On 2012/09/25 18:52, Jan Kara wrote:

On Tue 25-09-12 18:42:16, Fernando Luis Vazquez Cao wrote:

quoted

On 2012年09月25日 18:11, Jan Kara wrote:

quoted

On Fri 14-09-12 15:45:04, Fernando Luis Vázquez Cao wrote:

quoted

iterate_supers() calls a function provided by the caller with the s_umount
semaphore taken in read mode. However, there may be cases where write mode
is preferable, so we add __iterate_supers(), which lets one
specify the mode of the lock, and replace iterate_supers with two helpers
around __iterate_supers(), iterate_supers_read() and iterate_supers_write().

This will be used to fix the emergency thaw (filesystem unfreeze) code, which
iterates over the list of superblocks but needs to hold the s_umount semaphore
in _write_ mode bebore carrying out the actual thaw operation.

This patch introduces no semantic changes since iterate_supers() users become
iterate_supers_read() which is equivalent.

Cc: Josef Bacik <redacted>
Cc: Eric Sandeen <redacted>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <redacted>
Signed-off-by: Fernando Luis Vazquez Cao <redacted>
---

...

quoted

diff -urNp linux-3.6-rc5-orig/fs/super.c linux-3.6-rc5/fs/super.c

--- linux-3.6-rc5-orig/fs/super.c	2012-09-14 11:53:43.416703312 +0900
+++ linux-3.6-rc5/fs/super.c	2012-09-14 12:30:52.188833193 +0900

@@ -537,14 +537,22 @@ void drop_super(struct super_block *sb)
  EXPORT_SYMBOL(drop_super);
  /**
- *	iterate_supers - call function for all active superblocks
+ *	__iterate_supers - call function for all active superblocks
   *	@f: function to call
   *	@arg: argument to pass to it
+ *	@wlock: mode of superblock lock (false->read lock, true->write lock)
   *
   *	Scans the superblock list and calls given function, passing it
   *	locked superblock and given argument.
+ *
+ *	When the caller asks for the superblock lock (s_umount semaphore) to be
+ *	taken in write mode, the lock is taken but not released because the
+ *	function provided by the caller may deactivate the superblock itself.
+ *	It is that function's job to unlock the superblock as needed in such a
+ *	case.
   */
-void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
+static void __iterate_supers(void (*f)(struct super_block *, void *), void *arg,
+			     bool wlock)
  {
  	struct super_block *sb, *p = NULL;

@@ -555,10 +563,19 @@ void iterate_supers(void (*f)(struct sup
  		sb->s_count++;
  		spin_unlock(&sb_lock);
-		down_read(&sb->s_umount);
+		if (wlock)
+			down_write(&sb->s_umount);
+		else
+			down_read(&sb->s_umount);
+
  		if (sb->s_root && (sb->s_flags & MS_BORN))
  			f(sb, arg);
-		up_read(&sb->s_umount);
+
+		/* When the semaphore was taken in write mode the function
+		 * provided by the caller takes care of unlocking it as
+		 * needed. See explanation above for details. */
+		if (!wlock)
+			up_read(&sb->s_umount);
  		spin_lock(&sb_lock);
  		if (p)

   These locking rules are ugly and counterintuitive. People will easily
get them wrong and create bugs. I'd rather see emergency thaw retake the
s_umount semaphore so that iterate_supers() can drop it...

I guess you are referring to treating the write lock differently
and not dropping the lock inside __iterate_supers(). The
problem is that f() may release the last reference to the
superblock which in turn will go away, so letting
__iterate_supers() drop the lock is not safe (I added a
comment about this issue in the function itself).

   Well, except that iterate_supers() actually takes a passive reference
(s_count) of the superblock. Thus deactivate_locked_super() will never
really destroy it. So what I propose should be safe.

Good point. I missed the fact that we are taking a passive
reference there, which should simplifying locking. I will
take that into account for the next revision.

Thanks,
Fernando
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