On 01/20/2015 03:29 AM, Markos Chandras wrote:
On 01/20/2015 01:04 AM, Maciej W. Rozycki wrote:
On Fri, 16 Jan 2015, Markos Chandras wrote:
sub $reg, imm is not a real MIPS instruction. The assembler replaces
that with 'addi $reg, -imm'. However, addi has been removed from R6,
so we replace the 'sub' instruction with 'addiu' instead.
Signed-off-by: Markos Chandras <markos.chandras@xxxxxxxxxx>
---
arch/mips/include/asm/spinlock.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/mips/include/asm/spinlock.h b/arch/mips/include/asm/spinlock.h
index c6d06d383ef9..500050d3bda6 100644
--- a/arch/mips/include/asm/spinlock.h
+++ b/arch/mips/include/asm/spinlock.h
@@ -276,7 +276,7 @@ static inline void arch_read_unlock(arch_rwlock_t *rw)
do {
__asm__ __volatile__(
"1: ll %1, %2 # arch_read_unlock \n"
- " sub %1, 1 \n"
+ " addiu %1, -1 \n"
" sc %1, %0 \n"
: "=" GCC_OFF12_ASM() (rw->lock), "=&r" (tmp)
: GCC_OFF12_ASM() (rw->lock)
This integer overflow trap is deliberate here -- have you seen the note
just above:
/* Note the use of sub, not subu which will make the kernel die with an
overflow exception if we ever try to unlock an rwlock that is already
unlocked or is being held by a writer. */
According to a comment on another thread from Ralf, this has been
observed in the wild only once. We can simplify the code and remove
that comment. Why not just use the ADDIU and be done with it?
There are many locking and atomic primitives that don't have any such
error checking. What makes the read lock so special that it needs this
extra protection?
David Daney
