Bug 1326472

Description of problem:

While auditing the diff between the normal RHEL7 kernel and the RHEL7 kernel-rt tree (git diff kernel-3.10.0-376.el7..kernel-rt-3.10.0-376.rt56.253.el7) I found this gem:

diff --git a/fs/fscache/page.c b/fs/fscache/page.c
index e1500bda3bfc..966aa4376037 100644
--- a/fs/fscache/page.c
+++ b/fs/fscache/page.c
@@ -887,6 +887,7 @@ void fscache_invalidate_writes(struct fscache_cookie *cookie)

                for (i = n - 1; i >= 0; i--)
                        page_cache_release(results[i]);
+               spin_lock(&cookie->stores_lock);
        }

        _leave("");

This will lead to a deadlock, since normal RHEL7 kernel has also merged an
equivalent of db7a50353, and takes the spinlock as the first thing when going around the loop:

        for (;;) {
                spin_lock(&cookie->stores_lock);
                n = radix_tree_gang_lookup_tag(&cookie->stores, results, 0,
                                               ARRAY_SIZE(results),
                                               FSCACHE_COOKIE_PENDING_TAG);
                if (n == 0) {
                        spin_unlock(&cookie->stores_lock);
                        break;
                }

                for (i = n - 1; i >= 0; i--) {
                        page = results[i];
                        radix_tree_delete(&cookie->stores, page->index);
                }

                spin_unlock(&cookie->stores_lock);

                for (i = n - 1; i >= 0; i--)
                        page_cache_release(results[i]);
                spin_lock(&cookie->stores_lock);
        }


Version-Release number of selected component (if applicable):

kernel-rt-3.10.0-376.rt56.253.el7