membarrier fixes

membarrier()'s MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE is documented as syncing the core on all sibling threads but not necessarily the calling thread. This behavior is fundamentally buggy and cannot be used safely. Suppose a user program has two threads. Thread A is on CPU 0 and thread B is on CPU 1. Thread A modifies some text and calls membarrier(MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE). Then thread B executes the modified code. If, at any point after membarrier() decides which CPUs to target, thread A could be preempted and replaced by thread B on CPU 0. This could even happen on exit from the membarrier() syscall. If this happens, thread B will end up running on CPU 0 without having synced.

In principle, this could be fixed by arranging for the scheduler to sync_core_before_usermode() whenever switching between two threads in the same mm if there is any possibility of a concurrent membarrier() call, but this would have considerable overhead. Instead, make membarrier() sync the calling CPU as well.

As an optimization, this avoids an extra smp_mb() in the default barrier-only mode.

Cc [email protected]

Signed-off-by Andy Lutomirski <[email protected]>

---
kernel/sched/membarrier.c | 49 +++++++++++++++++++++++++--------------

1 file changed, 32 insertions(+), 17 deletions(-)

kernel/sched/membarrier.c +0 -0

index 01538b31f27e..7df7c0e60647 100644

kernel/sched/membarrier.c +0 -2

 		if (cpu_id >= nr_cpu_ids || !cpu_online(cpu_id))
 			goto out;
-		if (cpu_id == raw_smp_processor_id())
-			goto out;
 		rcu_read_lock();
 		p = rcu_dereference(cpu_rq(cpu_id)->curr);

		if (!p || p->mm != mm) {

kernel/sched/membarrier.c +0 -10

@@ -368,16 +366,6 @@ static int membarrier_private_expedited(int flags, int
 cpu_id)
 		for_each_online_cpu(cpu) {
 			struct task_struct *p;
 
-			/*
-			 * Skipping the current CPU is OK even through we can be
-			 * migrated at any point. The current CPU, at the point
-			 * where we read raw_smp_processor_id(), is ensured to
-			 * be in program order with respect to the caller
-			 * thread. Therefore, we can skip this CPU from the
-			 * iteration.
-			 */
-			if (cpu == raw_smp_processor_id())
-				continue;
 			p = rcu_dereference(cpu_rq(cpu)->curr);
 			if (p && p->mm == mm)

				__cpumask_set_cpu(cpu, tmpmask);

kernel/sched/membarrier.c +13 -5

@@ -385,12 +373,39 @@ static int membarrier_private_expedited(int flags, int
 cpu_id)
 		rcu_read_unlock();
 	}
 
-	preempt_disable();
-	if (cpu_id >= 0)
+	if (cpu_id >= 0) {
+		/*
+		 * smp_call_function_single() will call ipi_func() if cpu_id
+		 * is the calling CPU.
+		 */
 		smp_call_function_single(cpu_id, ipi_func, NULL, 1);
-	else
-		smp_call_function_many(tmpmask, ipi_func, NULL, 1);
-	preempt_enable();
+	} else {
+		/*
+		 * For regular membarrier, we can save a few cycles by
+		 * skipping the current cpu -- we're about to do smp_mb()
+		 * below, and if we migrate to a different cpu, this cpu
+		 * and the new cpu will execute a full barrier in the
+		 * scheduler.
+		 *

kernel/sched/membarrier.c +9 -0

+		 */
+		if (ipi_func == ipi_mb) {
+			preempt_disable();
+			smp_call_function_many(tmpmask, ipi_func, NULL, true);
+			preempt_enable();
+		} else {
+			on_each_cpu_mask(tmpmask, ipi_func, NULL, true);
+		}
+	}
 

out:

	if (cpu_id < 0)

--
2.28.0

It seems to me that most RSEQ membarrier users will expect any stores done before the membarrier() syscall to be visible to the target task(s). While this is extremely likely to be true in practice, nothing actually guarantees it by a strict reading of the x86 manuals. Rather than providing this guarantee by accident and potentially causing a problem down the road, just add an explicit barrier.

Cc [email protected]

Signed-off-by Andy Lutomirski <[email protected]>

---
kernel/sched/membarrier.c | 8 ++++++++

1 file changed, 8 insertions(+)

kernel/sched/membarrier.c +8 -1

index 5a40b3828ff2..6251d3d12abe 100644
@@ -168,6 +168,14 @@ static void ipi_mb(void *info)
 
 static void ipi_rseq(void *info)
 {
+	/*
+	 * Ensure that all stores done by the calling thread are visible
+	 * to the current task before the current task resumes.  We could
+	 * probably optimize this away on most architectures, but by the
+	 * time we've already sent an IPI, the cost of the extra smp_mb()
+	 * is negligible.
+	 */
+	smp_mb();
 	rseq_preempt(current);
 }
 
--

2.28.0

membarrier() does not explicitly sync_core() remote CPUs; instead, it relies on the assumption that an IPI will result in a core sync. On x86, I think this may be true in practice, but it's not architecturally reliable. In particular, the SDM and APM do not appear to guarantee that interrupt delivery is serializing. While IRET does serialize, IPI return can schedule, thereby switching to another task in the same mm that was sleeping in a syscall. The new task could then SYSRET back to usermode without ever executing IRET.

Make this more robust by explicitly calling sync_core_before_usermode() on remote cores. (This also helps people who search the kernel tree for instances of sync_core() and sync_core_before_usermode() -- one might be surprised that the core membarrier code doesn't currently show up in a such a search.)

Cc [email protected]

Signed-off-by Andy Lutomirski <[email protected]>

---
kernel/sched/membarrier.c | 18 ++++++++++++++++++

1 file changed, 18 insertions(+)

kernel/sched/membarrier.c +18 -0

index 6251d3d12abe..01538b31f27e 100644
@@ -166,6 +166,23 @@ static void ipi_mb(void *info)
 	smp_mb();	/* IPIs should be serializing but paranoid. */
 }
 
+static void ipi_sync_core(void *info)
+{
+	/*
+	 * The smp_mb() in membarrier after all the IPIs is supposed to
+	 * ensure that memory on remote CPUs that occur before the IPI
+	 * become visible to membarrier()'s caller -- see scenario B in
+	 * the big comment at the top of this file.
+	 *
+	 * A sync_core() would provide this guarantee, but
+	 * sync_core_before_usermode() might end up being deferred until
+	 * after membarrier()'s smp_mb().
+	 */
+	smp_mb();	/* IPIs should be serializing but paranoid. */
+
+	sync_core_before_usermode();
+}
+
 static void ipi_rseq(void *info)
 {
 	/*

@@ -301,6 +318,7 @@ static int membarrier_private_expedited(int flags, int
 cpu_id)
 		if (!(atomic_read(&mm->membarrier_state) &
 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
 			return -EPERM;
+		ipi_func = ipi_sync_core;
 	} else if (flags == MEMBARRIER_FLAG_RSEQ) {
 		if (!IS_ENABLED(CONFIG_RSEQ))

			return -EINVAL;

--
2.28.0