[PATCH 1/2] lib: sbi_init: Avoid thundering hurd problem with coldbook_lock

[Atish Patra]

On Sat, Aug 15, 2020 at 10:01 AM Anup Patel <anup.patel at wdc.com> wrote:
>
> We can have thundering hurd problem with coldboot_lock where the
> boot HART can potentially starve trying to acquire coldboot_lock
> because some of the non-boot HARTs are continuously acquiring and
> releasing coldboot_lock. This can happen if WFI is a NOP OR if
> MIP.MSIP bit is already set for some of the non-boot HARTs.
>
> To avoid thundering hurd problem for coldboot_lock, we convert
> coldboot_done flag into atomic variable and using coldboot_lock
> only for coldboot_wait_hmask.
>
> Signed-off-by: Anup Patel <anup.patel at wdc.com>
> Tested-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
> ---
>  lib/sbi/sbi_init.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
> index a7fb848..6b58983 100644
> --- a/lib/sbi/sbi_init.c
> +++ b/lib/sbi/sbi_init.c
> @@ -85,9 +85,10 @@ static void sbi_boot_prints(struct sbi_scratch *scratch, u32 hartid)
>  }
>
>  static spinlock_t coldboot_lock = SPIN_LOCK_INITIALIZER;
> -static unsigned long coldboot_done = 0;
>  static struct sbi_hartmask coldboot_wait_hmask = { 0 };
>
> +static atomic_t coldboot_done = ATOMIC_INITIALIZER(0);
> +
>  static void wait_for_coldboot(struct sbi_scratch *scratch, u32 hartid)
>  {
>         unsigned long saved_mie, cmip;
> @@ -105,16 +106,20 @@ static void wait_for_coldboot(struct sbi_scratch *scratch, u32 hartid)
>         /* Mark current HART as waiting */
>         sbi_hartmask_set_hart(hartid, &coldboot_wait_hmask);
>
> +       /* Release coldboot lock */
> +       spin_unlock(&coldboot_lock);
> +
>         /* Wait for coldboot to finish using WFI */
> -       while (!coldboot_done) {
> -               spin_unlock(&coldboot_lock);
> +       while (!atomic_read(&coldboot_done)) {
>                 do {
>                         wfi();
>                         cmip = csr_read(CSR_MIP);
>                  } while (!(cmip & MIP_MSIP));
> -               spin_lock(&coldboot_lock);
>         };
>
> +       /* Acquire coldboot lock */
> +       spin_lock(&coldboot_lock);
> +
>         /* Unmark current HART as waiting */
>         sbi_hartmask_clear_hart(hartid, &coldboot_wait_hmask);
>
> @@ -132,12 +137,12 @@ static void wake_coldboot_harts(struct sbi_scratch *scratch, u32 hartid)
>  {
>         const struct sbi_platform *plat = sbi_platform_ptr(scratch);
>
> +       /* Mark coldboot done */
> +       atomic_write(&coldboot_done, 1);
> +
>         /* Acquire coldboot lock */
>         spin_lock(&coldboot_lock);
>
> -       /* Mark coldboot done */
> -       coldboot_done = 1;
> -
>         /* Send an IPI to all HARTs waiting for coldboot */
>         for (int i = 0; i <= sbi_scratch_last_hartid(); i++) {
>                 if ((i != hartid) &&
> --
> 2.25.1
>
>
> --
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Reviewed-by: Atish Patra <atish.patra at wdc.com>

-- 
Regards,
Atish