gdb: Do not interrupt atomic sequences for ARC

gdb: Do not interrupt atomic sequences for ARC

When stepping over thread-lock related codes (in uClibc), the inferior process
gets stuck and never manages to enter the critical section:

------8<-------
 1 size_t fwrite(const void * __restrict ptr, size_t size,
 2               size_t nmemb, register FILE * __restrict stream)
 3 {
 4     size_t retval;
 5     __STDIO_AUTO_THREADLOCK_VAR;
 6
 7 >   __STDIO_AUTO_THREADLOCK(stream);
 8
 9     retval = fwrite_unlocked(ptr, size, nmemb, stream);
10
11     __STDIO_AUTO_THREADUNLOCK(stream);
12
13     return retval;
14 }
------>8-------

Here, we are at line 7.  Using the "next" command leads no where.
However, setting a breakpoint on line 9 and issuing "continue" works.

Looking at the assembly instructions reveals that we're dealing with the
critical section entry code [1] that should never be interrupted, in this
case by the debugger's implicit breakpoints:

------8<-------
  ...
1 add_s   r0,r13,0x38
2 mov_s   r3,1
3 llock   r2,[r0]        <-.
4 brne.nt r2,0,14     --.  |
5 scond   r3,[r0]       |  |
6 bne     -10         --|--'
7 brne_s  r2,0,84     <-'
  ...
------>8-------

Lines 3 until 5 (inclusive) are supposed to be executed atomically.
Therefore, GDB should never (implicitly) insert a breakpoint on lines
4 and 5, else the program will try to acquire the lock again by jumping
back to line 3 and gets stuck in an infinite loop.

The solution is to make GDB aware of these patterns so it inserts
breakpoints after the sequence -- line 6 in this example.

[1]
https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/tree/libc/sysdeps/linux/arc/bits/atomic.h#n46
------8<-------
  ({									\
	__typeof(oldval) prev;						\
									\
	__asm__ __volatile__(						\
	"1:	llock   %0, [%1]	\n"				\
	"	brne    %0, %2, 2f	\n"				\
	"	scond   %3, [%1]	\n"				\
	"	bnz     1b		\n"				\
	"2:				\n"				\
	: "=&r"(prev)							\
	: "r"(mem), "ir"(oldval),					\
	  "r"(newval) /* can't be "ir". scond can't take limm for "b" */\
	: "cc", "memory");						\
									\
	prev;								\
  })
------>8-------
"llock" (Load Locked) loads the 32-bit word pointed by the source
operand.  If the load is completed without any interruption or
exception, the physical address is remembered, in Lock Physical Address
(LPA), and the Lock Flag (LF) is set to 1.  LF is a non-architecturally
visible flag and is cleared whenever an interrupt or exception takes
place.  LF is also cleared (atomically) whenever another process writes
to the LPA.

"scond" (Store Conditional) will write to the destination address if
and only if the LF is set to 1.  When finished, with or without a write,
it atomically copies the LF value to ZF (Zero Flag).

These two instructions together provide the mechanism for entering a
critical section.  The code snippet above comes from uClibc:
-----------------------

v3 (after Tom's remarks[2]):
 handle_atomic_sequence()
  - no need to initialize the std::vector with "{}"
  - fix typo in comments: "conditial" -> "conditional"
  - add braces to the body of "if" condition because of the comment line
 arc_linux_software_single_step()
  - make the performance slightly more efficient by moving a few
    variables after the likely "return" point.

v2 (after Simon's remarks[3]):
- handle_atomic_sequence() gets a copy of an instruction instead of
  a reference.
- handle_atomic_sequence() asserts if the given instruction is an llock.

[2]
https://sourceware.org/pipermail/gdb-patches/2021-February/175805.html

[3]
https://sourceware.org/pipermail/gdb-patches/2021-January/175487.html

gdb/ChangeLog:

	PR tdep/27369
	* arc-linux-tdep.c (handle_atomic_sequence): New.
	(arc_linux_software_single_step): Call handle_atomic_sequence().