Re: [PATCH v4] x86/sgx: Fix the call order of synchronize_srcu() in sgx_release()

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On Wed, 20 Jan 2021 18:26:56 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:

On Wed, Jan 20, 2021 at 09:34:10AM -0800, Dave Hansen wrote:
On 1/20/21 6:43 AM, Jarkko Sakkinen wrote:
>> So why do you need the synchronize_srcu() call when this process sees an
>> empty mm_list already?
>>
>> Thx.
> The other process aka some process using the enclave calls list_del_rcu()
> (and synchronize_srcu()), which starts a new grace period. If we don't
> do it, then the cleanup_srcu() will race with that grace period.

To me, this is only a partial explanation.

That goal of synchronize_srcu() is to wait for the completion of a
*previous* grace period: one that might have observed the old state of
the list.

Could you explain the *actual* effects of the misplaced
synchronize_srcu()?  If the race _occurs_, what is the side-effect?

As I haven't been able to reproduce this regression myself, I need
to take steps back and try to reproduce the it with Graphene.

WARN_ON()'s trigger inside cleanup_srcu_struct(), which causes a memory
leak since free_percpu() gets never called. If I recall correctly, it
was srcu_readers_active() but unfortunately I don't have a log available.

Perhaps Haitao could provide us one.

/Jarkko

Sorry I lost those logs too as our email server automatically clean up old emails. I have been re-running the tests but have not been able to reproduce the same issue.

Haitao



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