linux-kernel - [PATCH 4/5] iommu/iova: don't disable preempt around this_cpu

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Message-Id: <20161021170626.31750-4-bigeasy@linutronix.de>
Date:   Fri, 21 Oct 2016 19:06:25 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org,
        Chris Wilson <chris@...is-wilson.co.uk>
Subject: [PATCH 4/5] iommu/iova: don't disable preempt around this_cpu_ptr()

Commit 583248e6620a ("iommu/iova: Disable preemption around use of
this_cpu_ptr()") disables preemption while accessing a per-CPU variable.
This does keep lockdep quiet. However I don't see the point why it is
bad if we get migrated after its access to another CPU.
__iova_rcache_insert() and __iova_rcache_get() immediately locks the
variable after obtaining it - before accessing its members.
_If_ we get migrated away after retrieving the address of cpu_rcache
before taking the lock then the *other* task on the same CPU will
retrieve the same address of cpu_rcache and will spin on the lock.

alloc_iova_fast() disables preemption while invoking
free_cpu_cached_iovas() on each CPU. The function itself uses
per_cpu_ptr() which does not trigger a warning (like this_cpu_ptr()
does) because it assumes the caller knows what he does because he might
access the data structure from a different CPU (which means he needs
protection against concurrent access).

Cc: Joerg Roedel <joro@...tes.org>
Cc: iommu@...ts.linux-foundation.org
Cc: Chris Wilson <chris@...is-wilson.co.uk>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 drivers/iommu/iova.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index e23001bfcfee..359d5d169ec0 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -22,6 +22,7 @@
 #include <linux/slab.h>
 #include <linux/smp.h>
 #include <linux/bitops.h>
+#include <linux/cpu.h>
 
 static bool iova_rcache_insert(struct iova_domain *iovad,
 			       unsigned long pfn,
@@ -420,10 +421,8 @@ alloc_iova_fast(struct iova_domain *iovad, unsigned long size,
 
 		/* Try replenishing IOVAs by flushing rcache. */
 		flushed_rcache = true;
-		preempt_disable();
 		for_each_online_cpu(cpu)
 			free_cpu_cached_iovas(cpu, iovad);
-		preempt_enable();
 		goto retry;
 	}
 
@@ -751,7 +750,7 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
 	bool can_insert = false;
 	unsigned long flags;
 
-	cpu_rcache = get_cpu_ptr(rcache->cpu_rcaches);
+	cpu_rcache = raw_cpu_ptr(rcache->cpu_rcaches);
 	spin_lock_irqsave(&cpu_rcache->lock, flags);
 
 	if (!iova_magazine_full(cpu_rcache->loaded)) {
@@ -781,7 +780,6 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
 		iova_magazine_push(cpu_rcache->loaded, iova_pfn);
 
 	spin_unlock_irqrestore(&cpu_rcache->lock, flags);
-	put_cpu_ptr(rcache->cpu_rcaches);
 
 	if (mag_to_free) {
 		iova_magazine_free_pfns(mag_to_free, iovad);
@@ -815,7 +813,7 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
 	bool has_pfn = false;
 	unsigned long flags;
 
-	cpu_rcache = get_cpu_ptr(rcache->cpu_rcaches);
+	cpu_rcache = raw_cpu_ptr(rcache->cpu_rcaches);
 	spin_lock_irqsave(&cpu_rcache->lock, flags);
 
 	if (!iova_magazine_empty(cpu_rcache->loaded)) {
@@ -837,7 +835,6 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
 		iova_pfn = iova_magazine_pop(cpu_rcache->loaded, limit_pfn);
 
 	spin_unlock_irqrestore(&cpu_rcache->lock, flags);
-	put_cpu_ptr(rcache->cpu_rcaches);
 
 	return iova_pfn;
 }
-- 
2.9.3