ARM: 7659/1: mm: make mm->context.id an atomic64_t variable

mm->context.id is updated under asid_lock when a new ASID is allocated
to an mm_struct. However, it is also read without the lock when a task
is being scheduled and checking whether or not the current ASID
generation is up-to-date.

If two threads of the same process are being scheduled in parallel and
the bottom bits of the generation in their mm->context.id match the
current generation (that is, the mm_struct has not been used for ~2^24
rollovers) then the non-atomic, lockless access to mm->context.id may
yield the incorrect ASID.

This patch fixes this issue by making mm->context.id and atomic64_t,
ensuring that the generation is always read consistently. For code that
only requires access to the ASID bits (e.g. TLB flushing by mm), then
the value is accessed directly, which GCC converts to an ldrb.

Cc: <stable@vger.kernel.org> # 3.8
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 13 insertions, 8 deletions

diff --git a/arch/arm/mm/context.c b/arch/arm/mm/context.c
index 03ba181e359c..44d4ee52f3e2 100644
--- a/arch/arm/mm/context.c
+++ b/arch/arm/mm/context.c
@@ -152,9 +152,9 @@ static int is_reserved_asid(u64 asid)
 	return 0;
 }
 
-static void new_context(struct mm_struct *mm, unsigned int cpu)
+static u64 new_context(struct mm_struct *mm, unsigned int cpu)
 {
-	u64 asid = mm->context.id;
+	u64 asid = atomic64_read(&mm->context.id);
 	u64 generation = atomic64_read(&asid_generation);
 
 	if (asid != 0 && is_reserved_asid(asid)) {
@@ -181,13 +181,14 @@ static void new_context(struct mm_struct *mm, unsigned int cpu)
 		cpumask_clear(mm_cpumask(mm));
 	}
 
-	mm->context.id = asid;
+	return asid;
 }
 
 void check_and_switch_context(struct mm_struct *mm, struct task_struct *tsk)
 {
 	unsigned long flags;
 	unsigned int cpu = smp_processor_id();
+	u64 asid;
 
 	if (unlikely(mm->context.vmalloc_seq != init_mm.context.vmalloc_seq))
 		__check_vmalloc_seq(mm);
@@ -198,19 +199,23 @@ void check_and_switch_context(struct mm_struct *mm, struct task_struct *tsk)
 	 */
 	cpu_set_reserved_ttbr0();
 
-	if (!((mm->context.id ^ atomic64_read(&asid_generation)) >> ASID_BITS)
-	    && atomic64_xchg(&per_cpu(active_asids, cpu), mm->context.id))
+	asid = atomic64_read(&mm->context.id);
+	if (!((asid ^ atomic64_read(&asid_generation)) >> ASID_BITS)
+	    && atomic64_xchg(&per_cpu(active_asids, cpu), asid))
 		goto switch_mm_fastpath;
 
 	raw_spin_lock_irqsave(&cpu_asid_lock, flags);
 	/* Check that our ASID belongs to the current generation. */
-	if ((mm->context.id ^ atomic64_read(&asid_generation)) >> ASID_BITS)
-		new_context(mm, cpu);
+	asid = atomic64_read(&mm->context.id);
+	if ((asid ^ atomic64_read(&asid_generation)) >> ASID_BITS) {
+		asid = new_context(mm, cpu);
+		atomic64_set(&mm->context.id, asid);
+	}
 
 	if (cpumask_test_and_clear_cpu(cpu, &tlb_flush_pending))
 		local_flush_tlb_all();
 
-	atomic64_set(&per_cpu(active_asids, cpu), mm->context.id);
+	atomic64_set(&per_cpu(active_asids, cpu), asid);
 	cpumask_set_cpu(cpu, mm_cpumask(mm));
 	raw_spin_unlock_irqrestore(&cpu_asid_lock, flags);