xfs: clean up filestreams helpers

Move xfs_filestream_peek_ag, xxfs_filestream_get_ag and xfs_filestream_put_ag
from xfs_filestream.h to xfs_filestream.c where it's only callers are, and
remove the inline marker while we're at it to let the compiler decide on the
inlining.  Also don't return a value from xfs_filestream_put_ag because
we don't need it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

1 files changed, 77 insertions, 3 deletions

diff --git a/fs/xfs/xfs_filestream.c b/fs/xfs/xfs_filestream.c
index d34b9e8d2d3..9b715dce569 100644
--- a/fs/xfs/xfs_filestream.c
+++ b/fs/xfs/xfs_filestream.c
@@ -123,6 +123,82 @@ typedef struct fstrm_item
         xfs_inode_t     *pip;   /* Parent directory inode pointer. */
 } fstrm_item_t;
 
+/*
+ * Allocation group filestream associations are tracked with per-ag atomic
+ * counters.  These counters allow _xfs_filestream_pick_ag() to tell whether a
+ * particular AG already has active filestreams associated with it. The mount
+ * point's m_peraglock is used to protect these counters from per-ag array
+ * re-allocation during a growfs operation.  When xfs_growfs_data_private() is
+ * about to reallocate the array, it calls xfs_filestream_flush() with the
+ * m_peraglock held in write mode.
+ *
+ * Since xfs_mru_cache_flush() guarantees that all the free functions for all
+ * the cache elements have finished executing before it returns, it's safe for
+ * the free functions to use the atomic counters without m_peraglock protection.
+ * This allows the implementation of xfs_fstrm_free_func() to be agnostic about
+ * whether it was called with the m_peraglock held in read mode, write mode or
+ * not held at all.  The race condition this addresses is the following:
+ *
+ *  - The work queue scheduler fires and pulls a filestream directory cache
+ *    element off the LRU end of the cache for deletion, then gets pre-empted.
+ *  - A growfs operation grabs the m_peraglock in write mode, flushes all the
+ *    remaining items from the cache and reallocates the mount point's per-ag
+ *    array, resetting all the counters to zero.
+ *  - The work queue thread resumes and calls the free function for the element
+ *    it started cleaning up earlier.  In the process it decrements the
+ *    filestreams counter for an AG that now has no references.
+ *
+ * With a shrinkfs feature, the above scenario could panic the system.
+ *
+ * All other uses of the following macros should be protected by either the
+ * m_peraglock held in read mode, or the cache's internal locking exposed by the
+ * interval between a call to xfs_mru_cache_lookup() and a call to
+ * xfs_mru_cache_done().  In addition, the m_peraglock must be held in read mode
+ * when new elements are added to the cache.
+ *
+ * Combined, these locking rules ensure that no associations will ever exist in
+ * the cache that reference per-ag array elements that have since been
+ * reallocated.
+ */
+static int
+xfs_filestream_peek_ag(
+        xfs_mount_t     *mp,
+        xfs_agnumber_t  agno)
+{
+        struct xfs_perag *pag;
+        int             ret;
+
+        pag = xfs_perag_get(mp, agno);
+        ret = atomic_read(&pag->pagf_fstrms);
+        xfs_perag_put(pag);
+        return ret;
+}
+
+static int
+xfs_filestream_get_ag(
+        xfs_mount_t     *mp,
+        xfs_agnumber_t  agno)
+{
+        struct xfs_perag *pag;
+        int             ret;
+
+        pag = xfs_perag_get(mp, agno);
+        ret = atomic_inc_return(&pag->pagf_fstrms);
+        xfs_perag_put(pag);
+        return ret;
+}
+
+static void
+xfs_filestream_put_ag(
+        xfs_mount_t     *mp,
+        xfs_agnumber_t  agno)
+{
+        struct xfs_perag *pag;
+
+        pag = xfs_perag_get(mp, agno);
+        atomic_dec(&pag->pagf_fstrms);
+        xfs_perag_put(pag);
+}
 
 /*
  * Scan the AGs starting at startag looking for an AG that isn't in use and has
@@ -351,16 +427,14 @@ xfs_fstrm_free_func(
 {
         fstrm_item_t    *item  = (fstrm_item_t *)data;
         xfs_inode_t     *ip = item->ip;
-        int ref;
 
         ASSERT(ip->i_ino == ino);
 
         xfs_iflags_clear(ip, XFS_IFILESTREAM);
 
         /* Drop the reference taken on the AG when the item was added. */
-        ref = xfs_filestream_put_ag(ip->i_mount, item->ag);
+        xfs_filestream_put_ag(ip->i_mount, item->ag);
 
-        ASSERT(ref >= 0);
         TRACE_FREE(ip->i_mount, ip, item->pip, item->ag,
                 xfs_filestream_peek_ag(ip->i_mount, item->ag));