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Date: Thu, 10 Jan 2013 01:51:39 +0100
From: David Sterba <dsterba@...e.cz>
To: zwu.kernel@...il.com
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
viro@...iv.linux.org.uk, david@...morbit.com,
darrick.wong@...cle.com, andi@...stfloor.org, hch@...radead.org,
linuxram@...ux.vnet.ibm.com, wuzhy@...ux.vnet.ibm.com
Subject: Re: [PATCH RESEND v1 03/16] vfs: add I/O frequency update function
On Thu, Dec 20, 2012 at 10:43:22PM +0800, zwu.kernel@...il.com wrote:
> --- a/fs/hot_tracking.c
> +++ b/fs/hot_tracking.c
> @@ -164,6 +164,135 @@ static void hot_inode_tree_exit(struct hot_info *root)
> spin_unlock(&root->lock);
> }
>
> +struct hot_inode_item
> +*hot_inode_item_lookup(struct hot_info *root, u64 ino)
> +{
> + struct rb_node **p = &root->hot_inode_tree.map.rb_node;
> + struct rb_node *parent = NULL;
> + struct hot_comm_item *ci;
> + struct hot_inode_item *entry;
> +
> + /* walk tree to find insertion point */
> + spin_lock(&root->lock);
> + while (*p) {
> + parent = *p;
> + ci = rb_entry(parent, struct hot_comm_item, rb_node);
> + entry = container_of(ci, struct hot_inode_item, hot_inode);
> + if (ino < entry->i_ino)
> + p = &(*p)->rb_left;
> + else if (ino > entry->i_ino)
> + p = &(*p)->rb_right;
style comment: put { } around the all if/else blocks
> + else {
> + spin_unlock(&root->lock);
> + kref_get(&entry->hot_inode.refs);
jumping forwards in the series, the spin_unlock and kref_get get swapped
later, and I think that's the right order. Otherwise there's a small
window where the entry does not get the reference and could be
potentially freed by racing kref_put, no?
<lookup entry E>
spin_unlock(tree)
spin_lock(tree)
<lookup entry E>
kref_put(E) or via hot_inode_item_put(E) (1)
kref_get(E) (2)
if the reference count at (1) was 1, it's freed and (2) hits a free
memory. hot_inode_item_put can be called from filesystem or via seq
print of the respective /proc files, so I think there are chances to hit
the problem.
> + return entry;
> + }
> + }
> + spin_unlock(&root->lock);
> +
> + entry = kmem_cache_zalloc(hot_inode_item_cachep, GFP_NOFS);
> + if (!entry)
> + return ERR_PTR(-ENOMEM);
> +
> + spin_lock(&root->lock);
> + hot_inode_item_init(entry, ino, &root->hot_inode_tree);
> + rb_link_node(&entry->hot_inode.rb_node, parent, p);
> + rb_insert_color(&entry->hot_inode.rb_node,
> + &root->hot_inode_tree.map);
> + spin_unlock(&root->lock);
> +
> + kref_get(&entry->hot_inode.refs);
Similar here, the entry is inserted into the tree but there's no
refcount yet. And the order of spin_unlock/kref_get remains unchanged.
> + return entry;
> +}
> +EXPORT_SYMBOL_GPL(hot_inode_item_lookup);
> +
> +static struct hot_range_item
> +*hot_range_item_lookup(struct hot_inode_item *he,
> + loff_t start)
> +{
> + struct rb_node **p = &he->hot_range_tree.map.rb_node;
> + struct rb_node *parent = NULL;
> + struct hot_comm_item *ci;
> + struct hot_range_item *entry;
> +
> + /* walk tree to find insertion point */
> + spin_lock(&he->lock);
> + while (*p) {
> + parent = *p;
> + ci = rb_entry(parent, struct hot_comm_item, rb_node);
> + entry = container_of(ci, struct hot_range_item, hot_range);
> + if (start < entry->start)
> + p = &(*p)->rb_left;
> + else if (start > hot_range_end(entry))
> + p = &(*p)->rb_right;
if { ...}
else if { ... }
> + else {
> + spin_unlock(&he->lock);
> + kref_get(&entry->hot_range.refs);
same here
> + return entry;
> + }
> + }
> + spin_unlock(&he->lock);
> +
> + entry = kmem_cache_zalloc(hot_range_item_cachep, GFP_NOFS);
> + if (!entry)
> + return ERR_PTR(-ENOMEM);
> +
> + spin_lock(&he->lock);
> + hot_range_item_init(entry, start, he);
> + rb_link_node(&entry->hot_range.rb_node, parent, p);
> + rb_insert_color(&entry->hot_range.rb_node,
> + &he->hot_range_tree.map);
> + spin_unlock(&he->lock);
> +
> + kref_get(&entry->hot_range.refs);
and here
> + return entry;
> +}
> +
> +/*
> + * This function does the actual work of updating
> + * the frequency numbers, whatever they turn out to be.
Can this function be described a bit better? This comment did not help.
> + */
> +static void hot_rw_freq_calc(struct timespec old_atime,
> + struct timespec cur_time, u64 *avg)
> +{
> + struct timespec delta_ts;
> + u64 new_delta;
> +
> + delta_ts = timespec_sub(cur_time, old_atime);
> + new_delta = timespec_to_ns(&delta_ts) >> FREQ_POWER;
> +
> + *avg = (*avg << FREQ_POWER) - *avg + new_delta;
> + *avg = *avg >> FREQ_POWER;
> +}
> +
> +static void hot_freq_data_update(struct hot_freq_data *freq_data, bool write)
> +{
> + struct timespec cur_time = current_kernel_time();
> +
> + if (write) {
> + freq_data->nr_writes += 1;
The preferred style is
freq_data->nr_writes++
> + hot_rw_freq_calc(freq_data->last_write_time,
> + cur_time,
> + &freq_data->avg_delta_writes);
> + freq_data->last_write_time = cur_time;
> + } else {
> + freq_data->nr_reads += 1;
(...)
> + hot_rw_freq_calc(freq_data->last_read_time,
> + freq_data->last_read_time,
> + cur_time,
> + &freq_data->avg_delta_reads);
> + freq_data->last_read_time = cur_time;
> + }
> +}
> +
david
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