Re: [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds tables

Dave Hansen
2014-10-28T17:43:48Z

On 10/24/2014 05:08 AM, Thomas Gleixner wrote:
> On Sun, 12 Oct 2014, Qiaowei Ren wrote:
>> + /*
>> + * Go poke the address of the new bounds table in to the
>> + * bounds directory entry out in userspace memory. Note:
>> + * we may race with another CPU instantiating the same table.
>> + * In that case the cmpxchg will see an unexpected
>> + * 'actual_old_val'.
>> + */
>> + ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry,
>> + expected_old_val, bt_addr);
>
> This is fully preemptible non-atomic context, right?
>
> So this wants a proper comment, why using
> user_atomic_cmpxchg_inatomic() is the right thing to do here.

Hey Thomas,

How's this for a new comment? Does this cover the points you think need
clarified?

==

The kernel has allocated a bounds table and needs to point the
(userspace-allocated) directory to it. The directory entry is the
*only* place we track that this table was allocated, so we essentially
use it instead of an kernel data structure for synchronization. A
copy_to_user()-style function would not give us the atomicity that we need.

If two threads race to instantiate a table, the cmpxchg ensures we know
which one lost the race and that the loser frees the table that they
just allocated.