On Mon, Dec 02, 2013 at 06:58:57PM -0800, Linus Torvalds wrote:
> In other words, it's unsafe to protect reference counts inside objects
> with anything but spinlocks and/or atomic refcounts. Or you have to
> have the lock *outside* the object you're protecting (which is often
> what you want for other reasons anyway, notably lookup).
>
> So having a non-atomic refcount protected inside a sleeping lock is a
> bug, and that's really the bug that ended up biting us for pipes.
>
> Now, the question is: do we have other such cases? How do we document
> this? Do we try to make mutexes and other locks safe to use for things
> like this?
Umm... AFAICS, in VFS proper we have
files_struct - atomic_dec_and_test
fs_struct - spinlock + int
file - atomic_long_dec_and_test (with delays after that, including
RCU).
super_block - global spinlock + int (s_count); the mutex in there
(->s_umount) can be taken by anybody who holds an active ref *or* has
bumped ->s_count while holding sb_lock. Exactly to prevent that kind of
unpleasantness. Freeing RCU-delayed.
vfsmount - percpu counter + flag + global seqlock, with quite a bit of
contortions for the sake of avoiding cross-CPU stores on fastpath; discussed
back in October, concluded to be safe. Freeing RCU-delayed.
dentry - lockref, with RCU-delayed actual freeing.
file_system_type, nls_table, linux_binfmt - module refcount of "owner";
search structures protected by global spinlocks or rwlocks, exiting module
is responsible for unregistering first.
inode - atomic_dec_and_lock, with actual freeing RCU-delayed (and
evicting code waiting for pagecache references to be gone, with the rest
being responsibility of fs method called before we free the sucker)
block_device - part of bdevfs inode
These should be safe, but damnit, we really need the lifecycle documented for
all objects - the above is only a part of it (note that for e.g. superblocks
we have additional rules re "->s_active can't be incremented for any reason
once it drops to zero, it can't be incremented until superblock had been
marked 'born' and it crosses over to zero only with ->s_umount held"; there's
6 stages in life cycle of struct super_block and we had interesting bugs due
to messing the transitions up). The trouble is, attempt to write those down
tends to stray into massive grep session, with usual results - some other
crap gets found (e.g. in some odd driver) and needs to be dealt with ;-/
Sigh...
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