Re: [Crash-utility] [tools-team] Questions on multi-thread for crash

Tao Liu <ltao(a)redhat.com&gt; writes: > CC the discussion to tools-team > > Hi lianbo, > > Thanks for the reply. Yes I agree by filtering out the symbols which > do not exist for current kernel will decrease the startup work and > shorten the waiting time. It is a different technical path, however I > believe multi-thread can achieve more, including symbol resolve. For > example in main.c, several xx_init() functions can be parallelled as > well after careful arrangement. > > Hi tools team, > > Sorry for the interruption, I don't know if it is a topic worthy of > discussion, is gdb a thread-safe debugger, or can it be thread-safe? I > can provide a little more background here. We are using crash utility > for kernel vmcore debugging, and crash is built upon gdb (By patching > some code to gdb source and compiling and linking them together). Hi Tao, I'd like to try and answer your question. Apologies if I have misunderstood you, but, it sounds like you are somehow combining GDB with some additional code to create a new "crash" utility. Almost like you are trying to use GDB as a debugger library.

If this is the case, then no, GDB is certainly not thread safe. GDB isn't a library, and was not written as one. I guess it'll work just fine if you only try to use a single instance of GDB within "crash", but if you have multiple threads calling into different parts of GDB then things are going to go wrong quickly.

> > I notice gdb already support multi-thread somehow (in > gdb-10.2/gdbsupport/thread-pool.cc, and "GDB may use multiple threads > to speed up certain CPU-intensive operations, such as demangling > symbol names" as I quoted from gdb-10.2/gdb/maint.c). So it seems to > me at least some part of gdb is thread safe. GDB does make use of multiple threads to speed up some aspects of the DWARF loading and symbol table building. But this is really a very limited corner of GDB (though definitely something that is time consuming so benefits from parallelism), the core of GDB, the bit that uses the symbol table to provide a debugger to the user, is all single threaded.

> The problem is I don't > know the thread safe boundary, so segfault and broken stack are easily > triggered when multithread enabled in crash utility. I really don't think what you want is going to work with GDB, so I've not dug through to find the exact code that runs in the worker threads, but I know its limited to DWARF parsing and symbol table creation.

> > In addition, if any other modern debuggers, such as lldb, can provide > more multi-thread support, then I can give it a try, but I lack > related info. Any suggestions and comments are welcomed, thanks in > advance! I don't know if any other debuggers are written more as a library, so can't help here, sorry.

Thanks, Andrew > > Thanks, > Tao Liu > > On Thu, Feb 9, 2023 at 5:28 PM lijiang <lijiang(a)redhat.com&gt; wrote: >> >> On Wed, Feb 8, 2023 at 4:26 PM <crash-utility-request(a)redhat.com&gt; wrote: >>> >>> Message: 1 >>> Date: Wed, 8 Feb 2023 12:34:32 +0800 >>> From: Tao Liu <ltao(a)redhat.com&gt; >>> To: crash-utility(a)redhat.com >>> Subject: [Crash-utility] Questions on multi-thread for crash >>> Message-ID: <Y+MmWBi+kq+ZSDqn(a)localhost.localdomain&gt; >>> 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. >>> >> >> Good question, Tao. >> >>> >>> 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! >>> >> >> Can you try to load some symbols on demand when crash initializes? And later, load and cache >> these symbols in crash when we execute a crash command for the first time, but it may have another >> issue, the crash command might be slow for the first time. >> >> In addition, can you also try to filter out some old and unuseful symbols? For example: >> >> Some kernel symbols have been removed from the latest kernel, if the current vmcore >> generated by the latest kernel version, crash won't need to check or search for these old >> kernel symbols when initializing. Otherwise, still load these old kernel symbols. Maybe it >> may save the initializing time. >> >> Thanks. >> Lianbo >> >> >>> Thanks! >>> Tao Liu