On Tue, Jan 19, 2010 at 02:03:35AM -0800, Roland McGrath wrote: > > It is also not clear to me if disabling pre-emption for the user-space > > (albeit for a very tiny time-window) is incorrect and if their side-effects > > are known. If otherwise, I think we should choose to operate in pre-empt > > safe mode and avoid all costs associated when done without it. > > I never really gave much consideration to returning to user mode with > preemption disabled. It would not really have occurred to me that was > even possible. I can't say it seems to me like it could ever be a very > good idea. I find it hard even to start listing the cans of worms that > might be opened by that. Perhaps the powerpc maintainers have a clearer > picture of it than I do. > > What does it mean when there is something that prevents it from returning > to user mode? i.e., TIF_SIGPENDING or TIF_NEED_RESCHED, or whatever. It > could do a lot in the kernel before it gets back to user mode. What if in > there somewhere it blocks voluntarily? > > Similarly, what does it mean if you get to user mode but the single-stepped > instruction is a load/store that gets a page fault? What if it blocks in > the page fault handler? > > For that matter, what about a page fault for the kernel-mode case? > > Perhaps I'm imagining gremlins where there aren't any, but I just cannot > really get my head around this "disable preemption while running some > unknown instruction that normally runs with preemption enabled" thing--let > alone "disable preemption while returning to user mode". > > > Thanks, > Roland
I posted a patch which re-enables pre-emption after a hw-breakpoint is processed (linuxppc-dev ref: 20100121084640.ga3...@in.ibm.com). It does lead to clumsiness (due to the new variables to track states, prior breakpoints, etc.) but with the reasons you pointed out, it is much better than having uncertain/incorrect code. Thanks for your comments. -- K.Prasad _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
