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Subject[PATCH v2] x86/mm: Improve switch_mm barrier comments
Date
My previous comments were still a bit confusing and there was a
typo. Fix it up.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Fixes: 71b3c126e611 ("x86/mm: Add barriers and document switch_mm()-vs-flush synchronization")
Cc: stable@vger.kernel.org
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---

Changes from v1: Totally different.

arch/x86/include/asm/mmu_context.h | 15 ++++++++-------
1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 1edc9cd198b8..4fcae1e066f3 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -132,14 +132,16 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
* be sent, and CPU 0's TLB will contain a stale entry.)
*
* The bad outcome can occur if either CPU's load is
- * reordered before that CPU's store, so both CPUs much
+ * reordered before that CPU's store, so both CPUs must
* execute full barriers to prevent this from happening.
*
* Thus, switch_mm needs a full barrier between the
* store to mm_cpumask and any operation that could load
- * from next->pgd. This barrier synchronizes with
- * remote TLB flushers. Fortunately, load_cr3 is
- * serializing and thus acts as a full barrier.
+ * from next->pgd. TLB fills are special and can happen
+ * due to instruction fetches or for no reason at all,
+ * and neither LOCK nor MFENCE orders them.
+ * Fortunately, load_cr3 is serializing and gives the
+ * ordering guarantee we need.
*
*/
load_cr3(next->pgd);
@@ -188,9 +190,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
* tlb flush IPI delivery. We must reload CR3
* to make sure to use no freed page tables.
*
- * As above, this is a barrier that forces
- * TLB repopulation to be ordered after the
- * store to mm_cpumask.
+ * As above, load_cr3 is serializing and orders TLB
+ * fills with respect to the mm_cpumask write.
*/
load_cr3(next->pgd);
trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
--
2.5.0
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