Re: [PATCH v9 6/7] iommu/arm-smmu-v3: Add arm_smmu_invs based arm_smmu_domain_inv_range()

Nicolin Chen
2026-01-27T18:07:41Z

On Tue, Jan 27, 2026 at 01:08:37PM -0400, Jason Gunthorpe wrote:
> On Tue, Jan 27, 2026 at 08:38:31AM -0800, Nicolin Chen wrote:
> > Hi Pranjal,
> >
> > Sorry, I missed this!
> >
> > On Fri, Jan 23, 2026 at 09:48:37AM +0000, Pranjal Shrivastava wrote:
> > > On Fri, Dec 19, 2025 at 12:11:28PM -0800, Nicolin Chen wrote:
> > > > + /*
> > > > + * Avoid locking unless ATS is being used. No ATC invalidation can be
> > > > + * going on after a domain is detached.
> > > > + */
> > > > + if (invs->has_ats) {
> > > > + read_lock(&invs->rwlock);
> > >
> > > Shouldn't these be read_lock_irqsave for all rwlock variants here?
> > > Invalidations might happen in IRQ context as well..
> > >
> > > > + __arm_smmu_domain_inv_range(invs, iova, size, granule, leaf);
> > > > + read_unlock(&invs->rwlock);
> >
> > It was kept from the older versions where we had a trylock. Jason
> > had an insight about this, mainly for less latency on invalidation
> > threads.
> >
> > Yet, now we have a plain locking. TBH, I can't find a good reason
> > justifying this. And it does look a bit unsafe to me. So, I think
> > I will just change to the _irqsave version. (Jason?)
>
> 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 :-/

> 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()"..

Thanks
Nicolin