linux-kernel - Re: [PATCH V2] MIPS: implement smp_cond_load

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Date:   Tue, 10 Jul 2018 12:54:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Huacai Chen <chenhc@...ote.com>
Cc:     Paul Burton <paul.burton@...s.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
        Fuxin Zhang <zhangfx@...ote.com>,
        Zhangjin Wu <wuzhangjin@...il.com>,
        stable <stable@...r.kernel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] MIPS: implement smp_cond_load_acquire() for Loongson-3

On Tue, Jul 10, 2018 at 11:36:37AM +0200, Peter Zijlstra wrote:

> So now explain why the cpu_relax() hack that arm did doesn't work for
> you?

So below is the patch I think you want; if not explain in detail how
this is wrong.

diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h
index af34afbc32d9..e59773de6528 100644
--- a/arch/mips/include/asm/processor.h
+++ b/arch/mips/include/asm/processor.h
@@ -386,7 +386,17 @@ unsigned long get_wchan(struct task_struct *p);
 #define KSTK_ESP(tsk) (task_pt_regs(tsk)->regs[29])
 #define KSTK_STATUS(tsk) (task_pt_regs(tsk)->cp0_status)
 
+#ifdef CONFIG_CPU_LOONGSON3
+/*
+ * Loongson-3 has a CPU bug where the store buffer gets starved when stuck in a
+ * read loop. Since spin loops of any kind should have a cpu_relax() in them,
+ * force a store-buffer flush from cpu_relax() such that any pending writes
+ * will become available as expected.
+ */
+#define cpu_relax()	smp_mb()
+#else
 #define cpu_relax()	barrier()
+#endif
 
 /*
  * Return_address is a replacement for __builtin_return_address(count)