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: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16355/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
This commit is contained in:
Paul Burton 2017-06-09 17:26:40 -07:00 committed by Ralf Baechle
parent b70eb30056
commit 3ba7f44d2b
2 changed files with 64 additions and 0 deletions

View File

@ -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);

View File

@ -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;
}
}