Smp recrsive spinlock confusion in RP2040

Kernel
1

Hello!
I am trying to port freertos smp and reference RP2040

I have confusion about acquiring lock:

        if( uxAcquire )
        {
            if( __builtin_expect( !*pxSpinLock, 0 ) )
            {
                if( ucOwnedByCore[ulCoreNum] & ulLockBit )
                {
                    configASSERT(ucRecursionCountByLock[ulLockNum] != 255u );
                    ucRecursionCountByLock[ulLockNum]++;
                    return;
                }
                while ( __builtin_expect( !*pxSpinLock, 0 ) );
            }
            __mem_fence_acquire();
            configASSERT(ucRecursionCountByLock[ulLockNum] == 0 );
             /* there needs a operation to mark the pxSpinLock? */
            ucRecursionCountByLock[ulLockNum] = 1;
            ucOwnedByCore[ulCoreNum] |= ulLockBit;

When a task acquires lock successfully，it should change ‘ pxSpinLock’ value to 0 to indicating the lock is acquired.

Besides, I do not think this spinlock implement is mutual exclusion.

Best regards

2

You do not need to do that explicitly as these are hardware spin locks. Look at the hardware spinlocks section in the datasheet.

Would you please elaborate your thinking.

1 Like
3

Thank you for your reply.

The spinlock from RP2040 is implemented by hardwire. I thought if is is implemented by soft, it may require mutual exclusion operation to ensure atomic.