[edk2-devel] [RFC] Incorrect memory ordering in ReleaseSpinLock()

[Bin, Sung-Uk (Bin) via groups.io]

Dear, Ard and maintainers

We are concerning that ReleaseSpinLock() does not have a memory barrier. This is reported to https://bugzilla.tianocore.org/show_bug.cgi?id=3005<https://bugzilla.tianocore.org/show_bug.cgi?id=3005>.  We’d like to hear from you whether current implementation needs improvement or not.

The concern comes from 'weak memory ordering' and multi-core. (we are using AARCH64) And the scenario that we’re concerning is like below:

AcquireSpinLock(); // contains ‘dmb sy’ and prevents "a = *b" from moving up (and unnecessarily prevents other things from moving down)
a = *b;
a = a + 1;
*b = a;
ReleaseSpinLock(); // No write barrier here, so "*b = a" can move down. Another core acquires the spinlock and can read stale data


Please let me know if it would be helpful to add MemoryFence like below:

SPIN_LOCK *
EFIAPI
ReleaseSpinLock (
  IN OUT  SPIN_LOCK                 *SpinLock
  )
{
  SPIN_LOCK    LockValue;

  ASSERT (SpinLock != NULL);

  MemoryFence();

  LockValue = *SpinLock;
  ASSERT (SPIN_LOCK_ACQUIRED == LockValue || SPIN_LOCK_RELEASED == LockValue);

  *SpinLock = SPIN_LOCK_RELEASED;
  return SpinLock;
}

MemoryFence is implemented with 'dmb', but I just wonder if it is okay to not implement it with 'dsb'.

* Attaching linux documentation describing SMP barrier pairing
https://github.com/torvalds/linux/blob/master/Documentation/memory-barriers.txt

SMP BARRIER PAIRING
-------------------

When dealing with CPU-CPU interactions, certain types of memory barrier should
always be paired.  A lack of appropriate pairing is almost certainly an error.

General barriers pair with each other, though they also pair with most
other types of barriers, albeit without multicopy atomicity.  An acquire
barrier pairs with a release barrier, but both may also pair with other
barriers, including of course general barriers.  A write barrier pairs
with a data dependency barrier, a control dependency, an acquire barrier,
a release barrier, a read barrier, or a general barrier.  Similarly a
read barrier, control dependency, or a data dependency barrier pairs
with a write barrier, an acquire barrier, a release barrier, or a
general barrier:

       CPU 1               CPU 2
       ===============            ===============
       WRITE_ONCE(a, 1);
       <write barrier>
       WRITE_ONCE(b, 2);     x = READ_ONCE(b);
                           <read barrier>
                           y = READ_ONCE(a);

Or:

       CPU 1               CPU 2
       ===============            ===============================
       a = 1;
       <write barrier>
       WRITE_ONCE(b, &a);    x = READ_ONCE(b);
                           <data dependency barrier>
                           y = *x;

Or even:

       CPU 1               CPU 2
       ===============            ===============================
       r1 = READ_ONCE(y);
       <general barrier>
       WRITE_ONCE(x, 1);     if (r2 = READ_ONCE(x)) {
                              <implicit control dependency>
                              WRITE_ONCE(y, 1);
                           }

       assert(r1 == 0 || r2 == 0);

Basically, the read barrier always has to be there, even though it can be of
the "weaker" type.

[!] Note that the stores before the write barrier would normally be expected to
match the loads after the read barrier or the data dependency barrier, and vice
versa:

       CPU 1                               CPU 2
       ===================                 ===================
       WRITE_ONCE(a, 1);    }----   --->{  v = READ_ONCE(c);
       WRITE_ONCE(b, 2);    }    \ /    {  w = READ_ONCE(d);
       <write barrier>            \        <read barrier>
       WRITE_ONCE(c, 3);    }    / \    {  x = READ_ONCE(a);
       WRITE_ONCE(d, 4);    }----   --->{  y = READ_ONCE(b);




Thanks,
Bin

From: bugzilla-daemon at bugzilla.tianocore.org <bugzilla-daemon at bugzilla.tianocore.org>
Sent: Wednesday, November 4, 2020 10:44 AM
To: Bin, Sung-Uk (Bin) <sunguk-bin at hp.com>
Subject: [Bug 3005] ReleaseSpinLock() requires a barrier at the beginning

https://bugzilla.tianocore.org/show_bug.cgi?id=3005<https://bugzilla.tianocore.org/show_bug.cgi?id=3005>

gaoliming at byosoft.com.cn<mailto:gaoliming at byosoft.com.cn> changed:

What |Removed |Added
----------------------------------------------------------------------------
Priority|Lowest |Normal
Status|UNCONFIRMED |CONFIRMED
CC| |leif at nuviainc.com<mailto:|leif at nuviainc.com>
Assignee|unassigned at tianocore.org<mailto:Assignee|unassigned at tianocore.org> |ard.biesheuvel at arm.com<mailto:|ard.biesheuvel at arm.com>
Ever confirmed|0 |1

--- Comment #5 from gaoliming at byosoft.com.cn<mailto:gaoliming at byosoft.com.cn> ---
Ard: can you help check it? This issue in AARCH64.

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