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

[Dave Hansen]

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.
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