[PATCH 1/1] lib: sbi: relax scoldboot_lock spinlock

[Heinrich Schuchardt]

On 13.08.20 18:36, Jessica Clarke wrote:
> On 13 Aug 2020, at 17:24, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>>
>> OpenSBI hangs in wake_coldboot_harts() indefinitely on the MaixDuino board
>> when trying to boot U-Boot as FW_PAYLOAD.
>
> This smells of there being a more fundamental issue that's not being
> addressed which this manages to work around, either reliably or by
> chance. What is the actual deadlock condition you observe?

There are only two function using coldboot_lock spinlock:

* wake_coldboot_harts()
* wait_for_coldboot()

wake_coldboot_harts() was entered but never left without patching one of
the functions.

The Kendryte K210 has two harts. One goes the cold boot path while the
other is sent to the warm boot path.

>
>> Even if reading and writing coldboot_done is not atomic it is safe to check
>> it without using a spinlock.
>
> Maybe microarchitecturally on the specific hardware in question (though
> I wouldn't even be so certain about that), but definitely not in C. You
> now have a data race in reading coldboot_done, which isn't even
> volatile^, so a sufficiently smart compiler would be completely within
> its right to do whatever it wants to the code, including things like
> delete the loop entirely (or, slightly less dramatic, turn it into an
> if). This patch introduces undefined behaviour.
>
> Jess
>
> ^ Please do not make it volatile as a workaround, that is not the right
>   way to "fix" such issues.

What do you suggest Jessica?

Best regards

Heinrich

>
>> Signed-off-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
>> ---
>> lib/sbi/sbi_init.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/lib/sbi/sbi_init.c b/lib/sbi/sbi_init.c
>> index a7fb848..a0e4f11 100644
>> --- a/lib/sbi/sbi_init.c
>> +++ b/lib/sbi/sbi_init.c
>> @@ -106,14 +106,14 @@ static void wait_for_coldboot(struct sbi_scratch *scratch, u32 hartid)
>> 	sbi_hartmask_set_hart(hartid, &coldboot_wait_hmask);
>>
>> 	/* Wait for coldboot to finish using WFI */
>> +	spin_unlock(&coldboot_lock);
>> 	while (!coldboot_done) {
>> -		spin_unlock(&coldboot_lock);
>> 		do {
>> 			wfi();
>> 			cmip = csr_read(CSR_MIP);
>> 		 } while (!(cmip & MIP_MSIP));
>> -		spin_lock(&coldboot_lock);
>> 	};
>> +	spin_lock(&coldboot_lock);
>>
>> 	/* Unmark current HART as waiting */
>> 	sbi_hartmask_clear_hart(hartid, &coldboot_wait_hmask);
>> --
>> 2.28.0
>>
>>
>> --
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