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Commit 3ba7f44d authored by Paul Burton's avatar Paul Burton Committed by Ralf Baechle
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MIPS: cmpxchg: Implement 1 byte & 2 byte cmpxchg() 



Implement support for 1 & 2 byte cmpxchg() using read-modify-write atop
a 4 byte cmpxchg(). This allows us to support these atomic operations
despite the MIPS ISA only providing 4 & 8 byte atomic operations.

This is required in order to support queued rwlocks (qrwlock) in a later
patch, since these make use of a 1 byte cmpxchg() in their slow path.

Signed-off-by: default avatarPaul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16355/


Signed-off-by: default avatarRalf Baechle <ralf@linux-mips.org>
parent b70eb300

arch/mips/include/asm/cmpxchg.h

+7 −0
Original line number Diff line number Diff line
@@ -142,10 +142,17 @@ static inline unsigned long __xchg(unsigned long x, volatile void * ptr, int siz 
	__ret;								\

})



extern unsigned long __cmpxchg_small(volatile void *ptr, unsigned long old,

				     unsigned long new, unsigned int size);



static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,

				      unsigned long new, unsigned int size)

{

	switch (size) {

	case 1:

	case 2:

		return __cmpxchg_small(ptr, old, new, size);



	case 4:

		return __cmpxchg_asm("ll", "sc", (volatile u32 *)ptr, old, new);

arch/mips/kernel/cmpxchg.c

+57 −0
Original line number Diff line number Diff line
@@ -50,3 +50,60 @@ unsigned long __xchg_small(volatile void *ptr, unsigned long val, unsigned int s 


	return (load32 & mask) >> shift;

}



unsigned long __cmpxchg_small(volatile void *ptr, unsigned long old,

			      unsigned long new, unsigned int size)

{

	u32 mask, old32, new32, load32;

	volatile u32 *ptr32;

	unsigned int shift;

	u8 load;



	/* Check that ptr is naturally aligned */

	WARN_ON((unsigned long)ptr & (size - 1));



	/* Mask inputs to the correct size. */

	mask = GENMASK((size * BITS_PER_BYTE) - 1, 0);

	old &= mask;

	new &= mask;



	/*

	 * Calculate a shift & mask that correspond to the value we wish to

	 * compare & exchange within the naturally aligned 4 byte integer

	 * that includes it.

	 */

	shift = (unsigned long)ptr & 0x3;

	if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN))

		shift ^= sizeof(u32) - size;

	shift *= BITS_PER_BYTE;

	mask <<= shift;



	/*

	 * Calculate a pointer to the naturally aligned 4 byte integer that

	 * includes our byte of interest, and load its value.

	 */

	ptr32 = (volatile u32 *)((unsigned long)ptr & ~0x3);

	load32 = *ptr32;



	while (true) {

		/*

		 * Ensure the byte we want to exchange matches the expected

		 * old value, and if not then bail.

		 */

		load = (load32 & mask) >> shift;

		if (load != old)

			return load;



		/*

		 * Calculate the old & new values of the naturally aligned

		 * 4 byte integer that include the byte we want to exchange.

		 * Attempt to exchange the old value for the new value, and

		 * return if we succeed.

		 */

		old32 = (load32 & ~mask) | (old << shift);

		new32 = (load32 & ~mask) | (new << shift);

		load32 = cmpxchg(ptr32, old32, new32);

		if (load32 == old32)

			return old;

	}

}