Date:	Wed, 28 May 2008 11:18:59 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, pbadari@...ibm.com,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH-v2] JBD: Fix race between free buffer and commit
	trasanction

On Sun, 2008-05-25 at 00:44 +0200, Jan Kara wrote:
> > On Wed, 2008-05-21 at 01:53 +0200, Jan Kara wrote:
> > > >  fs/jbd/transaction.c |   55 +++++++++++++++++++++++++++++++++++++++++++++++++--
> > > >  mm/filemap.c         |    3 --
> > > >  2 files changed, 54 insertions(+), 4 deletions(-)
> > > > 
> > > > Index: linux-2.6.26-rc2/fs/jbd/transaction.c
> > > > ===================================================================
> > > > --- linux-2.6.26-rc2.orig/fs/jbd/transaction.c	2008-05-11 17:09:41.000000000 -0700
> > > > +++ linux-2.6.26-rc2/fs/jbd/transaction.c	2008-05-19 16:16:41.000000000 -0700
> > > > @@ -1648,12 +1648,39 @@ out:
> > > >  	return;
> > > >  }
> > > >  
> > > > +/*
> > > > + * journal_try_to_free_buffers() could race with journal_commit_transaction()
> > > > + * The later might still hold the reference count to the buffers when inspecting
> > > > + * them on t_syncdata_list or t_locked_list.
> > > > + *
> > > > + * Journal_try_to_free_buffers() will call this function to
> > > > + * wait for the current transaction to finish syncing data buffers, before
> > > > + * try to free that buffer.
> > > > + *
> > > > + * Called with journal->j_state_lock hold.
> > > > + */
> > > > +static void journal_wait_for_transaction_sync_data(journal_t *journal)
> > > > +{
> > > > +	transaction_t *transaction = NULL;
> > > > +	tid_t tid;
> > > > +
> > > > +	transaction = journal->j_committing_transaction;
> > > > +
> > > > +	if (!transaction)
> > > > +		return;
> > > > +
> > > > +	tid = transaction->t_tid;
> > > > +	spin_unlock(&journal->j_state_lock);
> > > > +	log_wait_commit(journal, tid);
> > > > +	spin_lock(&journal->j_state_lock);
> > > > +}
> > >   What is actually the point of entering the function with j_state_lock
> > > held and also keeping it after return? It should be enough to take it
> > > and release it just inside this function, shouldn't it?
> > > 
> > 
> > I was worried about the case when we call try_to_free_buffers() again,
> > it races with the current transaction commit again. Is it possible? I
> > guess the question is whether it is possible to have buffers on the same
> > page attached to different transaction.  If so, I think we need to keep
> > the journal state lock while retry try_to_free_buffers(), so that the
> > retry won't race with the commit transaction again...
>   Well, but by the time log_wait_commit() finishes, it may well
> happen that a new transaction is already started so your lock doesn't
> help you much. And the page you are called on is actually locked, so
> noone can really mess with it until you unlock it... So I think you can
> just use the lock for obtaining tid and then drop it.
> 

You are right that the page was locked during the process we are trying
to free the buffer. so I agree it's safe to drop the lock. 

> 								Honza
> 
> PS: For JBD2 you'd need to be a bit more careful because you cannot call
> log_wait_commit() while holding page lock (we have reversed locking
> order for ext4) - but ordered-mode rewrite patch actually fixes this
> problem and I'm going to submit the splitted patches on Monday or
> Tuesday (I only need to test them that I didn't do something stupid
> while porting them to ext4)...
> 
Thanks for pointing this out. I think when we put back the reversed
locking order and new ordered mode the jbd2 patch could go away...

Updated patch for JBD (take 4) below.
Mingming

JBD: fix race between journal_try_to_free_buffers() and jbd commit transaction

From: Mingming Cao <cmm@...ibm.com>

journal_try_to_free_buffers() could race with jbd commit transaction when
the later is holding the buffer reference while waiting for the data buffer
to flush to disk. If the caller of journal_try_to_free_buffers() request
tries hard to release the buffers, it will treat the failure as error and return
back to the caller. We have seen the directo IO failed due to this race. 
Some of the caller of releasepage() also expecting the buffer to be dropped
when passed with GFP_KERNEL mask to the releasepage()->journal_try_to_free_buffers().

With this patch, if the caller is passing the GFP_KERNEL to indicating this
call could wait, in case of try_to_free_buffers() failed, let's waiting for
journal_commit_transaction() to finish commit the current committing transaction
, then try to free those buffers again with journal locked.

