x86: clean up rwsem type system

The fast version of the rwsems (the code that uses xadd) has
traditionally only worked on x86-32, and as a result it mixes different
kinds of types wildly - they just all happen to be 32-bit.  We have
"long", we have "__s32", and we have "int".

To make it work on x86-64, the types suddenly matter a lot more.  It can
be either a 32-bit or 64-bit signed type, and both work (with the caveat
that a 32-bit counter will only have 15 bits of effective write
counters, so it's limited to 32767 users).  But whatever type you
choose, it needs to be used consistently.

This makes a new 'rwsem_counter_t', that is a 32-bit signed type.  For a
64-bit type, you'd need to also update the BIAS values.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <alpine.LFD.2.00.1001121755220.17145@localhost.localdomain>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 15 insertions, 10 deletions

diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index 413620024768..5f9af3081d66 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -55,6 +55,9 @@ extern asmregparm struct rw_semaphore *
 
 /*
  * the semaphore definition
+ *
+ * The bias values and the counter type needs to be extended to 64 bits
+ * if we want to have more than 32767 potential readers/writers
  */
 
 #define RWSEM_UNLOCKED_VALUE            0x00000000
@@ -64,8 +67,10 @@ extern asmregparm struct rw_semaphore *
 #define RWSEM_ACTIVE_READ_BIAS          RWSEM_ACTIVE_BIAS
 #define RWSEM_ACTIVE_WRITE_BIAS         (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS)
 
+typedef signed int rwsem_count_t;
+
 struct rw_semaphore {
-        signed long             count;
+        rwsem_count_t           count;
         spinlock_t              wait_lock;
         struct list_head        wait_list;
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
@@ -121,7 +126,7 @@ static inline void __down_read(struct rw_semaphore *sem)
  */
 static inline int __down_read_trylock(struct rw_semaphore *sem)
 {
-        __s32 result, tmp;
+        rwsem_count_t result, tmp;
         asm volatile("# beginning __down_read_trylock\n\t"
                      "  mov          %0,%1\n\t"
                      "1:\n\t"
@@ -143,7 +148,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
  */
 static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
 {
-        int tmp;
+        rwsem_count_t tmp;
 
         tmp = RWSEM_ACTIVE_WRITE_BIAS;
         asm volatile("# beginning down_write\n\t"
@@ -170,9 +175,9 @@ static inline void __down_write(struct rw_semaphore *sem)
  */
 static inline int __down_write_trylock(struct rw_semaphore *sem)
 {
-        signed long ret = cmpxchg(&sem->count,
-                                  RWSEM_UNLOCKED_VALUE,
-                                  RWSEM_ACTIVE_WRITE_BIAS);
+        rwsem_count_t ret = cmpxchg(&sem->count,
+                                    RWSEM_UNLOCKED_VALUE,
+                                    RWSEM_ACTIVE_WRITE_BIAS);
         if (ret == RWSEM_UNLOCKED_VALUE)
                 return 1;
         return 0;
@@ -183,7 +188,7 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
  */
 static inline void __up_read(struct rw_semaphore *sem)
 {
-        __s32 tmp = -RWSEM_ACTIVE_READ_BIAS;
+        rwsem_count_t tmp = -RWSEM_ACTIVE_READ_BIAS;
         asm volatile("# beginning __up_read\n\t"
                      LOCK_PREFIX "  xadd      %1,(%2)\n\t"
                      /* subtracts 1, returns the old value */
@@ -201,7 +206,7 @@ static inline void __up_read(struct rw_semaphore *sem)
  */
 static inline void __up_write(struct rw_semaphore *sem)
 {
-        unsigned long tmp;
+        rwsem_count_t tmp;
         asm volatile("# beginning __up_write\n\t"
                      LOCK_PREFIX "  xadd      %1,(%2)\n\t"
                      /* tries to transition
@@ -245,9 +250,9 @@ static inline void rwsem_atomic_add(int delta, struct rw_semaphore *sem)
 /*
  * implement exchange and add functionality
  */
-static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
+static inline rwsem_count_t rwsem_atomic_update(int delta, struct rw_semaphore *sem)
 {
-        int tmp = delta;
+        rwsem_count_t tmp = delta;
 
         asm volatile(LOCK_PREFIX "xadd %0,%1"
                      : "+r" (tmp), "+m" (sem->count)