linux-kernel - Re: [PATCH v3 0/7] Allow setting caching mode in arch_add

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Date:   Thu, 27 Feb 2020 10:54:40 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-ia64@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, linux-mm@...ck.org,
        Dan Williams <dan.j.williams@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Badger <ebadger@...aio.com>
Subject: Re: [PATCH v3 0/7] Allow setting caching mode in arch_add_memory()
 for P2PDMA



On 2020-02-27 10:43 a.m., Jason Gunthorpe wrote:
> Hm, AFAIK WC memory is not compatible with the spinlocks/mutexs/etc in
> Linux, so while it is true the memory has no side effects, there would
> be surprising concurrency risks if anything in the kernel tried to
> write to it.
> 
> Not compatible means the locks don't contain stores to WC memory the
> way you would expect. AFAIK on many CPUs extra barriers are required
> to keep WC stores ordered, the same way ARM already has extra barriers
> to keep UC stores ordered with locking..
> 
> The spinlocks are defined to contain UC stores though.
> 
> If there is no actual need today for WC I would suggest using UC as
> the default.

Ok, that sounds sensible. I'll do that in the next revision.

Thanks,

Logan