linux-kernel - Re: [PATCH v3 02/21] coda_flag_children(): cope with dentries turning negative

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Message-ID: <6435833a-bdcb-4114-b29d-28b7f436d47d@paulmck-laptop>
Date:   Fri, 24 Nov 2023 14:58:55 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
        Christian Brauner <brauner@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 02/21] coda_flag_children(): cope with dentries
 turning negative

On Fri, Nov 24, 2023 at 01:22:19PM -0800, Linus Torvalds wrote:
> On Thu, 23 Nov 2023 at 22:04, Al Viro <viro@...iv.linux.org.uk> 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.

Yes, spinlocks are implied RCU read-side critical sections.  Even in -rt,
where non-raw spinlocks are preemptible, courtesy of this:

	static __always_inline void __rt_spin_lock(spinlock_t *lock)
	{
		rtlock_might_resched();
		rtlock_lock(&lock->lock);
		rcu_read_lock();
		migrate_disable();
	}

So given -rt's preemptible spinlocks still being RCU readers, I need to
explicitly call this out in the documentation.

How about as shown below for a start?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst
index 659d5913784d..2524dcdadde2 100644
--- a/Documentation/RCU/rcu_dereference.rst
+++ b/Documentation/RCU/rcu_dereference.rst
@@ -408,7 +408,10 @@ member of the rcu_dereference() to use in various situations:
 	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.
+	preemption.  Please note that spinlock critical sections
+	are also implied RCU read-side critical sections, even when
+	they are preemptible, as they are in kernels built with
+	CONFIG_PREEMPT_RT=y.
 
 2.	If the access might be within an RCU read-side critical section
 	on the one hand, or protected by (say) my_lock on the other,