Re: [PATCH 0/8] io-pgtable lock removal

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

Ray,

On Wed, Jun 28, 2017 at 10:02:35AM -0700, Ray Jui wrote:
> On 6/28/17 4:46 AM, Will Deacon wrote:
> > Robin and I have been bashing our heads against the tlb_sync_pending flag
> > this morning, and we reckon it could have something to do with your timeouts
> > on MMU-500.
> > 
> > On Tue, Jun 27, 2017 at 09:43:19AM -0700, Ray Jui wrote:
> >>>> Also, in a few occasions, I observed the following message during the
> >>>> test, when multiple cores are involved:
> >>>>
> >>>> arm-smmu 64000000.mmu: TLB sync timed out -- SMMU may be deadlocked
> > 
> > The tlb_sync_pending logic was written under the assumption of a global
> > page-table lock, so it assumes that it only has to care about syncing
> > flushes from the current CPU/context. That's not true anymore, and the
> > current code can accidentally skip syncs and (what I think is happening in
> > your case) allow concurrent syncs, which will potentially lead to timeouts
> > if a CPU is unlucky enough to keep missing the Ack.
> > 
> > Please can you try the diff below and see if it fixes things for you?
> > This applies on top of my for-joerg/arm-smmu/updates branch, but note
> > that I've only shown it to the compiler. Not tested at all.
> > 
> > Will
> > 
> 
> Thanks for looking into this. I'm a bit busy at work but will certainly
> find time to test the diff below. I hopefully will get to it later this
> week.

It would be really handy if you could test this, since I think it could
cause some nasty problems if we don't get it fixed. Updated patch (with
commit message) below.

Will

--->8

>From eeb11dab63fcdd698b671a3a8c63516005caa9ec Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>
Date: Thu, 29 Jun 2017 15:08:09 +0100
Subject: [PATCH] iommu/io-pgtable: Fix tlb_sync_pending flag access from
 concurrent CPUs

The tlb_sync_pending flag is used to elide back-to-back TLB sync operations
for two reasons:

  1. TLB sync operations can be expensive, and so avoiding them where we
     can is a good idea.

  2. Some hardware (mtk_iommu) locks up if it sees a TLB sync without an
     unsync'd TLB flush preceding it

The flag is set on an ->add_flush callback and cleared on a ->sync callback,
which worked nicely when all map/unmap operations where protected by a
global lock.

Unfortunately, moving to a lockless implementation means that we suddenly
have races on the flag: updates can go missing and we can end up with
back-to-back syncs once again.

This patch resolves the problem by making the tlb_sync_pending flag an
atomic_t and sorts out the ordering with respect to TLB callbacks.
Now, the flag is set with release semantics after adding a flush and
checked with an xchg operation (and subsequent control dependency) when
performing the sync. We could consider using a cmpxchg here, but we'll
likely just hit our local update to the flag anyway.

Cc: Ray Jui <ray.jui-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>
Cc: Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
Fixes: 2c3d273eabe8 ("iommu/io-pgtable-arm: Support lockless operation")
Signed-off-by: Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>
---
 drivers/iommu/io-pgtable.c |  1 +
 drivers/iommu/io-pgtable.h | 12 ++++++------
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/iommu/io-pgtable.c b/drivers/iommu/io-pgtable.c
index 127558d83667..cd8d7aaec161 100644
--- a/drivers/iommu/io-pgtable.c
+++ b/drivers/iommu/io-pgtable.c
@@ -59,6 +59,7 @@ struct io_pgtable_ops *alloc_io_pgtable_ops(enum io_pgtable_fmt fmt,
 	iop->cookie	= cookie;
 	iop->cfg	= *cfg;
 
+	atomic_set(&iop->tlb_sync_pending, 0);
 	return &iop->ops;
 }
 
diff --git a/drivers/iommu/io-pgtable.h b/drivers/iommu/io-pgtable.h
index 524263a7ae6f..b64580c9d03d 100644
--- a/drivers/iommu/io-pgtable.h
+++ b/drivers/iommu/io-pgtable.h
@@ -1,5 +1,7 @@
 #ifndef __IO_PGTABLE_H
 #define __IO_PGTABLE_H
+
+#include <linux/atomic.h>
 #include <linux/bitops.h>
 
 /*
@@ -165,7 +167,7 @@ void free_io_pgtable_ops(struct io_pgtable_ops *ops);
 struct io_pgtable {
 	enum io_pgtable_fmt	fmt;
 	void			*cookie;
-	bool			tlb_sync_pending;
+	atomic_t		tlb_sync_pending;
 	struct io_pgtable_cfg	cfg;
 	struct io_pgtable_ops	ops;
 };
@@ -175,22 +177,20 @@ struct io_pgtable {
 static inline void io_pgtable_tlb_flush_all(struct io_pgtable *iop)
 {
 	iop->cfg.tlb->tlb_flush_all(iop->cookie);
-	iop->tlb_sync_pending = true;
+	atomic_set_release(&iop->tlb_sync_pending, 1);
 }
 
 static inline void io_pgtable_tlb_add_flush(struct io_pgtable *iop,
 		unsigned long iova, size_t size, size_t granule, bool leaf)
 {
 	iop->cfg.tlb->tlb_add_flush(iova, size, granule, leaf, iop->cookie);
-	iop->tlb_sync_pending = true;
+	atomic_set_release(&iop->tlb_sync_pending, 1);
 }
 
 static inline void io_pgtable_tlb_sync(struct io_pgtable *iop)
 {
-	if (iop->tlb_sync_pending) {
+	if (atomic_xchg_relaxed(&iop->tlb_sync_pending, 0))
 		iop->cfg.tlb->tlb_sync(iop->cookie);
-		iop->tlb_sync_pending = false;
-	}
 }
 
 /**
-- 
2.1.4