From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 16479 invoked by alias); 19 Jul 2010 16:01:42 -0000 Mailing-List: contact archer-help@sourceware.org; run by ezmlm Sender: Precedence: bulk List-Post: List-Help: List-Subscribe: List-Id: Received: (qmail 16102 invoked by uid 22791); 19 Jul 2010 16:01:40 -0000 X-SWARE-Spam-Status: No, hits=-5.2 required=5.0 tests=AWL,BAYES_00,FAKE_REPLY_C,RCVD_IN_DNSWL_HI,SPF_HELO_PASS,T_RP_MATCHES_RCVD X-Spam-Check-By: sourceware.org Date: Mon, 19 Jul 2010 16:01:00 -0000 From: Jan Kratochvil To: Oleg Nesterov Cc: archer@sourceware.org Subject: Re: Q: mutlithreaded tracees && clone/exit Message-ID: <20100719160127.GA13331@host1.dyn.jankratochvil.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100718174851.GA15528@redhat.com> <20100716205147.GA26313@redhat.com> User-Agent: Mutt/1.5.20 (2009-12-10) X-SW-Source: 2010-q3/txt/msg00038.txt.bz2 On Fri, 16 Jul 2010 22:51:47 +0200, Oleg Nesterov wrote: > In case this matters, I used gdb-7.1 for testing. FSF GDB (not Fedora/RHEL GDB) probably. > Q1: if gsbstub reported that the tracee has exited (say, we sent > '$X09#..' to gdb), can gsbstub assume it can forget about this thread? `X' is about processes, not threads ('W'=TARGET_WAITKIND_EXITED, 'X'=TARGET_WAITKIND_SIGNALLED). Threads death is handled by GDB-driven 'T' packet (remote_thread_alive). (I just mostly read the GDB sources, I am intact by the remote GDB stuff.) > I mean, can it assume that gdb won't send something like 'D;EXITED_PID'? TARGET_WAITKIND_EXITED and TARGET_WAITKIND_SIGNALLED in handle_inferior_event() call target_mourn_inferior(), this is very terminal. > Looking at gdb sources/behaviour, I think the answer is yes, it can > forget. But I'd like to have the confirmation. Yes, I also think so. I cannot give the confirmation. > And. I'd like to let you know that gdb is buggy ;) Please file those bugs while discussing them here: http://sourceware.org/bugzilla/enter_bug.cgi?product=gdb > The user presses ^C, gdb sends 3 and waits for reply. Suppose that > gdbstub doesn't reply immediately. IMHO this remote GDB protocol and non-stop mode are primarily tested with Eclipse-over-MI. Bugs faced by GDB CLI are going to be very common. > I noticed this bug when I found another problem, gdb+gdbserver doesn't > work correctly if the main thread exits. But let's forget about this > problem for now. This issue does not work well even with linux-nat.c (local GDB), in the current development stage of ugdb I believe we do not have to solve it before linux-nat.c gets fixed first: GDB hangs with simple multi-threaded program on linux http://sourceware.org/ml/gdb/2010-07/msg00045.html > The main question is, I do not understand how gdbstub should handle the > multithreaded targets. [...] > (gdb) file test1 > (gdb) target extended-remote :2000 > (gdb) attach 16927 > Attached to process 16927 > ... > 0x00000033af60e57d in pause () from /lib64/libpthread.so.0 > (gdb) > > OK. gdbserver ptraces both threads. But afaics gdb doesn't now this > program is multithreaded, > Q2: Shouldn't gdbstub let debugger know about sub-threads somehow? gdb did not ask for it so why gdbserver should tell gdbserver it? (gdb) info threads [New Thread 14739.14740] <-- GDB has notified it now. 2 Thread 14739.14740 0x000000349e8a6a6d in nanosleep () at ../sysdeps/unix/syscall-template.S:82 * 1 Thread 14739.14739 0x000000349f007fbd in pthread_join (threadid=140515741927184, thread_return=0x0) at pthread_join.c:89 Eclipse apparently does `info threads' over MI so it is not a problem. Also as you state in non-stop mode gdb asks for the thread list anyway. > gdbserver resumes both threads. Press enter, the sub-thread exits. > > And nothing happens! gdbserver sends nothing to gdb, it just reaps > the tracee silently: ... > Q3: is it correct? shouldn't we inform the debugger? GDB will sooner or later use the 'T' packet (remote_thread_alive) to reclaim dead threads. With libpthread_db (linux-thread-db.c) it just sets thread_info->private->dying = 1; on TD_DEATH anyway and continues tracking the threads before its kernel task finally dies. > So. Afaics, gdb can only find the new thread if the user does > "info threads", or if this thread reports somehow about itself > (say, it gets a signal and gdbserver sends "$T..." with its tid). Yes, GDB is master of the remote protocol communication. Not the gdbserver. > Also. gdb can't know the sub-thread has exited unless the user > does "info threads" again, or something like "$TpPID.TGID" gets > "E01" in reply. > > Correct? > > Q4: is this what we want to implement? IMO yes, we should first get ugdb a bit on-par with linux-nat.c, don't we? > I am asking because that I thought that gdb+gdbserver should > try to work the same way as it works without gdbserver, and > thus it should see clone/exit. GDB has two lists of "threads": Real libpthread / libthread_db / linux-thread-db.c / struct thread_info * which is primarily used. Name is displayed by thread_db_pid_to_str(). Then there are also kernel tasks / linux-nat.c / struct lwp_info * which are provided when libthread_db is not available. This second category IIRC does not work so well as linux-thread-db.c is more commonly in use (I do not have a fail testcase offhand, it may work). Name is displayed by linux_nat_pid_to_str(). Both types of "threads" are displayed by GDB CLI `info threads'. Their name format differs a bit, according to the display name function (to_pid_to_str). > However, gdbserver sends nothing to gdb if the tracee does > pthread_create() or pthread_exit(). yes On Sun, 18 Jul 2010 19:48:51 +0200, Oleg Nesterov wrote: > gdbserver tracks PTRACE_EVENT_CLONE, yes. But it doesn't inform gdb. IMO we can tune the non-libpthread mode later, AFAIK it does not work well with linux-nat.c anyway. > > gdb also uses higher-level knowledge read from user memory > > (libthread_db) for some aspects of thread tracking. > > Well, yes and no (if I understood your message correctly). > > I have already looked at this code in horror. I really hope this magic > is not needed for our purposes. > > It is gdbserver, not gdb, who uses libthread_db to find sub-threads and > do other things. > > gdbserver asks gdb what is the symbol's address (say, _thread_db_list_t_next) > via 'qSymbol'. i see this can be a problem for ugdb. Guessing we will need to change GDB to support new variant of proc-service.c working over the GDB protocol wire. > However, there is the complication I already mentioned. If the main > thread exits, this confuses gdbserver at least. Replied above, this is a GDB bug even with linux-nat.c first, it was fixed in Fedora GDB before but for some cases it apparently still does not work. Thanks, Jan