linux-kernel - Re: [PATCH v9 6/7] iommu/arm-smmu-v3: Add arm_smmu_invs based arm_smmu_domain_inv

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Message-ID: <aXkF+FcsXG928Qfc@Asurada-Nvidia>
Date: Tue, 27 Jan 2026 10:37:44 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Pranjal Shrivastava <praan@...gle.com>, <will@...nel.org>,
	<jean-philippe@...aro.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<balbirs@...dia.com>, <miko.lenczewski@....com>, <peterz@...radead.org>,
	<kevin.tian@...el.com>, <linux-arm-kernel@...ts.infradead.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 6/7] iommu/arm-smmu-v3: Add arm_smmu_invs based
 arm_smmu_domain_inv_range()

On Tue, Jan 27, 2026 at 02:23:48PM -0400, Jason Gunthorpe wrote:
> On Tue, Jan 27, 2026 at 10:07:09AM -0800, Nicolin Chen wrote:
> > > My understanding has been that this invalidation can run from an IRQ
> > > context - we permit the use of the DMA API from an interrupt handler?
> > > 
> > > I though that for rwsem the read side does not require the _irqsave,
> > > even if it is in an irq context, unless the write side runs from an
> > > IRQ. 
> > 
> > Hmm, is "rwsem" a typo? Because it's rwlock_t, which is spinlock :-/
> 
> Yeah, sorry
> 
> > > Here the write side always runs from a process context.
> > > 
> > > So the write side will block the IRQ which ensures we don't spin
> > > during read in an IRQ.
> > 
> > And, does write_lock_irqsave() disable global IRQ or local IRQ only?
> > 
> > Documentation/locking/locktypes.rst mentions "local_irq_disable()"..
> 
> It will only disable the local IRQ, since it is a spin type lock an IRQ on
> another CPU can spin until it is unlocked.
> 
> The main issue is if this CPU takes an IRQ while the write side is
> locked and spins, then it will never unlock.

Yea, that sounds unsafe. I'll send a v11 with read_lock_irqsave().

We can also pass in iommu_domain to arm_smmu_attach_prepare_invs()
that you pointed out in the followup series.

Thanks!
Nicolin