Messages in this thread | | | Date | Tue, 6 Mar 2007 17:56:57 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: 2.6.21-rc suspend regression: sysfs deadlock |
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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.
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