Message: 1
Date: Wed, 8 Feb 2023 12:34:32 +0800
From: Tao Liu <ltao@redhat.com>
To: crash-utility@redhat.com
Subject: [Crash-utility] Questions on multi-thread for crash
Message-ID: <Y+MmWBi+kq+ZSDqn@localhost.localdomain>
Content-Type: text/plain; charset=iso-8859-1

Hello,

Recently I made an attempt to introduce a thread pool for crash utility, to
optimize the performance of crash.

One obvious point which can benefit from multi-threading is memory.c:vm_init().
There are hundreds of MEMBER_OFFSET_INIT() related symbol resolving functions,
and most of the symbols are independent from each other, by careful arrangement,
they can be invoked parallelly. By doing so, we can shorten the waiting time of
crash starting.

The implementation is abstracted as the following:

Before multi-threading:
        MEMBER_OFFSET_INIT(task_struct_mm, "task_struct", "mm");
        MEMBER_OFFSET_INIT(mm_struct_mmap, "mm_struct", "mmap");

After multi-threading:
        create_threadpool(&pool, 3);
        ...
        MEMBER_OFFSET_INIT_PARA(pool, task_struct_mm, "task_struct", "mm");
        MEMBER_OFFSET_INIT_PARA(pool, mm_struct_mmap, "mm_struct", "mmap");
        ...
        wait_and_destroy_threadpool(pool);

MEMBER_OFFSET_INIT_PARA just append the task to the work queue of thread pool
and continues, it's up to the pool to schedule the worker thread to do the
symbol resolving work.

However, after enable multi-threading, I noticed there are always random errors
from gdb. From segfault to broken stack, it seems gdb is not thread safe at
all...

For example one error listed as follows:

        Thread 10 "crash" received signal SIGSEGV, Segmentation fault.
        [Switching to Thread 0x7fffc4f00640 (LWP 72950)]
        c_yylex () at /sources/up-crash/gdb-10.2/gdb/c-exp.y:3250
        3250 ? if (pstate->language ()->la_language != language_cplus
        (gdb) bt
        #0 ?c_yylex () at /sources/up-crash/gdb-10.2/gdb/c-exp.y:3250
        #1 ?c_yyparse () at /sources/up-crash/gdb-10.2/gdb/c-exp.c.tmp:2092
        #2 ?0x00000000006f62d7 in c_parse (par_state=<optimized out>) at /sources/
            up-crash/gdb-10.2/gdb/c-exp.y:3414
        #3 ?0x0000000000894eac in parse_exp_in_context (stringptr=0x7fffc4efeff8,
            pc=<optimized out>, block=<optimized out>, comma=0, out_subexp=0x0,
            tracker=0x7fffc4efef10, cstate=0x0, void_context_p=0) at parse.c:1122
        #4 ?0x00000000008951d6 in parse_exp_1 (tracker=0x0, comma=0, block=0x0,
            pc=0, stringptr=0x7fffc4efeff8) at parse.c:1031
        #5 ?parse_expression (string=<optimized out>, string@entry=0x7fffc4eff140
            "slab_s", tracker=tracker@entry=0x0) at parse.c:1166
        #6 ?0x000000000092039a in gdb_get_datatype (req=0x7fffc4eff720) at symtab.c:7239
        #7 ?gdb_command_funnel_1 (req=0x7fffc4eff720) at symtab.c:7018
        #8 ?0x00000000009206de in gdb_command_funnel (req=0x7fffc4eff720) at symtab.c:6956
        #9 ?0x00000000005ad137 in gdb_interface (req=0x7fffc4eff720) at gdb_interface.c:409
        #10 0x00000000005fe76c in datatype_info (name=0xab9700 "slab_s",
            member=0xaba8d8 "list", dm=0x0) at symbols.c:5708
        #11 0x0000000000517a85 in member_offset_init_slab_s_list_slab_s_list ()
            at memory.c:659
        #12 0x000000000068168f in group_routine (args=<optimized out>) at thpool.c:81
        #13 0x00007ffff7a48b17 in start_thread () from /lib64/libc.so.6
        ? #14 0x00007ffff7acd6c0 in clone3 () from /lib64/libc.so.6
        (gdb) p pstate
        $1 = (parser_state *) 0x0

        $ cat -n /sources/up-crash/gdb-10.2/gdb/c-exp.y
        66 ?/* The state of the parser, used internally when we are parsing the
        67 ? ? expression. ?*/
        68 ?
        69 ?static struct parser_state *pstate = NULL;

pstate is a global variable and not thread safe, the value must be changed by
someone else... 

Now the project has reached a dead end. Because making gdb thread safe is an
impossible mission to me. Is there any advice or suggestions? Thanks in advance!

Thanks!
Tao Liu