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Date:   Sat, 19 Jun 2021 13:14:42 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
To:     Bhaumik Bhatt <bbhatt@codeaurora.org>
Cc:     linux-arm-msm@vger.kernel.org, hemantk@codeaurora.org,
        jhugo@codeaurora.org, linux-kernel@vger.kernel.org,
        loic.poulain@linaro.org
Subject: Re: [PATCH] bus: mhi: core: Disable pre-emption for data events
 tasklet processing
Message-ID: <20210619074442.GC4889@workstation>
References: <1624054985-31365-1-git-send-email-bbhatt@codeaurora.org>
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On Fri, Jun 18, 2021 at 03:23:05PM -0700, Bhaumik Bhatt wrote:
> With spin_lock_bh(), it is possible that a tasklet processing data
> events gets scheduled out if another higher priority tasklet is
> ready to run. While the tasklet is sleeping, it can hold the event
> ring spinlock and block another tasklet, for example, one processing
> an M0 state change event, from acquiring it. This can starve the
> core which blocks in an attempt to acquire the spinlock until it
> gets released. Fix this issue by disabling pre-emption on the core
> processing data events and allow it to run to completion such that
> other tasklets do not block for long periods.
> 

Are you sure?

IIUC, the tasklets priority is only used while trying to schedule the
next pending tasklet. But I don't think a high priority tasklet can
preempt the low priority one.

Please correct me if I'm wrong.

Thanks,
Mani

> Fixes: 1d3173a3bae7 ("bus: mhi: core: Add support for processing events from client device")
> Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
> ---
>  drivers/bus/mhi/core/main.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/bus/mhi/core/main.c b/drivers/bus/mhi/core/main.c
> index 3775c77..02c8c09 100644
> --- a/drivers/bus/mhi/core/main.c
> +++ b/drivers/bus/mhi/core/main.c
> @@ -1036,11 +1036,12 @@ void mhi_ev_task(unsigned long data)
>  {
>  	struct mhi_event *mhi_event = (struct mhi_event *)data;
>  	struct mhi_controller *mhi_cntrl = mhi_event->mhi_cntrl;
> +	unsigned long flags;
>  
>  	/* process all pending events */
> -	spin_lock_bh(&mhi_event->lock);
> +	spin_lock_irqsave(&mhi_event->lock, flags);
>  	mhi_event->process_event(mhi_cntrl, mhi_event, U32_MAX);
> -	spin_unlock_bh(&mhi_event->lock);
> +	spin_unlock_irqrestore(&mhi_event->lock, flags);
>  }
>  
>  void mhi_ctrl_ev_task(unsigned long data)
> -- 
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
>