Messages in this thread Patch in this message | | | Date | Mon, 30 Jan 2006 20:08:14 +0530 | From | Balbir Singh <> | Subject | Re: [PATCH] shrink_dcache_parent() races against shrink_dcache_memory() |
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> > New patch attached below. Comments are welcome. > > Regards, > Jan > [snip]
> From: Jan Blunck <jblunck@suse.de> > Subject: Fix shrink_dcache_parent() against shrink_dcache_memory() race > References: 136310 > > Kirill Korotaev <dev@sw.ru> discovered a race between shrink_dcache_parent() > and shrink_dcache_memory() which leads to "Busy inodes after unmount". > When unmounting a file system shrink_dcache_parent() is racing against a > possible shrink_dcache_memory(). This might lead to the situation that > shrink_dcache_parent() is returning too early. In this situation the > super_block is destroyed before shrink_dcache_memory() could put the inode. > > This patch fixes the problem through introducing a prunes counter which is > incremented when a dentry is pruned but the corresponding inoded isn't put yet. > When the prunes counter is not null, shrink_dcache_parent() is waiting and > restarting its work. > > Signed-off-by: Jan Blunck <jblunck@suse.de> > > --- > > fs/dcache.c | 36 ++++++++++++++++++++++++++++++++++++ > fs/super.c | 4 +++- > include/linux/fs.h | 3 +++ > 3 files changed, 42 insertions(+), 1 deletion(-) > > Index: linux-2.6/fs/dcache.c > =================================================================== > --- linux-2.6.orig/fs/dcache.c > +++ linux-2.6/fs/dcache.c > @@ -364,17 +364,21 @@ restart: > */ > static inline void prune_one_dentry(struct dentry * dentry) > { > + struct super_block *sb = dentry->d_sb; > struct dentry * parent; > > __d_drop(dentry); > list_del(&dentry->d_u.d_child); > dentry_stat.nr_dentry--; /* For d_free, below */ > + sb->s_prunes++; > dentry_iput(dentry); > parent = dentry->d_parent; > d_free(dentry); > if (parent != dentry) > dput(parent); > spin_lock(&dcache_lock); > + sb->s_prunes--; > + wake_up(&sb->s_wait_prunes); > } >
We can think about optimizing this to if (!sb->sprunes) wake_up(&sb->s_wait_prunes);
> /** > @@ -623,6 +627,34 @@ out: > return found; > } > > +static int wait_on_prunes(struct super_block *sb) > +{ > + DEFINE_WAIT(wait); > + > + spin_lock(&dcache_lock); > + if (!sb->s_prunes) { > + spin_unlock(&dcache_lock); > + return 0; > + } > + > + printk(KERN_DEBUG "%s: waiting for %d prunes\n", __FUNCTION__, > + sb->s_prunes); > + > + while (1) { > + prepare_to_wait(&sb->s_wait_prunes, &wait, > + TASK_UNINTERRUPTIBLE); > + if (!sb->s_prunes) > + break; > + spin_unlock(&dcache_lock); > + schedule(); > + spin_lock(&dcache_lock); > + } > + > + finish_wait(&sb->s_wait_prunes, &wait); > + spin_unlock(&dcache_lock); > + return 1; > +} > + > /** > * shrink_dcache_parent - prune dcache > * @parent: parent of entries to prune > @@ -634,8 +666,12 @@ void shrink_dcache_parent(struct dentry > { > int found; > > + again: > while ((found = select_parent(parent)) != 0) > prune_dcache(found); > + > + if (wait_on_prunes(parent->d_sb)) > + goto again; > }
Is the goto again required? At this point select_parent() should have pruned all entries, except those missed due to the race. These should be captured by sb->s_prunes. Once the code comes out of wait_on_prunes() everything should be ok since a dput has happened on the missed parent dentries.
> > /** > Index: linux-2.6/fs/super.c > =================================================================== > --- linux-2.6.orig/fs/super.c > +++ linux-2.6/fs/super.c > @@ -80,6 +80,8 @@ static struct super_block *alloc_super(v > sema_init(&s->s_dquot.dqio_sem, 1); > sema_init(&s->s_dquot.dqonoff_sem, 1); > init_rwsem(&s->s_dquot.dqptr_sem); > + s->s_prunes = 0; > + init_waitqueue_head(&s->s_wait_prunes); > init_waitqueue_head(&s->s_wait_unfrozen); > s->s_maxbytes = MAX_NON_LFS; > s->dq_op = sb_dquot_ops; > @@ -230,8 +232,8 @@ void generic_shutdown_super(struct super > > if (root) { > sb->s_root = NULL; > - shrink_dcache_parent(root); > shrink_dcache_anon(&sb->s_anon); > + shrink_dcache_parent(root); > dput(root); > fsync_super(sb); > lock_super(sb); > Index: linux-2.6/include/linux/fs.h > =================================================================== > --- linux-2.6.orig/include/linux/fs.h > +++ linux-2.6/include/linux/fs.h > @@ -833,6 +833,9 @@ struct super_block { > struct list_head s_instances; > struct quota_info s_dquot; /* Diskquota specific options */ > > + int s_prunes;
Can this be an unsigned int? Perhaps you might to mention that is protected by the dcache_lock.
> + wait_queue_head_t s_wait_prunes; > + > int s_frozen; > wait_queue_head_t s_wait_unfrozen; >
Your fix seems correct at first sight and good to be included. But could you please do a correctness/speed/cost analysis of your fix with the fix I previously sent out?
Regards, Balbir - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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