[PATCH] mm: mmap_lock: Use local locks instead of disabling preemption

mmap_lock will explicitly disable/enable preemption upon manipulating
its local CPU variables. This is to be expected, but in this case, it
doesn't play well with PREEMPT_RT. The preemption disabled code section
also takes a spin-lock. Spin-locks in RT systems will try to schedule,
which is exactly what we're trying to avoid.

To mitigate this, convert the explicit preemption handling to
local_locks. Which are RT aware, and will disable migration instead of
preemption when PREEMPT_RT=y.

The faulty call trace looks like the following:
    __mmap_lock_do_trace_*()
      preempt_disable()
      get_mm_memcg_path()
        cgroup_path()
          kernfs_path_from_node()
            spin_lock_irqsave() /* Scheduling while atomic! */

Fixes: 2b5067a8143e3 ("mm: mmap_lock: add tracepoints around lock acquisition ")
Signed-off-by: Nicolas Saenz Julienne <redacted>
---
 mm/mmap_lock.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c
index dcdde4f722a4..2ae3f33b85b1 100644
--- a/mm/mmap_lock.c
+++ b/mm/mmap_lock.c

@@ -11,6 +11,7 @@
 #include <linux/rcupdate.h>
 #include <linux/smp.h>
 #include <linux/trace_events.h>
+#include <linux/local_lock.h>

 EXPORT_TRACEPOINT_SYMBOL(mmap_lock_start_locking);
 EXPORT_TRACEPOINT_SYMBOL(mmap_lock_acquire_returned);

@@ -39,21 +40,30 @@ static int reg_refcount; /* Protected by reg_lock. */
  */
 #define CONTEXT_COUNT 4

-static DEFINE_PER_CPU(char __rcu *, memcg_path_buf);
+struct memcg_path {
+       local_lock_t lock;
+       char __rcu *buf;
+       local_t buf_idx;
+};
+static DEFINE_PER_CPU(struct memcg_path, memcg_paths) = {
+       .lock = INIT_LOCAL_LOCK(lock),
+       .buf_idx = LOCAL_INIT(0),
+};
+
 static char **tmp_bufs;
-static DEFINE_PER_CPU(int, memcg_path_buf_idx);

 /* Called with reg_lock held. */
 static void free_memcg_path_bufs(void)
 {
+       struct memcg_path *memcg_path;
        int cpu;
        char **old = tmp_bufs;

        for_each_possible_cpu(cpu) {
-               *(old++) = rcu_dereference_protected(
-                       per_cpu(memcg_path_buf, cpu),
+               memcg_path = per_cpu_ptr(&memcg_paths, cpu);
+               *(old++) = rcu_dereference_protected(memcg_path->buf,
                        lockdep_is_held(&reg_lock));
-               rcu_assign_pointer(per_cpu(memcg_path_buf, cpu), NULL);
+               rcu_assign_pointer(memcg_path->buf, NULL);
        }

        /* Wait for inflight memcg_path_buf users to finish. */

@@ -88,7 +98,7 @@ int trace_mmap_lock_reg(void)
                new = kmalloc(MEMCG_PATH_BUF_SIZE * CONTEXT_COUNT, GFP_KERNEL);
                if (new == NULL)
                        goto out_fail_free;
-               rcu_assign_pointer(per_cpu(memcg_path_buf, cpu), new);
+               rcu_assign_pointer(per_cpu_ptr(&memcg_paths, cpu)->buf, new);
                /* Don't need to wait for inflights, they'd have gotten NULL. */
        }

@@ -122,23 +132,24 @@ void trace_mmap_lock_unreg(void)

 static inline char *get_memcg_path_buf(void)
 {
+       struct memcg_path *memcg_path = this_cpu_ptr(&memcg_paths);
        char *buf;
        int idx;

        rcu_read_lock();
-       buf = rcu_dereference(*this_cpu_ptr(&memcg_path_buf));
+       buf = rcu_dereference(memcg_path->buf);
        if (buf == NULL) {
                rcu_read_unlock();
                return NULL;
        }
-       idx = this_cpu_add_return(memcg_path_buf_idx, MEMCG_PATH_BUF_SIZE) -
+       idx = local_add_return(MEMCG_PATH_BUF_SIZE, &memcg_path->buf_idx) -
              MEMCG_PATH_BUF_SIZE;
        return &buf[idx];
 }

 static inline void put_memcg_path_buf(void)
 {
-       this_cpu_sub(memcg_path_buf_idx, MEMCG_PATH_BUF_SIZE);
+       local_sub(MEMCG_PATH_BUF_SIZE, &this_cpu_ptr(&memcg_paths)->buf_idx);
        rcu_read_unlock();
 }

@@ -179,14 +190,14 @@ static const char *get_mm_memcg_path(struct mm_struct *mm)
 #define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                                   \
        do {                                                                   \
                const char *memcg_path;                                        \
-               preempt_disable();                                             \
+               local_lock(&memcg_paths.lock);                                 \
                memcg_path = get_mm_memcg_path(mm);                            \
                trace_mmap_lock_##type(mm,                                     \
                                       memcg_path != NULL ? memcg_path : "",   \
                                       ##__VA_ARGS__);                         \
                if (likely(memcg_path != NULL))                                \
                        put_memcg_path_buf();                                  \
-               preempt_enable();                                              \
+               local_unlock(&memcg_paths.lock);                               \
        } while (0)

 #else /* !CONFIG_MEMCG */
--

2.31.1