[PATCH] riscv: locks: introduce ticket-based spinlock implementation
guoren at kernel.org
Wed Apr 14 06:54:18 BST 2021
On Tue, Apr 13, 2021 at 6:54 PM David Laight <David.Laight at aculab.com> wrote:
> From: Catalin Marinas
> > Sent: 13 April 2021 11:45
> > This indeed needs some care. IIUC RISC-V has similar restrictions as arm
> > here, no load/store instructions are allowed between LR and SC. You
> > can't guarantee that the compiler won't spill some variable onto the
> > stack.
> You can probably never guarantee the compiler won't spill to stack.
> Especially if someone compiles with -O0.
> Which probably means that anything using LR/SC must be written in
> asm and the C wrappers disabled.
Agree, and cmpxchg has been widely used in Linux. I think it's the
last requirement for complex atomic API, although cmpxchg has ABA
old32 = load32;
*ptr32 = new32_tmp;
*ptr32 = old32;
load32 = cmpxchg(ptr32, old32, new32); //still success
} while (load32 != old32);
That means cmpxhg only cares about the result but not the middle
situation. It's different from LR/SC or AMO instructions.
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
More information about the linux-riscv