linux-kernel - Re: [RFC][PATCH 08/12] s390: Replace cmpxchg

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Message-ID: <Y71LoCIl+IFdy9D8@FVFF77S0Q05N>
Date:   Tue, 10 Jan 2023 11:27:28 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Heiko Carstens <hca@...ux.ibm.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        torvalds@...ux-foundation.org, corbet@....net, will@...nel.org,
        boqun.feng@...il.com, catalin.marinas@....com, dennis@...nel.org,
        tj@...nel.org, cl@...ux.com, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, borntraeger@...ux.ibm.com,
        svens@...ux.ibm.com, Herbert Xu <herbert@...dor.apana.org.au>,
        davem@...emloft.net, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, joro@...tes.org, suravee.suthikulpanit@....com,
        robin.murphy@....com, dwmw2@...radead.org,
        baolu.lu@...ux.intel.com, Arnd Bergmann <arnd@...db.de>,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        Andrew Morton <akpm@...ux-foundation.org>, vbabka@...e.cz,
        roman.gushchin@...ux.dev, 42.hyeyoo@...il.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org,
        linux-crypto@...r.kernel.org, iommu@...ts.linux.dev,
        linux-arch@...r.kernel.org
Subject: Re: [RFC][PATCH 08/12] s390: Replace cmpxchg_double() with
 cmpxchg128()

On Tue, Jan 10, 2023 at 09:32:55AM +0100, Peter Zijlstra wrote:
> On Tue, Jan 10, 2023 at 08:23:05AM +0100, Heiko Carstens wrote:
> 
> > So, Alexander Gordeev reported that this code was already prior to your
> > changes potentially broken with respect to missing READ_ONCE() within the
> > cmpxchg_double() loops.
> 
> Unless there's an early exit, that shouldn't matter. If you managed to
> read garbage the cmpxchg itself will simply fail and the loop retries.

I don't think that's true; without READ_ONCE() the compiler could (but is
very unlikely to) read multiple times, and that could cause problems.

For example:

| 	prev = *ptr;
| 
| 	do {
| 		new = some_function_of(prev);
| 		old = cmpxchg(ptr, prev, new);
| 	} while (old != prev);

Could effectively become:

| 	prev1 = *ptr;
|	prev2 = *ptr;
|
| 	do {
| 		new = some_function_of(prev1)
| 		old = cmpxchg(ptr, prev2, new);
| 	} while (old != prev2);

... which would effectively udpate from a stale value, throwing away prev2.
That and the two generated reads could be in either order.

So I do think it's warranted to use READ_ONCE() for the prev value feeding into
a cmpxchg operation, even if that's only for the "once" part rather than lack
of tearing.

Thanks,
Mark.