RFR: 8234372: Investigate use of Thread::stack_base() and queries for "in stack"

[Thomas Stüfe]

Hi David,

I did not find anything wrong in your patch. Nice cleanup, and great
archaeological work :)

Only small nits and some bike shedding:

---

So, if I get this right:

is_in_stack -> is in live stack (base ... sp]
on_local_stack  -> is in (base...start] includes guard pages
is_in_usable_stack -> is in (base...start - guard] excludes guard pages

The naming is confusing but I saw you recommended renaming the functions in
the JBS comments, and I like all your suggestions better than what we have
now.

-----

http://cr.openjdk.java.net/~dholmes/8234372/webrev.v2/src/hotspot/cpu/aarch64/frame_aarch64.cpp.udiff.html

-  if (locals > thread->stack_base() || locals < (address) fp()) return
false;
+  if (locals >= thread->stack_base() || locals < (address) fp()) return
false;

This would be easier to read as a negated positive (also applies to all
other frame_xxx.cpp).

Just an idea, maybe we could add a function
Thread::is_in_stack_limited_by(ptr, arbitrary_end_ptr) which could compare
that ptr is between (base .. arbitrary_end_ptr] and based on that we could
implement the other three stack functions.

For cases like this we could then write:
if (!thread->is_in_stack_limited_by(locals, fp())

But I am unsure, maybe I overthink things.

-------

http://cr.openjdk.java.net/~dholmes/8234372/webrev.v2/src/hotspot/os/linux/os_linux.cpp.udiff.html

-  if (addr <  t->stack_base() && addr >= t->stack_reserved_zone_base()) {
+  if (t->is_in_usable_stack(addr)) {

First confused me but then I read Fredericks comment in JBS so I think it
is okay.

But it would be nice to be able to remove this manually-expand-stack-coding
altogether :)

-------

Cheers, Thomas

On Tue, Feb 11, 2020 at 6:40 AM David Holmes <david.holmes at oracle.com>
wrote:

> Bug: https://bugs.openjdk.java.net/browse/JDK-8234372
> webrev: http://cr.openjdk.java.net/~dholmes/8234372/webrev.v2/
>
> Following on from JDK-8215355 I checked all uses of Thread::stack_base()
> to watch for range tests that should be exclusive but are inclusive, and
> vice-versa. And in addition clarified and streamlined the various "in
> stack" checks that are made.
>
> Summary of changes:
>
> src/hotspot/cpu/aarch64/frame_aarch64.cpp
> src/hotspot/cpu/arm/frame_arm.cpp
> src/hotspot/cpu/ppc/frame_ppc.cpp
> src/hotspot/cpu/s390/frame_s390.cpp
> src/hotspot/cpu/sparc/frame_sparc.cpp
> src/hotspot/cpu/x86/frame_x86.cpp
>
> In terms of actual bugs the implementation of frame::safe_for_sender on
> all platforms except x86 and aarch64 was using the wrong range test in a
> number of cases, so these are all now correct and consistent.
>
> All platforms had an incorrect range check in relation to the "locals".
>
> All platforms now use is_in_usable_stack to check for a valid sp, rather
> than duplicating (sometimes incorrectly) that logic.
>
> --
>
> src/hotspot/os/linux/os_linux.cpp
> src/hotspot/os/windows/os_windows.cpp
>
> Replaced explicit range check with is_in_usable_stack
>
> src/hotspot/os_cpu/linux_arm/os_linux_arm.cpp
> src/hotspot/os_cpu/linux_s390/thread_linux_s390.cpp
>
> Replaced explicit range check with on_local_stack.
>
> ---
>
> src/hotspot/share/runtime/thread.?pp
>
> Moved is_in_usable_stack from Thread to JavaThread (guard regions are
> only relevant for JavaThreads).
>
> Clarified functionality and use of the three "in stack" variants.
>
> Removed redundant check from is_in_stack:
>
> !   // Allow non Java threads to call this without stack_base
> !   if (_stack_base == NULL) return true;
>
> As this is executed by the current thread, and the very first thing a
> thread does is set its stack base and size, it is impossible to find a
> NULL stack_base (which is already asserted inside stack_base()). [I
> tested this extensively just as a sanity check: tiers 1-5 plus hotspot
> runtime/serviceability/gc.]
>
> Misc cleanup to use stack_end() rather than recalulate it.
>
> ---
>
> There are some further possible cleanups here but I didn't want to go
> too far with things that would obscure the functional changes too much.
> As mentioned in the bug report the three "in stack" functions would
> benefit from some minor renamings so that their relationship is clearer.
> But I can leave that to a follow on RFE. Further, it may be possible to
> replace a lot of the remaining uses of stack_base() with a more
> constrained "in stack" function that takes a limit. For example, rather
> than something like:
>
> if (thread->stack_base() > fp && fp >= sp)
>
> have:
>
> if (thread->is_in_stack_range(fp /* addr*/, sp /*limit*/))
>
> which checks the given addr against stack_base and the limit, and checks
> the limit against stack_end(). The difficultly may lie in determining
> whether checking against the limit should be a > or >= test, as it will
> be dependent on the context. Again this seems like something for a
> second RFE.
>
> ---
>
> Testing:
>
> Thanks to Andrew Haley for taking the frame changes for a spin on
> ARM/PPC/Aarch64/S390(?).
>
> I also ran our tier 1 to 3 testing on x86 and sparc.
>
> Thanks,
> David
>