Re: [PATCH] [gdb/testsuite] Require hw watchpoint in gdb.ada/task_watch.exp

[Carl Love <cel@us.ibm.com>]

Tom:

On Mon, 2022-11-28 at 12:56 +0100, Tom de Vries wrote:
> From: Tom de Vries <tdevries@loganberry-1.arch.suse.de>
> 
> On powerpc64le-linux I run into:
> ...
> (gdb) PASS: gdb.ada/task_watch.exp: info tasks before inserting
> breakpoint
> watch -location value task 3^M
> Watchpoint 2: -location value^M
> (gdb) PASS: gdb.ada/task_watch.exp: watch -location value task 3
> continue^M
> Continuing.^M
> [Thread 0x7ffff7ccf170 (LWP 65550) exited]^M
> [Thread 0x7ffff7abf170 (LWP 65551) exited]^M
> FAIL: gdb.ada/task_watch.exp: continue to watchpoint (timeout)
> ...
> 
> On x86_64-linux (where the test-case passes), a hardware watchpoint
> is used:
> ...
> (gdb) PASS: gdb.ada/task_watch.exp: info tasks before inserting
> breakpoint
> watch -location value task 3^M
> Hardware watchpoint 2: -location value^M
> ...
> and after forcing "set can-use-hw-watchpoints 0" we can
> intermittently
> reproduce the same failure.
> 
> In the gdb documentation related to watchpoints in multi-threaded
> programs, we
> read:
> ...
> Warning: In multi-threaded programs, software watchpoints have only
> limited
> usefulness.  If GDB creates a software watchpoint, it can only watch
> the value
> of an expression in a single thread.  If you are confident that the
> expression
> can only change due to the current thread’s activity (and if you are
> also
> confident that no other thread can become current), then you can use
> software
> watchpoints as usual.  However, GDB may not notice when a non-current 
> thread’s
> activity changes the expression.  (Hardware watchpoints, in contrast,
> watch an
> expression in all threads.)
> ...
> 
> Since the ada task construct is mapped onto threads, it seems that
> the
> same limitation holds for tasks.
> 
> Fix this by using skip_hw_watchpoint_tests.
> 
> Tested on powerpc64-linux.
> ---
>  gdb/testsuite/gdb.ada/task_watch.exp | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/gdb/testsuite/gdb.ada/task_watch.exp
> b/gdb/testsuite/gdb.ada/task_watch.exp
> index b22a6204e90..697074ac164 100644
> --- a/gdb/testsuite/gdb.ada/task_watch.exp
> +++ b/gdb/testsuite/gdb.ada/task_watch.exp
> @@ -19,6 +19,8 @@ load_lib "ada.exp"
> 
>  if { [skip_ada_tests] } { return -1 }
> 
> +if { [skip_hw_watchpoint_tests] } { return -1 }
> +
>  standard_ada_testfile foo
> 
>  if {[gdb_compile_ada "${srcfile}" "${binfile}" executable {debug}]
> != ""} {

Thanks for looking at the ada test failures.  I have not gotten to
working on them yet.

This test passes on Power 10 with/without your patch.

The test fails on Power 9 since the hw watchpoint support is disabled
on Power 9 due to an issue.  The patch fixes the test by not running it
on Power 9.

                       Carl