linux-kernel - Re: [PATCH v2 2/2] bus: mhi: host: Take irqsave lock after TRE is generated

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Date:   Mon, 25 Sep 2023 12:08:44 +0800
From:   Qiang Yu <quic_qianyu@...cinc.com>
To:     Jeffrey Hugo <quic_jhugo@...cinc.com>, <mani@...nel.org>
CC:     <mhi@...ts.linux.dev>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <quic_cang@...cinc.com>,
        <quic_mrana@...cinc.com>, Hemant Kumar <quic_hemantk@...cinc.com>,
        "Lazarus Motha" <quic_lmotha@...cinc.com>
Subject: Re: [PATCH v2 2/2] bus: mhi: host: Take irqsave lock after TRE is
 generated

On 9/22/2023 10:50 PM, Jeffrey Hugo wrote:
> On 9/13/2023 2:47 AM, Qiang Yu wrote:
>> From: Hemant Kumar <quic_hemantk@...cinc.com>
>>
>> Take irqsave lock after TRE is generated to avoid deadlock due to core
>> getting interrupts enabled as local_bh_enable must not be called with
>> irqs disabled based on upstream patch.
>
> Where is local_bh_enable() being called?  What patch?  What is 
> upstream of the codebase you submitted this to?  Why is it safe to 
> call mhi_gen_tre() without the lock?

This patch is to fix the issue included by  "[PATCH v2 1/2] bus: mhi: 
host: Add spinlock to protect WP access when queueing TREs". In that 
patch, we add write_lock_bh/write_unlock_bh in mhi_gen_tre().

However, before mhi_gen_tre() is invoked, mhi_cntrl->pm_lock is getted, 
line 1125, and it is a spin lock. So it becomes we want to get and 
release bh lock after spin lock.  __local_bh_enable_ip is called as part 
of write_unlock_bh

and local_bh_enable. When CONFIG_TRACE_IRQFLAGS is enabled, irq will be 
enabled once __local_bh_enable_ip is called. The commit message is not 
clear and confusing, will change it in [patch v3].