[IA64] Fix race in mm-context wrap-around logic.

The patch below should fix a race which could cause stale TLB entries.
Specifically, when 2 CPUs ended up racing for entrance to
wrap_mmu_context().  The losing CPU would find that by the time it
acquired ctx.lock, mm->context already had a valid value, but then it
failed to (re-)check the delayed TLB flushing logic and hence could
end up using a context number when there were still stale entries in
its TLB.  The fix is to check for delayed TLB flushes only after
mm->context is valid (non-zero).  The patch also makes GCC v4.x
happier by defining a non-volatile variant of mm_context_t called
nv_mm_context_t.

Signed-off-by: David Mosberger-Tang <David.Mosberger@acm.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>

2 files changed, 37 insertions, 25 deletions

diff --git a/include/asm-ia64/mmu.h b/include/asm-ia64/mmu.h
index ae1525352a25..611432ba579c 100644
--- a/include/asm-ia64/mmu.h
+++ b/include/asm-ia64/mmu.h
@@ -2,10 +2,12 @@
 #define __MMU_H
 
 /*
- * Type for a context number.  We declare it volatile to ensure proper ordering when it's
- * accessed outside of spinlock'd critical sections (e.g., as done in activate_mm() and
- * init_new_context()).
+ * Type for a context number.  We declare it volatile to ensure proper
+ * ordering when it's accessed outside of spinlock'd critical sections
+ * (e.g., as done in activate_mm() and init_new_context()).
  */
 typedef volatile unsigned long mm_context_t;
 
+typedef unsigned long nv_mm_context_t;
+
 #endif
diff --git a/include/asm-ia64/mmu_context.h b/include/asm-ia64/mmu_context.h
index e3e5fededb04..0680d163be97 100644
--- a/include/asm-ia64/mmu_context.h
+++ b/include/asm-ia64/mmu_context.h
@@ -55,34 +55,46 @@ static inline void
 delayed_tlb_flush (void)
 {
         extern void local_flush_tlb_all (void);
+        unsigned long flags;
 
         if (unlikely(__ia64_per_cpu_var(ia64_need_tlb_flush))) {
-                local_flush_tlb_all();
-                __ia64_per_cpu_var(ia64_need_tlb_flush) = 0;
+                spin_lock_irqsave(&ia64_ctx.lock, flags);
+                {
+                        if (__ia64_per_cpu_var(ia64_need_tlb_flush)) {
+                                local_flush_tlb_all();
+                                __ia64_per_cpu_var(ia64_need_tlb_flush) = 0;
+                        }
+                }
+                spin_unlock_irqrestore(&ia64_ctx.lock, flags);
         }
 }
 
-static inline mm_context_t
+static inline nv_mm_context_t
 get_mmu_context (struct mm_struct *mm)
 {
         unsigned long flags;
-        mm_context_t context = mm->context;
-
-        if (context)
-                return context;
-
-        spin_lock_irqsave(&ia64_ctx.lock, flags);
-        {
-                /* re-check, now that we've got the lock: */
-                context = mm->context;
-                if (context == 0) {
-                        cpus_clear(mm->cpu_vm_mask);
-                        if (ia64_ctx.next >= ia64_ctx.limit)
-                                wrap_mmu_context(mm);
-                        mm->context = context = ia64_ctx.next++;
+        nv_mm_context_t context = mm->context;
+
+        if (unlikely(!context)) {
+                spin_lock_irqsave(&ia64_ctx.lock, flags);
+                {
+                        /* re-check, now that we've got the lock: */
+                        context = mm->context;
+                        if (context == 0) {
+                                cpus_clear(mm->cpu_vm_mask);
+                                if (ia64_ctx.next >= ia64_ctx.limit)
+                                        wrap_mmu_context(mm);
+                                mm->context = context = ia64_ctx.next++;
+                        }
                 }
+                spin_unlock_irqrestore(&ia64_ctx.lock, flags);
         }
-        spin_unlock_irqrestore(&ia64_ctx.lock, flags);
+        /*
+         * Ensure we're not starting to use "context" before any old
+         * uses of it are gone from our TLB.
+         */
+        delayed_tlb_flush();
+
         return context;
 }
 
@@ -104,7 +116,7 @@ destroy_context (struct mm_struct *mm)
 }
 
 static inline void
-reload_context (mm_context_t context)
+reload_context (nv_mm_context_t context)
 {
         unsigned long rid;
         unsigned long rid_incr = 0;
@@ -138,7 +150,7 @@ reload_context (mm_context_t context)
 static inline void
 activate_context (struct mm_struct *mm)
 {
-        mm_context_t context;
+        nv_mm_context_t context;
 
         do {
                 context = get_mmu_context(mm);
@@ -157,8 +169,6 @@ activate_context (struct mm_struct *mm)
 static inline void
 activate_mm (struct mm_struct *prev, struct mm_struct *next)
 {
-        delayed_tlb_flush();
-
         /*
          * We may get interrupts here, but that's OK because interrupt handlers cannot
          * touch user-space.