On Fri, Jul 21, 2023 at 08:34:55AM +0200, Andreas Schwab wrote:
On Jul 20 2023, Matthew Wilcox wrote:+static inline bool clear_bit_unlock_is_negative_byte(unsigned int nr, + volatile unsigned long *p) +{ + char result; + char mask = 1 << nr; /* nr guaranteed to be < 7 */ + + __asm__ __volatile__ ("eori %1, %2; smi %0"Why are you using XOR if you want to clear a bit? If it operates on a byte, why does it receive a pointer to long?
It's a clever hack. This function has exactly one user, and it's an important one -- folio_unlock(). Bit 7 is set if there are other threads waiting for this folio to be unlocked. There are two reasonable implementations, depending what kind of CPU you have; you can either load-locked; clear the bottom bit, store-conditional, test bit 7. Or x86 and m68k have the perfect instruction to clear a bit and set the Negative flag if bit 7 is set. As I said in the earlier email, BCLR doesn't affect the N flag, but EORI and ANDI do. We are guaranteed that the bit we're clearing is set, so EORI will work. ANDI would also work. Do you happen to know if __GCC_ASM_FLAG_OUTPUTS__ is implemented for m68k so we can save the SMI instruction?
