Re: [PATCH 1/1] iommu/vt-d: Fix lockdep splat in intel_pasid_get_entry()

[Lu Baolu]

Hi Joerg,

On 3/18/21 6:21 PM, Joerg Roedel wrote:
> On Wed, Mar 17, 2021 at 08:58:34AM +0800, Lu Baolu wrote:
> > The pasid_lock is used to synchronize different threads from modifying a
> > same pasid directory entry at the same time. It causes below lockdep splat.
> >
> > [   83.296538] ========================================================
> > [   83.296538] WARNING: possible irq lock inversion dependency detected
> > [   83.296539] 5.12.0-rc3+ #25 Tainted: G        W
> > [   83.296539] --------------------------------------------------------
> > [   83.296540] bash/780 just changed the state of lock:
> > [   83.296540] ffffffff82b29c98 (device_domain_lock){..-.}-{2:2}, at:
> >             iommu_flush_dev_iotlb.part.0+0x32/0x110
> > [   83.296547] but this lock took another, SOFTIRQ-unsafe lock in the past:
> > [   83.296547]  (pasid_lock){+.+.}-{2:2}
> > [   83.296548]
> >
> >             and interrupts could create inverse lock ordering between them.
> >
> > [   83.296549] other info that might help us debug this:
> > [   83.296549] Chain exists of:
> >                   device_domain_lock --> &iommu->lock --> pasid_lock
> > [   83.296551]  Possible interrupt unsafe locking scenario:
> >
> > [   83.296551]        CPU0                    CPU1
> > [   83.296552]        ----                    ----
> > [   83.296552]   lock(pasid_lock);
> > [   83.296553]                                local_irq_disable();
> > [   83.296553]                                lock(device_domain_lock);
> > [   83.296554]                                lock(&iommu->lock);
> > [   83.296554]   <Interrupt>
> > [   83.296554]     lock(device_domain_lock);
> > [   83.296555]
> >                  *** DEADLOCK ***
> >
> > Fix it by replacing the pasid_lock with an atomic exchange operation.
> >
> > Reported-and-tested-by: Dave Jiang <dave.jiang@xxxxxxxxx>
> > Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > ---
> >   drivers/iommu/intel/pasid.c | 14 ++++++--------
> >   1 file changed, 6 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
> > index 9fb3d3e80408..1ddcb8295f72 100644
> > --- a/drivers/iommu/intel/pasid.c
> > +++ b/drivers/iommu/intel/pasid.c
> > @@ -24,7 +24,6 @@
> >   /*
> >    * Intel IOMMU system wide PASID name space:
> >    */
> > -static DEFINE_SPINLOCK(pasid_lock);
> >   u32 intel_pasid_max_id = PASID_MAX;
> >   
> >   int vcmd_alloc_pasid(struct intel_iommu *iommu, u32 *pasid)
> > @@ -259,19 +258,18 @@ struct pasid_entry *intel_pasid_get_entry(struct device *dev, u32 pasid)
> >   	dir_index = pasid >> PASID_PDE_SHIFT;
> >   	index = pasid & PASID_PTE_MASK;
> >   
> > -	spin_lock(&pasid_lock);
> >   	entries = get_pasid_table_from_pde(&dir[dir_index]);
> >   	if (!entries) {
> >   		entries = alloc_pgtable_page(info->iommu->node);
> > -		if (!entries) {
> > -			spin_unlock(&pasid_lock);
> > +		if (!entries)
> >   			return NULL;
> > -		}
> >   
> > -		WRITE_ONCE(dir[dir_index].val,
> > -			   (u64)virt_to_phys(entries) | PASID_PTE_PRESENT);
> > +		if (cmpxchg64(&dir[dir_index].val, 0ULL,
> > +			      (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {
> > +			free_pgtable_page(entries);
> > +			entries = get_pasid_table_from_pde(&dir[dir_index]);
>
> This is racy, someone could have already cleared the pasid-entry again.

This code modifies the pasid directory entry. The pasid directory
entries are allocated on demand and will never be freed.

> What you need to do here is to retry the whole path by adding a goto
> to before  the first get_pasid_table_from_pde() call.

Yes. Retrying by adding a goto makes the code clearer.

> Btw, what makes sure that the pasid_entry does not go away when it is
> returned here?

As explained above, it handles the pasid directory table entry which
won't go away.

> Regards,
>
> 	Joerg

Best regards,
baolu