powerpc/mm: Fix some SMP issues with MMU context handling

This patch fixes a couple of issues that can happen as a result
of steal_context() dropping the context_lock when all possible
PIDs are ineligible for stealing (hopefully an extremely hard to
hit occurence).

This case exposes the possibility of a stale context_mm[] entry
to be seen since destroy_context() doesn't clear it and the free
map isn't re-tested. It also means steal_context() will not notice
a context freed while the lock was help, thus possibly trying to
steal a context when a free one was available.

This fixes it by always returning to the caller from steal_context
when it dropped the lock with a return value that causes the
caller to re-samble the number of free contexts, along with
properly clearing the context_mm[] array for destroyed contexts.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/arch/powerpc/mm/mmu_context_nohash.c b/arch/powerpc/mm/mmu_context_nohash.c
index 030d0005b4d2..c42858780cbd 100644
--- a/arch/powerpc/mm/mmu_context_nohash.c
+++ b/arch/powerpc/mm/mmu_context_nohash.c
@@ -73,7 +73,6 @@ static unsigned int steal_context_smp(unsigned int id)
         struct mm_struct *mm;
         unsigned int cpu, max;
 
- again:
         max = last_context - first_context;
 
         /* Attempt to free next_context first and then loop until we manage */
@@ -108,7 +107,9 @@ static unsigned int steal_context_smp(unsigned int id)
         spin_unlock(&context_lock);
         cpu_relax();
         spin_lock(&context_lock);
-        goto again;
+
+        /* This will cause the caller to try again */
+        return MMU_NO_CONTEXT;
 }
 #endif  /* CONFIG_SMP */
 
@@ -194,6 +195,8 @@ void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next)
                 WARN_ON(prev->context.active < 1);
                 prev->context.active--;
         }
+
+ again:
 #endif /* CONFIG_SMP */
 
         /* If we already have a valid assigned context, skip all that */
@@ -212,7 +215,8 @@ void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next)
 #ifdef CONFIG_SMP
                 if (num_online_cpus() > 1) {
                         id = steal_context_smp(id);
-                        goto stolen;
+                        if (id == MMU_NO_CONTEXT)
+                                goto again;
                 }
 #endif /* CONFIG_SMP */
                 id = steal_context_up(id);
@@ -286,8 +290,8 @@ void destroy_context(struct mm_struct *mm)
                 mm->context.id = MMU_NO_CONTEXT;
 #ifdef DEBUG_MAP_CONSISTENCY
                 mm->context.active = 0;
-                context_mm[id] = NULL;
 #endif
+                context_mm[id] = NULL;
                 nr_free_contexts++;
         }
         spin_unlock(&context_lock);