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From: "Billie Alsup (balsup)" <balsup@cisco.com>
To: Muni Sekhar <munisekharrms@gmail.com>
CC: 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: AQHaoiVG0oAXLW6sfEGq1UjTh9JUCbGPJhuAgAAFYz0=
Date: Thu, 9 May 2024 17:57:44 +0000
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>From:=A0Muni Sekhar <munisekharrms@gmail.com>=0A=
>In the scenario where an interrupt occurs while we are servicing the=0A=
>interrupt, and in the scenario where it doesn't occur while we are=0A=
>servicing the interrupt, when should we use the=0A=
>spin_lock_irqsave/spin_unlock_irqrestore APIs?=0A=
=0A=
In my experience, the interrupts are masked by the infrastructure before in=
voking the=0A=
interrupt service routine. =A0So unless you explicitly re-enable them, ther=
e shouldn't be=0A=
a nested interrupt for the same interrupt number.=0A=
=0A=
It is the code run at process context that must be protected using the irqs=
ave/irqrestore=0A=
versions. =A0You want to not only enter the critical section, but also prev=
ent=0A=
the interrupt from occurring (on the same cpu at least). =A0If you enter th=
e critical section in=0A=
process context, but then take an interrupt and attempt to again enter the=
=0A=
critical section, then your interrupt routine will deadlock. the interrupt =
routine will never=A0=0A=
be able to acquire the lock, and the process context code that was interrup=
ted will never be=0A=
able to complete to release the lock. =A0So the process context code requir=
es the=0A=
irqsave/irqrestore variant to not only take the lock, but also prevent a co=
mpeting interrupt=0A=
routine from being triggered while you hold the lock.=0A=
=0A=
Bottom line is that if a critical section can be entered via both process c=
ontext=0A=
and interrupt context, then the process context invocation should use the i=
rqsave/irqrestore=0A=
variants to disable the interrupt before taking the lock.  If it is common =
code shared between=0A=
process context and interrupt context, then there is no harm in calling the=
 irqsave/irqrestore=0A=
version from both contexts.=0A=
=0A=
Otherwise, the standard spin_lock/spin_unlock variants (without irqsave/irq=
restore) would be=0A=
used for a critical section shared by multiple threads (different cpus), or=
 when your code has=0A=
already (separately) handled disabling interrupts as needed before invoking=
 spin_lock.=0A=
=0A=
=0A=