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
On Tue, Jan 19, 2010 at 02:03:35AM -0800, Roland McGrath wrote:
quoted
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.GA3252@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