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Subject[RFC] revoke(2) and generic handling of things like remove_proc_entry()
After some digging in procfs logics for revoking further file IO after
remove_proc_entry() (and figuring out what to do for debugfs - it also
needs something similar), I think I've got something that has potential
to become a working revoke(2) with very low overhead. It will require
some preparations and there are several interesting questions regarding
the semantics, but it looks like there's a decent chance of making it
work:
* procfs and debugfs - definitely can use it
* sysfs - probably can switch to that from its home-grown mechanism
* tty hangup handling - might be possible to switch to that
mechanism.
* ALSA analog of hangup - ditto.
* revoke(2) - would be nice; might be doable, depending on the
degree of generality we want.

Below is an outline of implementation; discussion of the fun spots of
semantics is in the very end.

Two new objects are introduced -

struct revokable { // freeing of whatever contains it is RCU'd
atomic_t in_use; // number of threads in methods,
// negative => going away
spinlock_t lock;
hlist_head list; // protected by ->lock, goes through
// struct revoke->node.
struct completion *c;
void (*kick)(struct revokable *); // discussion in the end; NULL
// for procfs and friends.
};
struct revoke { // per-file, lives until __fput()
struct file *file; // never changes
struct revokable *revokable; // RCU protected
struct hlist_node list; // protected by ->revokable->lock
bool closing; // ditto
struct completion *c; // ditto
};
struct file gets a new field - struct revoke *f_revoke (next to ->f_op, never
changes after open, usually NULL).

New helpers:
int make_revokable(file, revokable) - to be used during ->open();
it'll allocate file->f_revoke and associate it with revokable. Should be
the last thing done by ->open() (see below for discussion - that's
one of the most inconvenient areas of this API)
inline bool start_using(file) - called before file method calls
except for ->release(); if it returns false, don't make a call, the
file has been revoked. Attempt to revoke will make sure no new
users can appear (i.e. that start_using() will fail from now on) and
wait until all of them are done. What to do on start_using() failure
is up to the caller - depends on the method (e.g. POLLERR is probably
the right one for ->poll(), -EIO - for ->write(), 0 might be right for
->read(), etc.)
inline void stop_using(file) - called after the method call.
void release_revoke(revoke) - __fput() calls that instead of calling
->release() if file->f_revoke is non-NULL.
void revoke_it(revokable) - revoke all files associated with this
revokable; ->release() will be called on each of those files and no method
calls will be issued after it returns. No references to revokable will
outlive the grace period.

static inline bool start_using(struct file *f)
{
struct revoke *revoke = f->f_revoke;
if (likely(!revoke))
return true; /* non-revokable file */
return __start_using(revoke);
}
static inline void stop_using(struct file *f)
{
struct revoke *revoke = f->f_revoke;
if (unlikely(revoke))
__stop_using(revoke);
}
}
bool __start_using(struct revoke *revoke)
{
struct revokable *r;
rcu_read_lock();
r = rcu_dereference(revoke->revokable);
if (unlikely(!r)) {
rcu_read_unlock();
return false; /* revoked */
}
if (likely(atomic_inc_unless_negative(&r->in_use))) {
rcu_read_unlock();
return true; /* we are using it now */
}
rcu_read_unlock();
return false; /* it's being revoked right now */
}
#define BIAS <large negative constant>

