linux-kernel - Re: [PATCH RFC 6/6] dcache: prevent flooding with negative dentries

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Message-ID: <YHZn/IFvZbMX9QTD@zeniv-ca.linux.org.uk>
Date:   Wed, 14 Apr 2021 03:56:44 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Gautham Ananthakrishna <gautham.ananthakrishna@...cle.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, matthew.wilcox@...cle.com,
        khlebnikov@...dex-team.ru
Subject: Re: [PATCH RFC 6/6] dcache: prevent flooding with negative dentries

On Thu, Jan 21, 2021 at 06:49:45PM +0530, Gautham Ananthakrishna wrote:

> +	spin_lock(&victim->d_lock);
> +	parent = lock_parent(victim);
> +
> +	rcu_read_unlock();

Similar story.  As soon as you hit that rcu_read_unlock(), the memory
pointed to by victim might be reused.  If you have hit __lock_parent(),
victim->d_lock had been dropped and regained.  Which means that freeing
might've been already scheduled.  Unlike #1/6, here you won't get
memory corruption in lock_parent() itself, but...

> +
> +	if (d_count(victim) || !d_is_negative(victim) ||
> +	    (victim->d_flags & DCACHE_REFERENCED)) {
> +		if (parent)
> +			spin_unlock(&parent->d_lock);
> +		spin_unlock(&victim->d_lock);

... starting from here you just might.