Re: [PATCH 00/13] [RFC] Rust support

[Matthew Wilcox]

On Fri, Apr 16, 2021 at 02:07:49PM +0100, Wedson Almeida Filho wrote:
> There is nothing in C forcing developers to actually use DEFINE_MUTEX_GUARD. So
> someone may simply forget (or not know that they need) to lock
> current->perf_event_mutex and directly access some field protected by it. This
> is unlikely to happen when one first writes the code, but over time as different
> people modify the code and invariants change, it is possible for this to happen.
> 
> In Rust, this isn't possible: the data protected by a lock is only accessible
> when the lock is locked. So developers cannot accidentally make mistakes of this
> kind. And since the enforcement happens at compile time, there is no runtime
> cost.

Well, we could do that in C too.

struct unlocked_inode {
	spinlock_t i_lock;
};

struct locked_inode {
	spinlock_t i_lock;
	unsigned short i_bytes;
	blkcnt_t i_blocks;
};

struct locked_inode *lock_inode(struct unlocked_inode *inode)
{
	spin_lock(&inode->i_lock);
	return (struct locked_inode *)inode;
}

There's a combinatoric explosion when you have multiple locks in a data
structure that protect different things in it (and things in a data
structure that are protected by locks outside that data structure),
but I'm not sufficiently familiar with Rust to know if/how it solves
that problem.

Anyway, my point is that if we believe this is a sufficiently useful
feature to have, and we're willing to churn the kernel, it's less churn
to do this than it is to rewrite in Rust.

> Another scenario: suppose within perf_event_task_enable you need to call a
> function that requires the mutex to be locked and that will unlock it for you on
> error (or unconditionally, doesn't matter). How would you do that in C? In Rust,
> there is a clean idiomatic way of transferring ownership of a guard (or any
> other object) such that the previous owner cannot continue to use it after
> ownership is transferred. Again, this is enforced at compile time. I'm happy to
> provide a small example if that would help.

I think we could do that too with an __attribute__((free)).  It isn't,
of course, actually freeing the pointer to the locked_inode, but it will
make the compiler think the pointer is invalid after the function returns.

(hm, looks like gcc doesn't actually have __attribute__((free)) yet.
that's unfortunate.  there's a potential solution in gcc-11 that might
do what we need)