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SubjectRe: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations
Date
Daniel Jordan <daniel.m.jordan@oracle.com> writes:

> 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.

Yes. I can use that function to reduce another line from the patch.
Thanks! Will do that.

>> + 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

Andrea has helped to explain this.

Best Regards,
Huang, Ying

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