[fedora-arm] ARMv5 and atomic operations

[Nicolas Pitre]

On Sun, 22 Apr 2012, Chris Tyler wrote:

> 6. There is the kernel's "user space atomic helper" (kuser_cmpxchg64) at
> 0xffff0f60, see Documentation/arm/kernel_user_helpers.txt. The kernel
> puts an instruction sequence here tuned for the current arch that can be
> called by userland to provide an atomic compare/exchange -- if it can be
> done natively, the instruction sequence does that, otherwise it does a
> syscall (with IRQ protection etc).

No, there is no syscall involved here.  Doing so would have killed 
performances for any trivial amount of atomic operations.

The way this works is to perform the operation, using non-atomic 
instructions as they're not available on ARMv5, and if an interrupt or a 
page fault occurs inside the critical region that the kernel controls, 
then the operation is rewinded and resumed the next time the user space 
task gets to run.  This way you get the same performance as with native 
atomic instructions on later processors, except for the added branch 
instruction.


Nicolas