xfs: fix race while discarding buffers [V4]

While xfs_buftarg_shrink() is freeing buffers from the dispose list (filled with
buffers from lru list), there is a possibility to have xfs_buf_stale() racing
with it, and removing buffers from dispose list before xfs_buftarg_shrink() does
it.

This happens because xfs_buftarg_shrink() handle the dispose list without
locking and the test condition in xfs_buf_stale() checks for the buffer being in
*any* list:

if (!list_empty(&bp->b_lru))

If the buffer happens to be on dispose list, this causes the buffer counter of
lru list (btp->bt_lru_nr) to be decremented twice (once in xfs_buftarg_shrink()
and another in xfs_buf_stale()) causing a wrong account usage of the lru list.

This may cause xfs_buftarg_shrink() to return a wrong value to the memory
shrinker shrink_slab(), and such account error may also cause an underflowed
value to be returned; since the counter is lower than the current number of
items in the lru list, a decrement may happen when the counter is 0, causing
an underflow on the counter.

The fix uses a new flag field (and a new buffer flag) to serialize buffer
handling during the shrink process. The new flag field has been designed to use
btp->bt_lru_lock/unlock instead of xfs_buf_lock/unlock mechanism.

dchinner, sandeen, aquini and aris also deserve credits for this.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

2 files changed, 28 insertions, 18 deletions

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index d7a9dd735e1e..933b7930b863 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -96,6 +96,7 @@ xfs_buf_lru_add(
                 atomic_inc(&bp->b_hold);
                 list_add_tail(&bp->b_lru, &btp->bt_lru);
                 btp->bt_lru_nr++;
+                bp->b_lru_flags &= ~_XBF_LRU_DISPOSE;
         }
         spin_unlock(&btp->bt_lru_lock);
 }
@@ -154,7 +155,8 @@ xfs_buf_stale(
                 struct xfs_buftarg *btp = bp->b_target;
 
                 spin_lock(&btp->bt_lru_lock);
-                if (!list_empty(&bp->b_lru)) {
+                if (!list_empty(&bp->b_lru) &&
+                    !(bp->b_lru_flags & _XBF_LRU_DISPOSE)) {
                         list_del_init(&bp->b_lru);
                         btp->bt_lru_nr--;
                         atomic_dec(&bp->b_hold);
@@ -1501,6 +1503,7 @@ xfs_buftarg_shrink(
                  */
                 list_move(&bp->b_lru, &dispose);
                 btp->bt_lru_nr--;
+                bp->b_lru_flags |= _XBF_LRU_DISPOSE;
         }
         spin_unlock(&btp->bt_lru_lock);
 
diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
index d03b73b9604e..7c0b6a0a1557 100644
--- a/fs/xfs/xfs_buf.h
+++ b/fs/xfs/xfs_buf.h
@@ -38,27 +38,28 @@ typedef enum {
         XBRW_ZERO = 3,                  /* Zero target memory */
 } xfs_buf_rw_t;
 
-#define XBF_READ        (1 << 0) /* buffer intended for reading from device */
-#define XBF_WRITE       (1 << 1) /* buffer intended for writing to device */
-#define XBF_READ_AHEAD  (1 << 2) /* asynchronous read-ahead */
-#define XBF_ASYNC       (1 << 4) /* initiator will not wait for completion */
-#define XBF_DONE        (1 << 5) /* all pages in the buffer uptodate */
-#define XBF_STALE       (1 << 6) /* buffer has been staled, do not find it */
+#define XBF_READ         (1 << 0) /* buffer intended for reading from device */
+#define XBF_WRITE        (1 << 1) /* buffer intended for writing to device */
+#define XBF_READ_AHEAD   (1 << 2) /* asynchronous read-ahead */
+#define XBF_ASYNC        (1 << 4) /* initiator will not wait for completion */
+#define XBF_DONE         (1 << 5) /* all pages in the buffer uptodate */
+#define XBF_STALE        (1 << 6) /* buffer has been staled, do not find it */
 
 /* I/O hints for the BIO layer */
-#define XBF_SYNCIO      (1 << 10)/* treat this buffer as synchronous I/O */
-#define XBF_FUA         (1 << 11)/* force cache write through mode */
-#define XBF_FLUSH       (1 << 12)/* flush the disk cache before a write */
+#define XBF_SYNCIO       (1 << 10)/* treat this buffer as synchronous I/O */
+#define XBF_FUA          (1 << 11)/* force cache write through mode */
+#define XBF_FLUSH        (1 << 12)/* flush the disk cache before a write */
 
 /* flags used only as arguments to access routines */
-#define XBF_TRYLOCK     (1 << 16)/* lock requested, but do not wait */
-#define XBF_UNMAPPED    (1 << 17)/* do not map the buffer */
+#define XBF_TRYLOCK      (1 << 16)/* lock requested, but do not wait */
+#define XBF_UNMAPPED     (1 << 17)/* do not map the buffer */
 
 /* flags used only internally */
-#define _XBF_PAGES      (1 << 20)/* backed by refcounted pages */
-#define _XBF_KMEM       (1 << 21)/* backed by heap memory */
-#define _XBF_DELWRI_Q   (1 << 22)/* buffer on a delwri queue */
-#define _XBF_COMPOUND   (1 << 23)/* compound buffer */
+#define _XBF_PAGES       (1 << 20)/* backed by refcounted pages */
+#define _XBF_KMEM        (1 << 21)/* backed by heap memory */
+#define _XBF_DELWRI_Q    (1 << 22)/* buffer on a delwri queue */
+#define _XBF_COMPOUND    (1 << 23)/* compound buffer */
+#define _XBF_LRU_DISPOSE (1 << 24)/* buffer being discarded */
 
 typedef unsigned int xfs_buf_flags_t;
 
@@ -72,12 +73,13 @@ typedef unsigned int xfs_buf_flags_t;
         { XBF_SYNCIO,           "SYNCIO" }, \
         { XBF_FUA,              "FUA" }, \
         { XBF_FLUSH,            "FLUSH" }, \
-        { XBF_TRYLOCK,          "TRYLOCK" },    /* should never be set */\
+        { XBF_TRYLOCK,          "TRYLOCK" },    /* should never be set */\
         { XBF_UNMAPPED,         "UNMAPPED" },   /* ditto */\
         { _XBF_PAGES,           "PAGES" }, \
         { _XBF_KMEM,            "KMEM" }, \
         { _XBF_DELWRI_Q,        "DELWRI_Q" }, \
-        { _XBF_COMPOUND,        "COMPOUND" }
+        { _XBF_COMPOUND,        "COMPOUND" }, \
+        { _XBF_LRU_DISPOSE,     "LRU_DISPOSE" }
 
 typedef struct xfs_buftarg {
         dev_t                   bt_dev;
@@ -124,7 +126,12 @@ typedef struct xfs_buf {
         xfs_buf_flags_t         b_flags;        /* status flags */
         struct semaphore        b_sema;         /* semaphore for lockables */
 
+        /*
+         * concurrent access to b_lru and b_lru_flags are protected by
+         * bt_lru_lock and not by b_sema
+         */
         struct list_head        b_lru;          /* lru list */
+        xfs_buf_flags_t         b_lru_flags;    /* internal lru status flags */
         wait_queue_head_t       b_waiters;      /* unpin waiters */
         struct list_head        b_list;
         struct xfs_perag        *b_pag;         /* contains rbtree root */