Messages in this thread Patch in this message | | | From | Sebastian Andrzej Siewior <> | Subject | [PATCH 4/5] iommu/iova: don't disable preempt around this_cpu_ptr() | Date | Fri, 21 Oct 2016 19:06:25 +0200 |
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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@8bytes.org> Cc: iommu@lists.linux-foundation.org Cc: Chris Wilson <chris@chris-wilson.co.uk> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.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
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