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    Subject[PATCH 4.14 151/165] MIPS: Change definition of cpu_relax() for Loongson-3
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Huacai Chen <chenhc@lemote.com>

    commit a30718868915fbb991a9ae9e45594b059f28e9ae upstream.

    Linux expects that if a CPU modifies a memory location, then that
    modification will eventually become visible to other CPUs in the system.

    Loongson 3 CPUs include a Store Fill Buffer (SFB) which sits between a
    core & its L1 data cache, queueing memory accesses & allowing for faster
    forwarding of data from pending stores to younger loads from the core.
    Unfortunately the SFB prioritizes loads such that a continuous stream of
    loads may cause a pending write to be buffered indefinitely. This is
    problematic if we end up with 2 CPUs which each perform a store that the
    other polls for - one or both CPUs may end up with their stores buffered
    in the SFB, never reaching cache due to the continuous reads from the
    poll loop. Such a deadlock condition has been observed whilst running
    qspinlock code.

    This patch changes the definition of cpu_relax() to smp_mb() for
    Loongson-3, forcing a flush of the SFB on SMP systems which will cause
    any pending writes to make it as far as the L1 caches where they will
    become visible to other CPUs. If the kernel is not compiled for SMP
    support, this will expand to a barrier() as before.

    This workaround matches that currently implemented for ARM when
    CONFIG_ARM_ERRATA_754327=y, which was introduced by commit 534be1d5a2da
    ("ARM: 6194/1: change definition of cpu_relax() for ARM11MPCore").

    Although the workaround is only required when the Loongson 3 SFB
    functionality is enabled, and we only began explicitly enabling that
    functionality in v4.7 with commit 1e820da3c9af ("MIPS: Loongson-3:
    Introduce CONFIG_LOONGSON3_ENHANCEMENT"), existing or future firmware
    may enable the SFB which means we may need the workaround backported to
    earlier kernels too.

    [paul.burton@mips.com:
    - Reword commit message & comment.
    - Limit stable backport to v3.15+ where we support Loongson 3 CPUs.]

    Signed-off-by: Huacai Chen <chenhc@lemote.com>
    Signed-off-by: Paul Burton <paul.burton@mips.com>
    References: 534be1d5a2da ("ARM: 6194/1: change definition of cpu_relax() for ARM11MPCore")
    References: 1e820da3c9af ("MIPS: Loongson-3: Introduce CONFIG_LOONGSON3_ENHANCEMENT")
    Patchwork: https://patchwork.linux-mips.org/patch/19830/
    Cc: Ralf Baechle <ralf@linux-mips.org>
    Cc: James Hogan <jhogan@kernel.org>
    Cc: linux-mips@linux-mips.org
    Cc: Fuxin Zhang <zhangfx@lemote.com>
    Cc: Zhangjin Wu <wuzhangjin@gmail.com>
    Cc: Huacai Chen <chenhuacai@gmail.com>
    Cc: stable@vger.kernel.org # v3.15+
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/mips/include/asm/processor.h | 13 +++++++++++++
    1 file changed, 13 insertions(+)

    --- a/arch/mips/include/asm/processor.h
    +++ b/arch/mips/include/asm/processor.h
    @@ -388,7 +388,20 @@ unsigned long get_wchan(struct task_stru
    #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 buffer writes indefinitely when a
    + * tight read loop is executed, because reads take priority over writes & the
    + * hardware (incorrectly) doesn't ensure that writes will eventually occur.
    + *
    + * Since spin loops of any kind should have a cpu_relax() in them, force an SFB
    + * flush from cpu_relax() such that any pending writes will become visible as
    + * expected.
    + */
    +#define cpu_relax() smp_mb()
    +#else
    #define cpu_relax() barrier()
    +#endif

    /*
    * Return_address is a replacement for __builtin_return_address(count)

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