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SubjectRe: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations
On Mon, Feb 11, 2019 at 04:38:46PM +0800, Huang, Ying wrote:
> +struct swap_info_struct *get_swap_device(swp_entry_t entry)
> +{
> + struct swap_info_struct *si;
> + unsigned long type, offset;
> +
> + if (!entry.val)
> + goto out;

> + type = swp_type(entry);
> + si = swap_type_to_swap_info(type);

These lines can be collapsed into swp_swap_info if you want.

> + if (!si)
> + goto bad_nofile;
> +
> + preempt_disable();
> + if (!(si->flags & SWP_VALID))
> + goto unlock_out;

After Hugh alluded to barriers, it seems the read of SWP_VALID could be
reordered with the write in preempt_disable at runtime. Without smp_mb()
between the two, couldn't this happen, however unlikely a race it is?

CPU0 CPU1

__swap_duplicate()
get_swap_device()
// sees SWP_VALID set
swapoff
p->flags &= ~SWP_VALID;
spin_unlock(&p->lock); // pair w/ smp_mb
...
stop_machine(...)
p->swap_map = NULL;
preempt_disable()
read NULL p->swap_map

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