On Thu, 23 Nov 2023 at 22:04, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> ->d_lock on parent does not stabilize ->d_inode of child.
> We don't do much with that inode in there, but we need
> at least to avoid struct inode getting freed under us...
Gaah. We've gone back and forth on this. Being non-preemptible is
already equivalent to rcu read locking.
>From Documentation/RCU/rcu_dereference.rst:
With the new consolidated
RCU flavors, an RCU read-side critical section is entered
using rcu_read_lock(), anything that disables bottom halves,
anything that disables interrupts, or anything that disables
preemption.
so I actually think the coda code is already mostly fine, because that
parent spin_lock may not stabilize d_child per se, but it *does* imply
a RCU read lock.
So I think you should drop the rcu_read_lock/rcu_read_unlock from that patch.
But that
struct inode *inode = d_inode_rcu(de);
conversion is required to get a stable inode pointer.
So half of this patch is unnecessary.
Adding Paul to the cc just to verify that the docs are up-to-date and
that we're still good here.
Because we've gone back-and-forth on the "spinlocks are an implied RCU
read-side critical section" a couple of times.
Linus
