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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().
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README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.