On Thu, Mar 03, 2016 at 08:15:13PM +0000, Peter Maydell wrote: > Hi Edgar -- I'm just looking back at these signal handling > race condition fix patches, and with this one I have a confusion > about the Microblaze Linux syscall code that I hope you can > clear up for me. > > Looking at the kernel entry.S code it looks to me like > the way syscalls work on microblaze is: > * syscall insn is brki r14 > * the insn itself saves the PC of the brki into r14 > * on entry the kernel advances r14 by 4 to skip the brki > * then SAVE_REGS saves r14 into the 'PC' slot in the pt_regs > struct > * for syscall restart handle_restart() may wind the PC > value in the pt_regs back by 4 > * in any case, on syscall exit we pull the PC value out of > pt_regs into r14, and do a return with rtbd r14, 0
Yes, that sounds right. > > I think what this implies is that: > * r14 is a "used by the kernel, may be corrupted at any > time, not to be touched by userspace" register Yes. r14 is not really usable by user-space, interrupts will for example clobber r14 at any time aswell. > * on exit from a syscall PC and r14 are always the same Yes that's how it works but as far as user-space is concerned r14 may have any value at any time as it's not really observable in a safe way. > * this includes do_sigreturn, ie "taking a signal" is one > of the things that can corrupt r14 Yes. > > Is that right? Yes, I think so. > (For context, the original patch is this one: > http://patchwork.ozlabs.org/patch/514879/ > and I now suspect my review comments at the time to be wrong.) I see. Functionally I think the patch is OK. It seems to have some whitespace fixes mixed with functional changes (nitpick). Either way: Reviewed-by: Edgar E. Iglesias <edgar.igles...@xilinx.com> Best regards, Edgar