Step causes GDB to spin in infinite loop when PC doesn't change

[Luis Machado]

Hi,

On 3/18/22 16:33, Denio, Mike via Gdb wrote:
> I am seeing an issue with the remote protocol  on GDB with multiple threads on a riscv32 using the step command. Not sure if it's a bug or just me, but it seems like a legitimate bug.
> 
> I am using a custom server. The behavior is the same on both GDB 11.1 and 11.2.
> 
> This GDB was configured as follows:
>     configure --host=x86_64-pc-linux-gnu --target=riscv32-unknown-elf
>               --with-auto-load-dir=$debugdir:$datadir/auto-load
>               --with-auto-load-safe-path=$debugdir:$datadir/auto-load
>               --with-expat
>               --with-gdb-datadir=/usr/local/share/gdb (relocatable)
>               --with-jit-reader-dir=/usr/local/lib/gdb (relocatable)
>               --without-libunwind-ia64
>               --with-lzma
>               --without-babeltrace
>               --without-intel-pt
>               --with-mpfr
>               --without-xxhash
>               --with-python=/usr
>               --with-python-libdir=/usr/lib
>               --without-debuginfod
>               --without-guile
>               --disable-source-highlight
>               --with-separate-debug-dir=/usr/local/lib/debug (relocatable)
> 
> I am running in non-stop mode. A full log of the test is at the end of the email. The GDB commands I execute after connecting are:
> (gdb) interrupt -a
> (gdb) s
> 
> The problem is that my code starts off in an infinite loop (endz: jal x0,endz), with the cores basically waiting for an interrupt. If I execute a 'step' command, GDB will basically keep sending "vCont;s:2" in an infinite loop, until the PC addr of thread 2 changes. It also occurs if I fall into an infinite loop while stepping. It doesn't need to be the first instruction.

That's what the step/next commands are designed to do. GDB will keep 
stepping until it sees a line change. I suppose a "continue" command 
here would be more appropriate, as GDB will send it once and keep 
waiting for the outcome. stepi/nexti would be undesirable given they 
will stop after a single instruction.