From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: linux-sgx@vger.kernel.org
Subject: Re: [PATCH for_v21 1/2] x86/sgx: Use SRCU to protect mm_list during reclaim
Date: Thu, 11 Jul 2019 14:25:49 -0700 [thread overview]
Message-ID: <20190711212549.GO15067@linux.intel.com> (raw)
In-Reply-To: <20190711211307.cymfaxeuq73v63xg@linux.intel.com>
On Fri, Jul 12, 2019 at 12:13:07AM +0300, Jarkko Sakkinen wrote:
> On Thu, Jul 11, 2019 at 09:16:24AM -0700, Sean Christopherson wrote:
> > Not taking a reference to encl_mm is not just an optimization, it's also
> > functionally necessary and a major motivation to moving to SRCu. Putting
> > the reference can invoke sgx_encl_mm_release(), which calls
> > synchronize_srcu() and will deadlock if done while holding the SRCU read
> > lock. Not taking a reference paves the way for additional refcounting
> > improvements that would be extremely difficult to implement when using
> > the custom walker due to cyclical dependencies on the refcount.
>
> I'm not sure I get this. The existing code does not have a call to
> synchronize_srcu().
Does this read any better?
Not taking a reference to encl_mm is not just an optimization, it's also
functionally necessary and a major motivation to moving to SRCU. From a
functional perspective, putting the encl_mm reference can invoke
sgx_encl_mm_release(), which now calls synchronize_srcu() and thus will
deadlock if triggered while holding the SRCU read lock. In terms of
motivation, not taking a reference paves the way for additional refcounting
improvements that would be extremely difficult to implement when using
the custom walker due to cyclical dependencies on the encl_mm's refcount.
> > - if (!encl)
> > - return;
> > + lockdep_assert_held_exclusive(&mm->mmap_sem);
>
> Just a question: what does it check (12:10AM too tired to check,
> apologies)?
Asserts that the caller has done down_write(&mmap_sem), i.e. guarantees
that sgx_encl_mm_add() cannot be called in parallel on the same mm_struct.
next prev parent reply other threads:[~2019-07-11 21:25 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-11 16:16 [PATCH for_v21 0/2] x86/sgx: Use SRCU and mmu_notifier Sean Christopherson
2019-07-11 16:16 ` [PATCH for_v21 1/2] x86/sgx: Use SRCU to protect mm_list during reclaim Sean Christopherson
2019-07-11 21:13 ` Jarkko Sakkinen
2019-07-11 21:25 ` Sean Christopherson [this message]
2019-07-12 3:44 ` Jarkko Sakkinen
2019-07-11 16:16 ` [PATCH for_v21 2/2] x86/sgx: Use mmu_notifier.release() instead of per-vma refcounting Sean Christopherson
2019-07-11 21:16 ` Jarkko Sakkinen
2019-07-11 18:01 ` [PATCH for_v21 0/2] x86/sgx: Use SRCU and mmu_notifier Jarkko Sakkinen
2019-07-11 21:51 ` Jarkko Sakkinen
2019-07-12 4:17 ` Jarkko Sakkinen
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