On Thu, Dec 18, 2014 at 10:53:05AM -0800, David Daney wrote: > On 12/18/2014 07:09 AM, Markos Chandras wrote: > >sub $reg, imm is not a real MIPS instruction. The assembler replaces > >that with 'addi $reg, -imm'. > > That is a bug right there. We cannot have faulting instructions like this > in the kernel. The instruction is meant to kill the kernel if a readlock gets unlocked more often than it was taken. Think of it as an efficient method of implementing a BUG_ON() for this condition. I've only seen that overflow exception once and honestly, the primary reason was my desparate search for a rarely used CPU feature. So if this ADDI is repaced by an ADDIU I won't be sad. Ralf
