fs: dcache avoid starvation in dcache multi-step operations

Long lived dcache "multi-step" operations which retry on rename seq can
be starved with a lot of rename activity. If they fail after the 1st pass,
take the rename_lock for writing to avoid further starvation.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

1 files changed, 42 insertions, 14 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 0dbae053b664..bf6294a20f0e 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -973,10 +973,11 @@ int have_submounts(struct dentry *parent)
         struct dentry *this_parent;
         struct list_head *next;
         unsigned seq;
+        int locked = 0;
 
-rename_retry:
-        this_parent = parent;
         seq = read_seqbegin(&rename_lock);
+again:
+        this_parent = parent;
 
         if (d_mountpoint(parent))
                 goto positive;
@@ -1021,7 +1022,7 @@ resume:
                 /* might go back up the wrong parent if we have had a rename
                  * or deletion */
                 if (this_parent != child->d_parent ||
-                                read_seqretry(&rename_lock, seq)) {
+                         (!locked && read_seqretry(&rename_lock, seq))) {
                         spin_unlock(&this_parent->d_lock);
                         rcu_read_unlock();
                         goto rename_retry;
@@ -1031,13 +1032,22 @@ resume:
                 goto resume;
         }
         spin_unlock(&this_parent->d_lock);
-        if (read_seqretry(&rename_lock, seq))
+        if (!locked && read_seqretry(&rename_lock, seq))
                 goto rename_retry;
+        if (locked)
+                write_sequnlock(&rename_lock);
         return 0; /* No mount points found in tree */
 positive:
-        if (read_seqretry(&rename_lock, seq))
+        if (!locked && read_seqretry(&rename_lock, seq))
                 goto rename_retry;
+        if (locked)
+                write_sequnlock(&rename_lock);
         return 1;
+
+rename_retry:
+        locked = 1;
+        write_seqlock(&rename_lock);
+        goto again;
 }
 EXPORT_SYMBOL(have_submounts);
 
@@ -1061,11 +1071,11 @@ static int select_parent(struct dentry * parent)
         struct list_head *next;
         unsigned seq;
         int found = 0;
+        int locked = 0;
 
-rename_retry:
-        this_parent = parent;
         seq = read_seqbegin(&rename_lock);
-
+again:
+        this_parent = parent;
         spin_lock(&this_parent->d_lock);
 repeat:
         next = this_parent->d_subdirs.next;
@@ -1127,7 +1137,7 @@ resume:
                 /* might go back up the wrong parent if we have had a rename
                  * or deletion */
                 if (this_parent != child->d_parent ||
-                                read_seqretry(&rename_lock, seq)) {
+                        (!locked && read_seqretry(&rename_lock, seq))) {
                         spin_unlock(&this_parent->d_lock);
                         rcu_read_unlock();
                         goto rename_retry;
@@ -1138,9 +1148,18 @@ resume:
         }
 out:
         spin_unlock(&this_parent->d_lock);
-        if (read_seqretry(&rename_lock, seq))
+        if (!locked && read_seqretry(&rename_lock, seq))
                 goto rename_retry;
+        if (locked)
+                write_sequnlock(&rename_lock);
         return found;
+
+rename_retry:
+        if (found)
+                return found;
+        locked = 1;
+        write_seqlock(&rename_lock);
+        goto again;
 }
 
 /**
@@ -2655,10 +2674,11 @@ void d_genocide(struct dentry *root)
         struct dentry *this_parent;
         struct list_head *next;
         unsigned seq;
+        int locked = 0;
 
-rename_retry:
-        this_parent = root;
         seq = read_seqbegin(&rename_lock);
+again:
+        this_parent = root;
         spin_lock(&this_parent->d_lock);
 repeat:
         next = this_parent->d_subdirs.next;
@@ -2703,7 +2723,7 @@ resume:
                 /* might go back up the wrong parent if we have had a rename
                  * or deletion */
                 if (this_parent != child->d_parent ||
-                                read_seqretry(&rename_lock, seq)) {
+                         (!locked && read_seqretry(&rename_lock, seq))) {
                         spin_unlock(&this_parent->d_lock);
                         rcu_read_unlock();
                         goto rename_retry;
@@ -2713,8 +2733,16 @@ resume:
                 goto resume;
         }
         spin_unlock(&this_parent->d_lock);
-        if (read_seqretry(&rename_lock, seq))
+        if (!locked && read_seqretry(&rename_lock, seq))
                 goto rename_retry;
+        if (locked)
+                write_sequnlock(&rename_lock);
+        return;
+
+rename_retry:
+        locked = 1;
+        write_seqlock(&rename_lock);
+        goto again;
 }
 
 /**