linux-kernel - [PATCH 0/6] V2: Clarify/standardize memory barriers for ipc

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Date:   Sat, 12 Oct 2019 07:49:52 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Waiman Long <longman@...hat.com>
Cc:     1vier1@....de, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 0/6] V2: Clarify/standardize memory barriers for ipc

Hi,

Updated series, based on input from Davidlohr:

- Mixing WRITE_ONCE(), when not holding a lock, and "normal" writes,
  when holding a lock, makes the code less readable.
  Thus use _ONCE() everywhere, for both WRITE_ONCE() and READ_ONCE().

- According to my understanding, wake_q_add() does not contain a barrier
  that protects the refount increase. Document that, and add the barrier
  to the ipc code

- and, based on patch review: The V1 patch for ipc/sem.c is incorrect,
  ->state must be set to "-EINTR", not EINTR.

>From V1:

The memory barriers in ipc are not properly documented, and at least
for some architectures insufficient:
Reading the xyz->status is only a control barrier, thus
smp_acquire__after_ctrl_dep() was missing in mqueue.c and msg.c
sem.c contained a full smp_mb(), which is not required.

Patches:
Patch 1: Document the barrier rules for wake_q_add().

Patch 2: remove code duplication
@Davidlohr: There is no "Signed-off-by" in your mail, otherwise I would
list you as author.

Patch 3-5: Update the ipc code, especially add missing
           smp_mb__after_ctrl_dep().

Clarify that smp_mb__{before,after}_atomic() are compatible with all
RMW atomic operations, not just the operations that do not return a value.

Patch 6: Documentation for smp_mb__{before,after}_atomic().

Open issues:
- Is my analysis regarding the refcount correct?

- Review other users of wake_q_add().

- More testing. I did some tests, but doubt that the tests would be
  sufficient to show issues with regards to incorrect memory barriers.

- Should I add a "Fixes:" or "Cc:stable"? The issues that I see are
  the missing smp_mb__after_ctrl_dep(), and WRITE_ONCE() vs.
  "ptr = NULL", and a risk regarding the refcount that I can't evaluate.

What do you think?

--
	Manfred