Signed-off-by: Mingming Cao <cmm@...ibm.com>
Reviewed-by: Badari Pulavarty <pbadari@...ibm.com> 
---
 fs/jbd/transaction.c |   57 +++++++++++++++++++++++++++++++++++++++++++++++++--
 mm/filemap.c         |    3 --
 2 files changed, 56 insertions(+), 4 deletions(-)

Index: linux-2.6.26-rc3/fs/jbd/transaction.c
===================================================================
--- linux-2.6.26-rc3.orig/fs/jbd/transaction.c	2008-05-28 10:55:37.000000000 -0700
+++ linux-2.6.26-rc3/fs/jbd/transaction.c	2008-05-28 10:57:32.000000000 -0700
@@ -1648,12 +1648,42 @@ out:
 	return;
 }
 
+/*
+ * journal_try_to_free_buffers() could race with journal_commit_transaction()
+ * The later might still hold the reference count to the buffers when inspecting
+ * them on t_syncdata_list or t_locked_list.
+ *
+ * Journal_try_to_free_buffers() will call this function to
+ * wait for the current transaction to finish syncing data buffers, before
+ * try to free that buffer.
+ *
+ * Called with journal->j_state_lock hold.
+ */
+static void journal_wait_for_transaction_sync_data(journal_t *journal)
+{
+	transaction_t *transaction = NULL;
+	tid_t tid;
+
+	spin_lock(&journal->j_state_lock);
+	transaction = journal->j_committing_transaction;
+
+	if (!transaction) {
+		spin_unlock(&journal->j_state_lock);
+		return;
+	}
+
+	tid = transaction->t_tid;
+	spin_unlock(&journal->j_state_lock);
+	log_wait_commit(journal, tid);
+}
 
 /**
  * int journal_try_to_free_buffers() - try to free page buffers.
  * @journal: journal for operation
  * @page: to try and free
- * @unused_gfp_mask: unused
+ * @gfp_mask: we use the mask to detect how hard should we try to release
+ * buffers. If __GFP_WAIT and __GFP_FS is set, we wait for commit code to
+ * release the buffers.
  *
  *
  * For all the buffers on this page,
@@ -1682,9 +1712,11 @@ out:
  * journal_try_to_free_buffer() is changing its state.  But that
  * cannot happen because we never reallocate freed data as metadata
  * while the data is part of a transaction.  Yes?
+ *
+ * Return 0 on failure, 1 on success
  */
 int journal_try_to_free_buffers(journal_t *journal,
-				struct page *page, gfp_t unused_gfp_mask)
+				struct page *page, gfp_t gfp_mask)
 {
 	struct buffer_head *head;
 	struct buffer_head *bh;
@@ -1713,7 +1745,28 @@ int journal_try_to_free_buffers(journal_
 		if (buffer_jbd(bh))
 			goto busy;
 	} while ((bh = bh->b_this_page) != head);
+
 	ret = try_to_free_buffers(page);
+
+ 	/*
+	 * There are a number of places where journal_try_to_free_buffers()
+	 * could race with journal_commit_transaction(), the later still
+	 * holds the reference to the buffers to free while processing them.
+	 * try_to_free_buffers() failed to free those buffers. Some of the
+	 * caller of releasepage() request page buffers to be dropped, otherwise
+	 * treat the fail-to-free as errors (such as generic_file_direct_IO())
+	 *
+	 * So, if the caller of try_to_release_page() wants the synchronous
+	 * behaviour(i.e make sure buffers are dropped upon return),
+	 * let's wait for the current transaction to finish flush of
+	 * dirty data buffers, then try to free those buffers again,
+	 * with the journal locked.
+	 */
+	if (ret == 0 && (gfp_mask & GFP_KERNEL == GFP_KERNEL)) {
+		journal_wait_for_transaction_sync_data(journal);
+		ret = try_to_free_buffers(page);
+	}
+
 busy:
 	return ret;
 }
Index: linux-2.6.26-rc3/mm/filemap.c
===================================================================
--- linux-2.6.26-rc3.orig/mm/filemap.c	2008-05-28 10:55:38.000000000 -0700
+++ linux-2.6.26-rc3/mm/filemap.c	2008-05-28 10:55:43.000000000 -0700
@@ -2581,9 +2581,8 @@ out:
  * Otherwise return zero.
  *
  * The @gfp_mask argument specifies whether I/O may be performed to release
- * this page (__GFP_IO), and whether the call may block (__GFP_WAIT).
+ * this page (__GFP_IO), and whether the call may block (__GFP_WAIT & __GFP_FS).
  *
- * NOTE: @gfp_mask may go away, and this function may become non-blocking.
  */
 int try_to_release_page(struct page *page, gfp_t gfp_mask)
 {


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