[PATCH v2 2/8] fix the broken lockdep logic in __sb_start_write()

Oleg Nesterov
2015-08-11T17:06:19Z

1. wait_event(frozen < level) without rwsem_acquire_read() is just
wrong from lockdep perspective. If we are going to deadlock
because the caller is buggy, lockdep detect this problem.

2. __sb_start_write() can race with thaw_super() + freeze_super(),
and after "goto retry" the 2nd acquire_freeze_lock() is wrong.

3. The "tell lockdep we are doing trylock" hack doesn't look nice.

I think this is correct, but this logic should be more explicit.
Yes, the recursive read_lock() is fine if we hold the lock on a
higher level. But we do not need to fool lockdep. If we can not
deadlock in this case then try-lock must not fail and we can use
use wait == F throughout this code.

Note: as Dave Chinner explains, the "trylock" hack and the fat comment
can be probably removed. But this needs a separate change and it will
be trivial: just kill __sb_start_write() and rename do_sb_start_write()
back to __sb_start_write().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
fs/super.c | 73 ++++++++++++++++++++++++++++++++---------------------------
1 files changed, 40 insertions(+), 33 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 928c20f..d0fdd49 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -1158,38 +1158,11 @@ void __sb_end_write(struct super_block *sb, int level)
}
EXPORT_SYMBOL(__sb_end_write);

-#ifdef CONFIG_LOCKDEP
-/*
- * We want lockdep to tell us about possible deadlocks with freezing but
- * it's it bit tricky to properly instrument it. Getting a freeze protection
- * works as getting a read lock but there are subtle problems. XFS for example
- * gets freeze protection on internal level twice in some cases, which is OK
- * only because we already hold a freeze protection also on higher level. Due
- * to these cases we have to tell lockdep we are doing trylock when we
- * already hold a freeze protection for a higher freeze level.
- */
-static void acquire_freeze_lock(struct super_block *sb, int level, bool trylock,
+static int do_sb_start_write(struct super_block *sb, int level, bool wait,
unsigned long ip)
{
- int i;
-
- if (!trylock) {
- for (i = 0; i < level - 1; i++)
- if (lock_is_held(&sb->s_writers.lock_map[i])) {
- trylock = true;
- break;
- }
- }
- rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, trylock, ip);
-}
-#endif
-
-/*
- * This is an internal function, please use sb_start_{write,pagefault,intwrite}
- * instead.
- */
-int __sb_start_write(struct super_block *sb, int level, bool wait)
-{
+ if (wait)
+ rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 0, ip);
retry:
if (unlikely(sb->s_writers.frozen >= level)) {
if (!wait)
@@ -1198,9 +1171,6 @@ retry:
sb->s_writers.frozen < level);
}

-#ifdef CONFIG_LOCKDEP
- acquire_freeze_lock(sb, level, !wait, _RET_IP_);
-#endif
percpu_counter_inc(&sb->s_writers.counter[level-1]);
/*
* Make sure counter is updated before we check for frozen.
@@ -1211,8 +1181,45 @@ retry:
__sb_end_write(sb, level);
goto retry;
}
+
+ if (!wait)
+ rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 1, ip);
return 1;
}
+
+/*
+ * This is an internal function, please use sb_start_{write,pagefault,intwrite}
+ * instead.
+ */
+int __sb_start_write(struct super_block *sb, int level, bool wait)
+{
+ bool force_trylock = false;
+ int ret;
+
+#ifdef CONFIG_LOCKDEP
+ /*
+ * We want lockdep to tell us about possible deadlocks with freezing
+ * but it's it bit tricky to properly instrument it. Getting a freeze
+ * protection works as getting a read lock but there are subtle
+ * problems. XFS for example gets freeze protection on internal level
+ * twice in some cases, which is OK only because we already hold a
+ * freeze protection also on higher level. Due to these cases we have
+ * to use wait == F (trylock mode) which must not fail.
+ */
+ if (wait) {
+ int i;
+
+ for (i = 0; i < level - 1; i++)
+ if (lock_is_held(&sb->s_writers.lock_map[i])) {
+ force_trylock = true;
+ break;
+ }
+ }
+#endif
+ ret = do_sb_start_write(sb, level, wait && !force_trylock, _RET_IP_);
+ WARN_ON(force_trylock & !ret);
+ return ret;
+}
EXPORT_SYMBOL(__sb_start_write);

/**
--
1.5.5.1