Re: 2.6.21-rc suspend regression: sysfs deadlock

Linus Torvalds
2007-03-07T02:00:30Z

On Tue, 6 Mar 2007, Hugh Dickins wrote:
>
> This comes from Oliver's commit 94bebf4d1b8e7719f0f3944c037a21cfd99a4af7
> Driver core: fix race in sysfs between sysfs_remove_file() and read()/write()
> in 2.6.21-rc1. It looks to me like sysfs_write_file downs buffer->sem
> while calling flush_write_buffer, and flushing that particular write
> buffer entails downing buffer->sem in orphan_all_buffers.

Gaah. What a crock.

I really don't see any alternative to just reverting the whole change.
Hugh's patch is simple, but rather pointless.

The fact is, the whole change is *bogus*.

We don't "lock" datastructures. We *reference count* them!

This is so fundamental that it's even mentioned in the file
Documentation/CodingStyle in "Chapter 11: Data structures".

The whole "orphaned" kind of locking is broken. It's stupid. The way we do
races between removal and use is that initial setup sets a reference count
of 1, and something really simple like:

static inline struct sysfs_buffer *get_sysfs_buffer(struct inode *inode)
{
struct sysfs_buffer *buffer = inode->i_private;

BUG_ON(!mutex_locked(&inode->i_mutex));
if (buffer)
atomic_inc(&buffer->count);
return buffer;
}

static inline void put_sysfs_buffer(struct sysfs_buffer *buffer)
{
if (atomic_dec_and_test(&buffer->count))
kfree(buffer);
}

and then the rule is:

- everybody uses "get_sysfs_buffer()" to follow the reference (and yes,
you obviously have to hold "inode->i_mutex" for this to be safe! I
added the BUG_ON() as an example)

- everybody uses "put_buffer()" to release it (and we simply don't *care*
whether somebody else released it too, since everybody has a reference
count)

- removing the buffer is now just

mutex_lock(&inode->i_mutex);
buffer = inode->i_private;
inode->i_private = NULL;
mutex_unlock(&inode->i_mutex);

put_sysfs_buffer(buffer);

- everybody is happy!

Anyway, I'm unable to revert the broken commit, since there are now other
changes that depend on it, but can somebody *please* do that? I'll apply
Hugh's silly patch in the meantime, just to avoid the lockup.

Linus