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

[David Holmes]

Hi Thomas,

On 11/02/2020 6:35 pm, Thomas Stüfe wrote:
> Hi David,
> 
> I did not find anything wrong in your patch. Nice cleanup, and great 
> archaeological work :)

Thanks for taking a look, and doing the additional testing.

> 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.

Right - fix this in the next round.

> -----
> 
> 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.

I made the same kind of suggestion :) Again something for round 2.

> -------
> 
> 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.

Yes not immediately obvious that the two expressions are the same but 
when you expand things out, they are.

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

:)

Thanks,
David
-----

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