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Date:   Tue, 5 Jul 2022 17:44:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        linux-kernel@vger.kernel.org, rjw@rjwysocki.net,
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        Will Deacon <will@kernel.org>, tj@kernel.org,
        linux-pm@vger.kernel.org, Richard Weinberger <richard@nod.at>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        linux-um@lists.infradead.org, Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>,
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        Jann Horn <jannh@google.com>, linux-ia64@vger.kernel.org,
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Subject: Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED,
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On Tue, Jun 28, 2022 at 10:39:59PM -0500, Eric W. Biederman wrote:

> > That is, the two paths should already be synchronized, and the memory
> > barriers will not help anything inside the locks. The locking should (and
> > must) handle all that.
> 
> I would presume so to.  However the READ_ONCE that is going astray
> does not look like it is honoring that.
> 
> So perhaps there is a bug in the s390 spin_lock barriers?  Perhaps there
> is a subtle detail in the barriers that spin locks provide that we are
> overlooking?

So the thing is, s390 is, like x86, a TSO architecture with SC atomics.
Or at least it used to be; I'm not entirely solid on the Z196 features.

I've been looking at this and I can't find anything obviously wrong.
arch_spin_trylock_once() has what seems a spurious barrier() but that's
not going to cause this.

Specifically, s390 is using a simple test-and-set spinlock based on
their Compare-and-Swap (CS) instruction (so no Z196 funnies around).

Except perhaps arch_spin_unlock(), I can't grok the magic there. It does
something weird before the presumably regular TSO store of 0 into the
lock word.

Ooohh.. /me finds arch_spin_lock_queued().. *urfh* because obviously a
copy of queued spinlocks is what we need.

rwlock_t OTOH is using __atomic_add_*() and that's all Z196 magic.

Sven, does all this still reproduce if you take out
CONFIG_HAVE_MARCH_Z196_FEATURES ? Also, could you please explain the
Z196 bits or point me to the relevant section in the PoO. Additionally,
what's that _niai[48] stuff?

And I'm assuming s390 has hardware fairness on competing CS ?