linux-kernel - Re: [PATCH] MIPS: Change definition of cpu

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Date:   Tue, 17 Jul 2018 20:46:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paul Burton <paul.burton@...s.com>
Cc:     Huacai Chen <chenhc@...ote.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>, linux-mips@...ux-mips.org,
        Fuxin Zhang <zhangfx@...ote.com>,
        Zhangjin Wu <wuzhangjin@...il.com>,
        Huacai Chen <chenhuacai@...il.com>, 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>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MIPS: Change definition of cpu_relax() for Loongson-3

On Tue, Jul 17, 2018 at 10:52:32AM -0700, Paul Burton wrote:
> On Fri, Jul 13, 2018 at 03:37:57PM +0800, Huacai Chen wrote:
> > Linux expects that if a CPU modifies a memory location, then that
> > modification will eventually become visible to other CPUs in the system.
> > 
> > On Loongson-3 processor with SFB (Store Fill Buffer), loads may be
> > prioritised over stores so it is possible for a store operation to be
> > postponed if a polling loop immediately follows it. If the variable
> > being polled indirectly depends on the outstanding store [for example,
> > another CPU may be polling the variable that is pending modification]
> > then there is the potential for deadlock if interrupts are disabled.
> > This deadlock occurs in qspinlock code.
> > 
> > This patch changes the definition of cpu_relax() to smp_mb() for
> > Loongson-3, forcing a flushing of the SFB on SMP systems before the
> > next load takes place. If the Kernel is not compiled for SMP support,
> > this will expand to a barrier() as before.
> > 
> > References: 534be1d5a2da940 (ARM: 6194/1: change definition of cpu_relax() for ARM11MPCore)
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Huacai Chen <chenhc@...ote.com>
> > ---
> >  arch/mips/include/asm/processor.h | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h
> > index af34afb..a8c4a3a 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's SFB (Store-Fill-Buffer) may get starved when stuck in a read
> > + * loop. Since spin loops of any kind should have a cpu_relax() in them, force
> > + * a Store-Fill-Buffer flush from cpu_relax() such that any pending writes will
> > + * become available as expected.
> > + */
> 
> I think "may starve writes" or "may queue writes indefinitely" would be
> clearer than "may get starved".

Agreed.

> > +#define cpu_relax()	smp_mb()
> > +#else
> >  #define cpu_relax()	barrier()
> > +#endif
> >  
> >  /*
> >   * Return_address is a replacement for __builtin_return_address(count)
> 
> Apart from the comment above though this looks better to me.
> 
> Re-copying the LKMM maintainers - are you happy(ish) with this?

Right, thanks for adding us back on :-)

Yes, this is much better, although I myself would also prefer explicit
mention that this is a work-around for a hardware bug.

But aside from the actual comment bike-shedding, this looks entirely
acceptible (also because ARM is already doing this -- and the Changelog
might want to refer to that particular patch).