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From: "Billie Alsup (balsup)" <balsup@cisco.com>
To: Muni Sekhar <munisekharrms@gmail.com>,
        kernelnewbies
	<kernelnewbies@kernelnewbies.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: Seeking Assistance with Spin Lock Usage and Resolving Hard LOCKUP
 Error
Thread-Topic: Seeking Assistance with Spin Lock Usage and Resolving Hard
 LOCKUP Error
Thread-Index: AQHaoiVG0oAXLW6sfEGq1UjTh9JUCQ==
Date: Thu, 9 May 2024 15:26:34 +0000
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>From:=A0Muni Sekhar <munisekharrms@gmail.com>=0A=
=0A=
>Here is a brief overview of how I have implemented spin locks in my module=
:=0A=
>=0A=
>spinlock_t my_spinlock; // Declare a spin lock=0A=
>=0A=
>// In ISR context (interrupt handler):=0A=
>spin_lock_irqsave(&my_spinlock, flags);=0A=
>// ... Critical section ...=0A=
>spin_unlock_irqrestore(&my_spinlock, flags);=0A=
>=0A=
>=0A=
>// In process context: (struct file_operations.read)=0A=
>spin_lock(&my_spinlock);=0A=
>// ... Critical section ...=0A=
>spin_unlock(&my_spinlock);=0A=
=0A=
from my understanding, you have the usage backwards.  It is the irqsave/irq=
restore versions that should be used within process context to prevent the =
interrupt from being handled on the same cpu while executing in your critic=
al section.=0A=
=0A=
The use of irqsave/irqrestore within the isr itself is ok, although perhaps=
 unnecessary.  It depends on whether the interrupt can occur again while yo=
u are servicing the interrupt (whether on this cpu or another).  Usually (?=
) the same interrupt does not nest, unless you have explicitly coded to all=
ow it (for example, by acknowledging and re-enabling the interrupt early in=
 your ISR). Certainly the spinlock is necessary to protect the critical sec=
tion from running in an isr on one cpu and process space on another cpu.=0A=
=0A=
From a lockup perspective, not doing the irqsave/irqrestore from process co=
ntext could explain your problem. Also look for code (anywhere!) that blind=
ly enables interrupts, rather than doing irqrestore from a prior irqsave.=
=0A=