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@xxxxxxxxxxxxxxx > > Signed-off-by: Huacai Chen <chenhc@xxxxxxxxxx> > > --- > > 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).
