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

[Jessica Clarke]

On 16 Aug 2020, at 15:02, Anup Patel <anup at brainfault.org> wrote:
> 
> On Sun, Aug 16, 2020 at 12:20 PM Bin Meng <bmeng.cn at gmail.com> wrote:
>> 
>> On Sun, Aug 16, 2020 at 3:49 AM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
>>> 
>>> On 15 Aug 2020, at 18:00, 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
>>> 
>>> That is neither necessary nor sufficient, it's solely based on
>>> whether the hart believes an M-mode software interrupt is pending?
>>> 
>>>> 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)) {
>>> 
>>> Personally I'd make this a relaxed read and then explicitly fence
>>> outside the loop, as otherwise if we end up with MSIP erroneously set
>>> there may be a lot of cache coherency traffic due to repeated
>>> unnecessary fences?
>>> 
>>>>              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);
>>> 
>>> This only needs to be a store release.
>> 
>> I believe relaxed read / write can work as well.
> 
> Even if we use relaxed read / write, we will still need explicit Acquire
> and Release barriers. Better to use __smp_store_release() and
> __smp_load_acquire().

Why not just use the C11 versions? Inline assembly is rarely needed
these days for atomics. Especially helpful given the large hammer that
is volatile inline assembly with a memory clobber causes highly
pessimistic code generation from the compiler. In general it should
only be necessary when needing to deal with I/O memory.

Jess