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

[Daniel D. Daugherty]

Hi David,

Very nice cleanup of some crufty/old code.


On 2/11/20 12:40 AM, David Holmes wrote:
> Bug: https://bugs.openjdk.java.net/browse/JDK-8234372
> webrev: http://cr.openjdk.java.net/~dholmes/8234372/webrev.v2/

src/hotspot/share/runtime/thread.hpp
     L691:   // Warning: the method can only be used on the running thread.
         Not fond of this existing comment wording. Perhaps:

             // Warning: Can only be called by the calling thread on itself.

src/hotspot/share/runtime/thread.cpp
     L1822: // Check for adr in the usable portion of our stack.
         Not fond of "our stack" here. To me it implies the caller's
         stack.  Perhaps:

            // Check for adr in the usable portion of the JavaThread's 
stack.

         This is the comment from the thread.hpp:

         L1733:   // Check if address is in the usable part of the stack 
(excludes protected
         L1734:   // guard pages). Can be applied to any thread and is 
an approximation for
         L1735:   // using is_in_stack when the query has to happen from 
another thread.

         It is much more clear, but also very long.


src/hotspot/os/linux/os_linux.cpp
     Note: Old code is this:

     L722:   if (addr <  t->stack_base() && addr >= 
t->stack_reserved_zone_base()) {

     so it is different than the Win* version. The new call is better.

src/hotspot/os/windows/os_windows.cpp
     Note: Old code is this:

     L2545:           if (addr > thread->stack_reserved_zone_base() && 
addr < thread->stack_base()) {

     so it is different than the Linux version. The new call is better.


src/hotspot/cpu/sparc/frame_sparc.cpp
     No comments.

src/hotspot/cpu/x86/frame_x86.cpp
     No comments.


src/hotspot/cpu/aarch64/frame_aarch64.cpp
     No comments.

src/hotspot/cpu/arm/frame_arm.cpp
     L144:       bool saved_fp_safe = ((address)saved_fp < 
thread->stack_base()) && (saved_fp > sender_sp);
     L174:       bool saved_fp_safe = ((address)saved_fp < 
thread->stack_base()) && (saved_fp >= sender_sp);
         L144 uses '>' and L174 uses '>=' with the same operands. Why?

src/hotspot/os_cpu/linux_arm/os_linux_arm.cpp
     No comments.


src/hotspot/cpu/ppc/frame_ppc.cpp
     L72:   bool fp_safe = (fp < thread->stack_base()) &&  (fp > sp);
         existing nit: extra space before second (...) expression.


src/hotspot/cpu/s390/frame_s390.cpp
     L76:   bool fp_safe = (fp < thread->stack_base()) &&  (fp > sp);
         existing nit: extra space before second (...) expression.

src/hotspot/os_cpu/linux_s390/thread_linux_s390.cpp
     No comments.


Other than one possible typo in src/hotspot/cpu/arm/frame_arm.cpp
everything else is a nit. I don't need to see another webrev.

Thumbs up!

As you like to say: The proof will be in the testing.
And I'll change that last word to "testing over time" since some
of these code paths might be rarely used...

Dan



>
> 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