linux-kernel - Re: [PATCH] MIPS: implement smp_cond_load

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Message-ID: <tencent_2BF857EC332473814C12A753@qq.com>
Date:   Wed, 20 Jun 2018 11:31:55 +0800
From:   "陈华才" <chenhc@...ote.com>
To:     "Peter Zijlstra" <peterz@...radead.org>
Cc:     "Paul Burton" <paul.burton@...s.com>,
        "Ralf Baechle" <ralf@...ux-mips.org>,
        "James Hogan" <james.hogan@...s.com>,
        "linux-mips" <linux-mips@...ux-mips.org>,
        "Fuxin Zhang" <zhangfx@...ote.com>,
        "wuzhangjin" <wuzhangjin@...il.com>,
        "Huacai Chen" <chenhuacai@...il.com>,
        "stable" <stable@...r.kernel.org>,
        "Alan Stern" <stern@...land.harvard.edu>,
        "AndreaParri" <andrea.parri@...rulasolutions.com>,
        "Will Deacon" <will.deacon@....com>,
        "Boqun Feng" <boqun.feng@...il.com>,
        "Nicholas Piggin" <npiggin@...il.com>,
        "David Howells" <dhowells@...hat.com>,
        "Jade Alglave" <j.alglave@....ac.uk>,
        "Luc Maranget" <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Akira Yokosawa" <akiyks@...il.com>,
        "linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: implement smp_cond_load_acquire() for Loongson-3

Hi, Peter and Paul,

Loongson-3's Store Fill Buffer is nearly the same as your "Store Buffer", and it increases the memory ordering weakness. So, smp_cond_load_acquire() only need a __smp_mb() before the loop, not after every READ_ONCE(). In other word, the following code is just OK:

#define smp_cond_load_acquire(ptr, cond_expr)                   \
({                                                              \
        typeof(ptr) __PTR = (ptr);                              \
        typeof(*ptr) VAL;                                       \
        __smp_mb();                                     \
        for (;;) {                                              \
                VAL = READ_ONCE(*__PTR);                        \
                if (cond_expr)                                  \
                        break;                                  \
                cpu_relax();                                    \
        }                                                       \
        __smp_mb();                                     \
        VAL;                                                    \
})

the __smp_mb() before loop is used to avoid "reads prioritised over writes", which is caused by SFB's weak ordering and similar to ARM11MPCore (mentioned by Will Deacon).

Huacai

------------------ Original ------------------
From:  "Peter Zijlstra"<peterz@...radead.org>;
Date:  Tue, Jun 19, 2018 03:22 PM
To:  "陈华才"<chenhc@...ote.com>;
Cc:  "Paul Burton"<paul.burton@...s.com>; "Ralf Baechle"<ralf@...ux-mips.org>; "James Hogan"<james.hogan@...s.com>; "linux-mips"<linux-mips@...ux-mips.org>; "Fuxin Zhang"<zhangfx@...ote.com>; "wuzhangjin"<wuzhangjin@...il.com>; "Huacai Chen"<chenhuacai@...il.com>; "stable"<stable@...r.kernel.org>; "Alan Stern"<stern@...land.harvard.edu>; "AndreaParri"<andrea.parri@...rulasolutions.com>; "Will Deacon"<will.deacon@....com>; "Boqun Feng"<boqun.feng@...il.com>; "Nicholas Piggin"<npiggin@...il.com>; "David Howells"<dhowells@...hat.com>; "Jade Alglave"<j.alglave@....ac.uk>; "Luc Maranget"<luc.maranget@...ia.fr>; "Paul E. McKenney"<paulmck@...ux.vnet.ibm.com>; "Akira Yokosawa"<akiyks@...il.com>; "linux-kernel"<linux-kernel@...r.kernel.org>;
Subject:  Re: [PATCH] MIPS: implement smp_cond_load_acquire() for Loongson-3
 
On Tue, Jun 19, 2018 at 02:40:14PM +0800, 陈华才 wrote:
> Hi, Paul,
> 
> First of all, could you please check why linux-mips reject e-mails
> from lemote.com? Of course I can send e-mails by gmail, but my gmail
> can't receive e-mails from linux-mips since March, 2018.

Could you please learn to use email? No top posting and wrap lines at 78
chars.

> I have already read Documentation/memory-barriers.txt, but I don't
> think we should define a smp_read_barrier_depends() for Loongson-3.
> Because Loongson-3's behavior isn't like Alpha, and in syntax, this is
> not a data-dependent issue.

Agreed, this is not a data-dependency issue.

> There is no document about Loongson-3's SFB. In my opinion, SFB looks
> like the L0 cache but sometimes it is out of cache-coherent machanism
> (L1 cache's cross-core coherency is maintained by hardware, but not
> always true for SFB). smp_mb() is needed for smp_cond_load_acquire(),
> but not every READ_ONCE().

Linux does _NOT_ support non-coherent SMP. If your system is not fully
coherent, you're out of luck.

But please, explain in excruciating detail what exactly you need that
smp_mb for. If, like I posited in my previous email, it is to ensure
remote store buffer flushes, then your machine is terminally broken.