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Date:   Wed, 5 Jun 2019 22:19:01 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        huang ying <huang.ying.caritas@gmail.com>
Subject: Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader
 optimistic spinning
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On Wed, Jun 05, 2019 at 02:13:27PM -0400, Waiman Long wrote:

> Using cmpxchg_double is actually more risky than I thought. I have been
> trying to try to use cmpxchg_double for down_write, but I kept getting
> kernel panics because the rwsem wasn't 16b-aligned. As rwsem is embedded
> in quite a large number of structures, they all have to align properly
> to make that work or the kernel will panic. That does seem too risky to
> me. So I am dropping the idea of trying to use it.

Urgh, that's another things that's been on the TODO list for a long long
time, write code to verify the alignment of allocations :/ I'm
suspecting quite a lot of that goes wrong all over the place.