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io-pgtable lock removal

Will Deacon linux-arm-kernel linux-iommu 1 version New

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Cover letter [PATCH 0/8] io-pgtable lock removal

Ray,

Quoted 22 lines · Ray Jui
Quoted 21 lines · Will Deacon

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.

Quoted 4 lines · Ray Jui
Quoted 4 lines
Quoted 4 lines

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.

Quoted 6 lines
Quoted 2 lines

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

Quoted 1 line

From eeb11dab63fcdd698b671a3a8c63516005caa9ec Mon Sep 17 00:00:00 2001

From: Will Deacon <[email protected]>
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 <[email protected]>
Cc Robin Murphy <[email protected]>
Fixes 2c3d273eabe8 ("iommu/io-pgtable-arm: Support lockless operation")
Signed-off-by Will Deacon <[email protected]>
---
 drivers/iommu/io-pgtable.c |  1 +
 drivers/iommu/io-pgtable.h | 12 ++++++------
 2 files changed, 7 insertions(+), 6 deletions(-)
drivers/iommu/io-pgtable.c +1 -0
index 127558d83667..cd8d7aaec161 100644
@@ -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;
 }
drivers/iommu/io-pgtable.h +6 -6
index 524263a7ae6f..b64580c9d03d 100644
@@ -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;
-	}
 }
 
 /**
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2.1.4

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