percpu: fix synchronization between chunk->map_extend_work and chunk destruction

Atomic allocations can trigger async map extensions which is serviced
by chunk->map_extend_work.  pcpu_balance_work which is responsible for
destroying idle chunks wasn't synchronizing properly against
chunk->map_extend_work and may end up freeing the chunk while the work
item is still in flight.

This patch fixes the bug by rolling async map extension operations
into pcpu_balance_work.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Reported-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: stable@vger.kernel.org # v3.18+
Fixes: 9c824b6a172c ("percpu: make sure chunk->map array has available space")

1 files changed, 36 insertions, 21 deletions

diff --git a/mm/percpu.c b/mm/percpu.c
index 0c59684f1ff2..b1d2a3844792 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -112,7 +112,7 @@ struct pcpu_chunk {
         int                     map_used;       /* # of map entries used before the sentry */
         int                     map_alloc;      /* # of map entries allocated */
         int                     *map;           /* allocation map */
-        struct work_struct      map_extend_work;/* async ->map[] extension */
+        struct list_head        map_extend_list;/* on pcpu_map_extend_chunks */
 
         void                    *data;          /* chunk data */
         int                     first_free;     /* no free below this */
@@ -166,6 +166,9 @@ static DEFINE_MUTEX(pcpu_alloc_mutex);	/* chunk create/destroy, [de]pop */
 
 static struct list_head *pcpu_slot __read_mostly; /* chunk list slots */
 
+/* chunks which need their map areas extended, protected by pcpu_lock */
+static LIST_HEAD(pcpu_map_extend_chunks);
+
 /*
  * The number of empty populated pages, protected by pcpu_lock.  The
  * reserved chunk doesn't contribute to the count.
@@ -395,13 +398,19 @@ static int pcpu_need_to_extend(struct pcpu_chunk *chunk, bool is_atomic)
 {
         int margin, new_alloc;
 
+        lockdep_assert_held(&pcpu_lock);
+
         if (is_atomic) {
                 margin = 3;
 
                 if (chunk->map_alloc <
-                    chunk->map_used + PCPU_ATOMIC_MAP_MARGIN_LOW &&
-                    pcpu_async_enabled)
-                        schedule_work(&chunk->map_extend_work);
+                    chunk->map_used + PCPU_ATOMIC_MAP_MARGIN_LOW) {
+                        if (list_empty(&chunk->map_extend_list)) {
+                                list_add_tail(&chunk->map_extend_list,
+                                              &pcpu_map_extend_chunks);
+                                pcpu_schedule_balance_work();
+                        }
+                }
         } else {
                 margin = PCPU_ATOMIC_MAP_MARGIN_HIGH;
         }
@@ -467,20 +476,6 @@ out_unlock:
         return 0;
 }
 
-static void pcpu_map_extend_workfn(struct work_struct *work)
-{
-        struct pcpu_chunk *chunk = container_of(work, struct pcpu_chunk,
-                                                map_extend_work);
-        int new_alloc;
-
-        spin_lock_irq(&pcpu_lock);
-        new_alloc = pcpu_need_to_extend(chunk, false);
-        spin_unlock_irq(&pcpu_lock);
-
-        if (new_alloc)
-                pcpu_extend_area_map(chunk, new_alloc);
-}
-
 /**
  * pcpu_fit_in_area - try to fit the requested allocation in a candidate area
  * @chunk: chunk the candidate area belongs to
@@ -740,7 +735,7 @@ static struct pcpu_chunk *pcpu_alloc_chunk(void)
         chunk->map_used = 1;
 
         INIT_LIST_HEAD(&chunk->list);
-        INIT_WORK(&chunk->map_extend_work, pcpu_map_extend_workfn);
+        INIT_LIST_HEAD(&chunk->map_extend_list);
         chunk->free_size = pcpu_unit_size;
         chunk->contig_hint = pcpu_unit_size;
 
@@ -1129,6 +1124,7 @@ static void pcpu_balance_workfn(struct work_struct *work)
                 if (chunk == list_first_entry(free_head, struct pcpu_chunk, list))
                         continue;
 
+                list_del_init(&chunk->map_extend_list);
                 list_move(&chunk->list, &to_free);
         }
 
@@ -1146,6 +1142,25 @@ static void pcpu_balance_workfn(struct work_struct *work)
                 pcpu_destroy_chunk(chunk);
         }
 
+        /* service chunks which requested async area map extension */
+        do {
+                int new_alloc = 0;
+
+                spin_lock_irq(&pcpu_lock);
+
+                chunk = list_first_entry_or_null(&pcpu_map_extend_chunks,
+                                        struct pcpu_chunk, map_extend_list);
+                if (chunk) {
+                        list_del_init(&chunk->map_extend_list);
+                        new_alloc = pcpu_need_to_extend(chunk, false);
+                }
+
+                spin_unlock_irq(&pcpu_lock);
+
+                if (new_alloc)
+                        pcpu_extend_area_map(chunk, new_alloc);
+        } while (chunk);
+
         /*
          * Ensure there are certain number of free populated pages for
          * atomic allocs.  Fill up from the most packed so that atomic
@@ -1644,7 +1659,7 @@ int __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
          */
         schunk = memblock_virt_alloc(pcpu_chunk_struct_size, 0);
         INIT_LIST_HEAD(&schunk->list);
-        INIT_WORK(&schunk->map_extend_work, pcpu_map_extend_workfn);
+        INIT_LIST_HEAD(&schunk->map_extend_list);
         schunk->base_addr = base_addr;
         schunk->map = smap;
         schunk->map_alloc = ARRAY_SIZE(smap);
@@ -1673,7 +1688,7 @@ int __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
         if (dyn_size) {
                 dchunk = memblock_virt_alloc(pcpu_chunk_struct_size, 0);
                 INIT_LIST_HEAD(&dchunk->list);
-                INIT_WORK(&dchunk->map_extend_work, pcpu_map_extend_workfn);
+                INIT_LIST_HEAD(&dchunk->map_extend_list);
                 dchunk->base_addr = base_addr;
                 dchunk->map = dmap;
                 dchunk->map_alloc = ARRAY_SIZE(dmap);