linux-kernel - Re: [PATCH 28/30] r/o bind mounts: track numbers of writers to mounts

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Message-Id: <E1JR8a0-0003sT-8d@pomaz-ex.szeredi.hu>
Date:	Mon, 18 Feb 2008 17:10:56 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	haveblue@...ibm.com
CC:	linux-kernel@...r.kernel.org, hch@....de, miklos@...redi.hu,
	akpm@...l.org, haveblue@...ibm.com
Subject: Re: [PATCH 28/30] r/o bind mounts: track numbers of writers to mounts

> @@ -327,7 +514,30 @@ static struct vfsmount *clone_mnt(struct
>  
>  static inline void __mntput(struct vfsmount *mnt)
>  {
> +	int cpu;
>  	struct super_block *sb = mnt->mnt_sb;
> +	/*
> +	 * We don't have to hold all of the locks at the
> +	 * same time here because we know that we're the
> +	 * last reference to mnt and that no new writers
> +	 * can come in.
> +	 */
> +	for_each_possible_cpu(cpu) {
> +		struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
> +		if (cpu_writer->mnt != mnt)
> +			continue;
> +		spin_lock(&cpu_writer->lock);
> +		atomic_add(cpu_writer->count, &mnt->__mnt_writers);
> +		cpu_writer->count = 0;

I think you should also add a

		cpu_writer->mnt = NULL;

here.  It's not a bug, but I had to think a bit about why it's not a bug.

Miklos
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