void __stop_using(struct revoke *revoke)
{
struct revokable *r;
r = rcu_dereference_protected(revoke->revokable, 1);
BUG_ON(!r);
if (atomic_dec_return(&r->in_use) == BIAS)
complete(r->c);
}
/* called with r->lock held by caller, unlocks it */
static void __release_revoke(struct revokable *r, struct revoke *revoke)
{
if (revoke->closing) {
DECLARE_COMPLETION_ONSTACK(c);
revoke->c = &c;
spin_unlock(&r->lock);
wait_for_completion(&c);
} else {
struct file *file;
revoke->closing = 1;
spin_unlock(&r->lock);
file = revoke->file;
if (file->f_op->release)
file->f_op->release(file_inode(file), file);
spin_lock(&r->lock);
hlist_del_init(&revoke->list);
rcu_assign_pointer(revoke->revokable, NULL);
rcu_read_lock(); /* prevent freeing of r */
if (revoke->c)
complete(revoke->c);
spin_unlock(&r->lock);
rcu_read_unlock();
}
}
void release_revoke(struct revoke *revoke)
{
struct revokable *r;
rcu_read_lock();
r = rcu_dereference(revoke->revokable);
if (!r) {
/* already closed by revokation */
rcu_read_unlock();
return;
}
spin_lock(&r->lock);
if (unlikely(hlist_unhashed(&revoke->node))) {
/* just been revoked */
spin_unlock(&r->lock);
rcu_read_unlock();
return;
}
/*
* OK, revoke_it() couldn't have been finished yet
* it'll have to get r->lock before it's through, so
* we can drop rcu_read_lock
*/
rcu_read_unlock();
__release_revoke(r, revoke);
kfree(revoke);
}
void revoke_it(struct revokable *r)
{
DECLARE_COMPLETION_ONSTACK(c);
r->c = &c;
if (atomic_add_return(BIAS, &r->in_use) != BIAS) {
if (r->kick)
r->kick(r);
wait_for_completion(c);
}
while (1) {
struct hlist_node *p;
spin_lock(&r->lock);
p = r->list.first;
if (!p)
break;
__release_revoke(r, hlist_entry(p, struct revoke, list));
}
spin_unlock(&r->lock);
}
int make_revokable(struct file *f, struct revokable *r)
{
struct revoke *revoke = kzalloc(sizeof(struct revoke), GFP_KERNEL);
if (!revoke)
return -ENOMEM;
if (!atomic_inc_unless_negative(&r->in_use)) {
kfree(revoke);
return -ENOENT;
}
revoke->file = f;
f->f_revoke = revoke;
spin_lock(&r->lock);
hlist_add_head(&revoke->list, &r->list);
spin_unlock(&r->lock);
__stop_using(revoke);
return 0;
}
procfs would have struct revokable embedded into proc_dir_entry, with
freeing of those guys RCUd. It would set ->f_op to ->proc_fops and
call make_revokable(file, &pde->revokable) in proc_reg_open(); no wrappers
for other methods needed anymore. All file_operations instances fed to
proc_create() et.al. would lose ->owner - it's already not needed for those,
actually. remove_proc_entry()/remove_proc_subtree() would call revoke_it()
on everything we are removing.

Ugly spots and questions re semantics:

1) ->open() instance calling make_revokable() would bloody better touch
nothing after make_revokable() succeeds, since at that point struct file
is visible to revoke_it() and ->release() may be called by it.

2) thou shalt not call revoke_it() from any methods of any file affected
by it. Deadlock for obvious reasons. Do it asynchronously if you really
need it.

3) that ->kick() thing: for something like procfs it's a non-issue, but
for anything like hangup/snd_card_disconnect/sys_revoke we'll need to
supply that method. It should terminate any indefinite waits for IO of
the stuff sitting in ->read()/->write()/etc.

4) nasty semantics issue - mmap() vs. revoke (of any sort, including
remove_proc_entry(), etc.). Suppose a revokable file had been mmapped;
now it's going away. What should we do to its VMAs? Right now sysfs
and procfs get away with that, but only because there's only one thing
that has ->mmap() there - /proc/bus/pci and sysfs equivalents. I've
no idea how does pci_mmap_page_range() interact with PCI hotplug (and
I'm not at all sure that whatever it does isn't racy wrt device removal),
but I suspect that it strongly depends on lack of ->fault() for those
VMAs, which makes killing all PTEs pointing to pages in question enough.
How generic do we want to make it? Anybody wanting to add more files
that could be mmapped in procfs/sysfs/debugfs deserves to be hurt, but
if we start playing with revoke(2), restriction might become inconvenient.
I'm not sure what kind of behaviour do we want there - *BSD at least
used to have revoke(2) only for character devices that had no mmap()...

5) another fun semantics question - what should revoke(2) do to open(2)
happening before it completes? In terms of implementation above, should
we put a new struct revokable in place before we'd finished revoke_it()
on the old one?

6) how do we get from revoke(2) to call of revoke_it() on the right object?
Note that revoke(2) is done by pathname; we might want an ...at() variant,
but all we'll have to play with will be inode, not an opened file. We probably
ought to put it into file_operations and use ->i_fop->revoke() to get there
(with cdev_revoke() looking for relevant struct cdev, etc.). Not sure; we'll
probably need to play with different variants once the generic part of
infrastructure is there.

7) we need to reduce the number of places where we do ->f_op->something();
it's not too horrible, but it's definitely more than I like. ->read() and
->write() are the main offenders in that respect and many of those are in
really ugly parts of tree...

8) if we convert vhangup(2) to that mechanism, serial consoles are going to
be a thing to watch out for. Extra ->release() and...


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