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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 24 Nov 2023 13:22:19 -0800
Message-ID: <CAHk-=whGKvjHCtJ6W4pQ0_h_k9fiFQ8V2GpM=BqYnB2X=SJ+XQ@mail.gmail.com>
Subject: Re: [PATCH v3 02/21] coda_flag_children(): cope with dentries turning negative
To:     Al Viro <viro@zeniv.linux.org.uk>,
        "Paul E. McKenney" <paulmck@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org,
        Christian Brauner <brauner@kernel.org>,
        linux-kernel@vger.kernel.org
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On Thu, 23 Nov 2023 at 22:04, Al Viro <viro@zeniv.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.

                  Linus