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Date:   Fri, 2 Jul 2021 18:11:44 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, linuxppc-dev@...ts.ozlabs.org,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [powerpc][5.13.0-next-20210701] Kernel crash while running
 ltp(chdir01) tests

On Fri, Jul 02, 2021 at 12:11:54PM -0400, Theodore Ts'o wrote:
> So it probably makes more sense to keep jbd2_journal_unregister_shrinker()
> in jbd2_destroy_journal(), since arguably the fact that we are using a
> shrinker is an internal implementation detail, and the users of jbd2
> ideally shouldn't need to be expected to know they have unregister
> jbd2's shirnkers.
> 
> Similarly, perhaps we should be moving jbd2_journal_register_shirnker()
> into jbd2_journal_init_common().  We can un-export the register and
> unshrink register functions, and declare them as static functions internal
> to fs/jbd2/journal.c.
> 
> What do you think?

Like this...

commit 8f9e16badb8fda3391e03146a47c93e76680efaf
Author: Theodore Ts'o <tytso@....edu>
Date:   Fri Jul 2 18:05:03 2021 -0400

    ext4: fix doubled call to jbd2_journal_unregister_shrinker()
    
    On Power and ARM platforms this was causing kernel crash when
    unmounting the file system, due to a percpu_counter getting destroyed
    twice.
    
    Fix this by cleaning how the jbd2 shrinker is initialized and
    uninitiazed by making it solely the responsibility of
    fs/jbd2/journal.c.
    
    Fixes: 4ba3fcdde7e3 ("jbd2,ext4: add a shrinker to release checkpointed buffers")
    Reported-by: Sachin Sant <sachinp@...ux.vnet.ibm.com>
    Signed-off-by: Theodore Ts'o <tytso@....edu>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index b8ff0399e171..dfa09a277b56 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1184,7 +1184,6 @@ static void ext4_put_super(struct super_block *sb)
 	ext4_unregister_sysfs(sb);
 
 	if (sbi->s_journal) {
-		jbd2_journal_unregister_shrinker(sbi->s_journal);
 		aborted = is_journal_aborted(sbi->s_journal);
 		err = jbd2_journal_destroy(sbi->s_journal);
 		sbi->s_journal = NULL;
@@ -5176,7 +5175,6 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_ea_block_cache = NULL;
 
 	if (sbi->s_journal) {
-		jbd2_journal_unregister_shrinker(sbi->s_journal);
 		jbd2_journal_destroy(sbi->s_journal);
 		sbi->s_journal = NULL;
 	}
@@ -5502,12 +5500,6 @@ static int ext4_load_journal(struct super_block *sb,
 		ext4_commit_super(sb);
 	}
 
-	err = jbd2_journal_register_shrinker(journal);
-	if (err) {
-		EXT4_SB(sb)->s_journal = NULL;
-		goto err_out;
-	}
-
 	return 0;
 
 err_out:
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 152880c298ca..2595703aca51 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -99,6 +99,8 @@ EXPORT_SYMBOL(jbd2_journal_begin_ordered_truncate);
 EXPORT_SYMBOL(jbd2_inode_cache);
 
 static int jbd2_journal_create_slab(size_t slab_size);
+static int jbd2_journal_register_shrinker(journal_t *journal);
+static void jbd2_journal_unregister_shrinker(journal_t *journal);
 
 #ifdef CONFIG_JBD2_DEBUG
 void __jbd2_debug(int level, const char *file, const char *func,
@@ -2043,7 +2045,8 @@ int jbd2_journal_load(journal_t *journal)
 		goto recovery_error;
 
 	journal->j_flags |= JBD2_LOADED;
-	return 0;
+
+	return jbd2_journal_register_shrinker(journal);
 
 recovery_error:
 	printk(KERN_WARNING "JBD2: recovery failed\n");
@@ -2099,7 +2102,7 @@ static unsigned long jbd2_journal_shrink_count(struct shrinker *shrink,
  * Init a percpu counter to record the checkpointed buffers on the checkpoint
  * list and register a shrinker to release their journal_head.
  */
-int jbd2_journal_register_shrinker(journal_t *journal)
+static int jbd2_journal_register_shrinker(journal_t *journal)
 {
 	int err;
 
@@ -2122,7 +2125,6 @@ int jbd2_journal_register_shrinker(journal_t *journal)
 
 	return 0;
 }
-EXPORT_SYMBOL(jbd2_journal_register_shrinker);
 
 /**
  * jbd2_journal_unregister_shrinker()
@@ -2130,12 +2132,13 @@ EXPORT_SYMBOL(jbd2_journal_register_shrinker);
  *
  * Unregister the checkpointed buffer shrinker and destroy the percpu counter.
  */
-void jbd2_journal_unregister_shrinker(journal_t *journal)
+static void jbd2_journal_unregister_shrinker(journal_t *journal)
 {
-	percpu_counter_destroy(&journal->j_jh_shrink_count);
-	unregister_shrinker(&journal->j_shrinker);
+	if (journal->j_shrinker.flags & SHRINKER_REGISTERED) {
+		percpu_counter_destroy(&journal->j_jh_shrink_count);
+		unregister_shrinker(&journal->j_shrinker);
+	}
 }
-EXPORT_SYMBOL(jbd2_journal_unregister_shrinker);
 
 /**
  * jbd2_journal_destroy() - Release a journal_t structure.
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 6cc035321562..632afbe4b18f 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1556,8 +1556,6 @@ extern int	   jbd2_journal_set_features
 		   (journal_t *, unsigned long, unsigned long, unsigned long);
 extern void	   jbd2_journal_clear_features
 		   (journal_t *, unsigned long, unsigned long, unsigned long);
-extern int	   jbd2_journal_register_shrinker(journal_t *journal);
-extern void	   jbd2_journal_unregister_shrinker(journal_t *journal);
 extern int	   jbd2_journal_load       (journal_t *journal);
 extern int	   jbd2_journal_destroy    (journal_t *);
 extern int	   jbd2_journal_recover    (journal_t *journal);

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