz@...nel.org,
        Gaston Gonzalez <gascoar@...il.com>,
        Ojaswin Mujoo <ojaswin98@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Phil Elwell <phil@...pberrypi.com>,
        bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH] staging: vc04_services: Fix RCU dereference check

Hi Padmanabha,

Am 29.12.21 um 16:32 schrieb Padmanabha Srinivasaiah:
> In service_callback path RCU dereference done without
> rcu_read_[lock/unlock] pair, fixing same by using them.
>
> [   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@...il.com>
> ---
>  drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> index 6759a6261500..ee1b48db9681 100644
> --- a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> +++ b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c
> @@ -1058,7 +1058,10 @@ service_callback(enum vchiq_reason reason, struct vchiq_header *header,
>  
>  	DEBUG_TRACE(SERVICE_CALLBACK_LINE);
>  
> +	rcu_read_lock();
>  	service = handle_to_service(handle);
> +	rcu_read_unlock();
> +
>  	if (WARN_ON(!service))
>  		return VCHIQ_SUCCESS;
>  

thanks for reporting this issue.

Could you please explain how to reproduce this issue?

What makes you sure that your patch fixes the issue and not just hiding
the warning?

Best regards