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Re: [PATCH] Stop threads when attaching to a PID that is the tgid
- From: Luis Machado <lgustavo at codesourcery dot com>
- To: Pedro Alves <pedro at codesourcery dot com>
- Cc: gdb-patches at sourceware dot org
- Date: Wed, 24 Aug 2011 09:22:23 -0300
- Subject: Re: [PATCH] Stop threads when attaching to a PID that is the tgid
- References: <4E53B25B.3030009@codesourcery.com> <201108231848.15068.pedro@codesourcery.com>
- Reply-to: lgustavo at codesourcery dot com
Hi,
On 08/23/2011 02:48 PM, Pedro Alves wrote:
On Tuesday 23 August 2011 14:59:55, Luis Machado wrote:
Hi,
Missing explanation as to _why_ we need this. Here's one:
On linux/ptrace, a ptracer needs to attach to each
and every thread of a process individually. Currently, when you
do `gdbserver --attach PID', GDBserver only attaches to the PID thread,
leaving all the other threads of the process running free. This is because
GDBserver relies on thread_db to list the threads of PID, and, in order
to activate thread_db, GDBserver needs to query GDB about some symbols,
which obviously can only work once GDB connects..
To fix this, this patch teaches GDBServer to list threads using /proc,
instead of relying on thread_db, and attach/stop all of them.
This patch teaches GDBServer how to stop threads from a TID that is also
the TGID, without having to rely on GDB to list and stop them upon
connection.
2011-08-23 Luis Machado<lgustavo@codesourcery.com>
* linux-low.c: Include linux-procfs.h.
(linux_attach_lwp_1): Update comments.
(linux_attach): Scan for existing threads when attaching to a
process that is the tgid.
--- .pc/stop_threads.diff/gdb/gdbserver/linux-low.c 2011-08-23 10:38:50.653049001 -0300
+++ gdb/gdbserver/linux-low.c 2011-08-23 10:58:02.817049000 -0300
@@ -26,6 +26,7 @@
#include<sys/param.h>
#include<sys/ptrace.h>
#include "linux-ptrace.h"
+#include "linux-procfs.h"
Sorry I missed this before. This needs an update to the linux-low.o
rule in Makefile.in. gdbserver doesn't do automatic header
dependencies... This is okay otherwise.
Thanks, i've checked the following in.
2011-08-24 Luis Machado <lgustavo@codesourcery.com>
* linux-low.c: Include linux-procfs.h.
(linux_attach_lwp_1): Update comments.
(linux_attach): Scan for existing threads when attaching to a
process that is the tgid.
* Makefile.in: Update dependencies.
--- .pc/stop_threads.diff/gdb/gdbserver/linux-low.c 2011-08-24 09:19:15.757049000 -0300
+++ gdb/gdbserver/linux-low.c 2011-08-24 09:19:33.965049000 -0300
@@ -26,6 +26,7 @@
#include <sys/param.h>
#include <sys/ptrace.h>
#include "linux-ptrace.h"
+#include "linux-procfs.h"
#include <signal.h>
#include <sys/ioctl.h>
#include <fcntl.h>
@@ -586,7 +587,9 @@ linux_attach_lwp_1 (unsigned long lwpid,
}
if (initial)
- /* NOTE/FIXME: This lwp might have not been the tgid. */
+ /* If lwp is the tgid, we handle adding existing threads later.
+ Otherwise we just add lwp without bothering about any other
+ threads. */
ptid = ptid_build (lwpid, lwpid, 0);
else
{
@@ -621,8 +624,10 @@ linux_attach_lwp_1 (unsigned long lwpid,
In this case we want the process thread to stop.
This is handled by having linux_attach set last_resume_kind ==
resume_stop after we return.
- ??? If the process already has several threads we leave the other
- threads running.
+
+ If the pid we are attaching to is also the tgid, we attach to and
+ stop all the existing threads. Otherwise, we attach to pid and
+ ignore any other threads in the same group as this pid.
3) GDB is connecting to gdbserver and is requesting an enumeration of all
existing threads.
@@ -646,9 +651,14 @@ linux_attach_lwp (unsigned long lwpid)
linux_attach_lwp_1 (lwpid, 0);
}
+/* Attach to PID. If PID is the tgid, attach to it and all
+ of its threads. */
+
int
linux_attach (unsigned long pid)
{
+ /* Attach to PID. We will check for other threads
+ soon. */
linux_attach_lwp_1 (pid, 1);
linux_add_process (pid, 1);
@@ -662,6 +672,65 @@ linux_attach (unsigned long pid)
thread->last_resume_kind = resume_stop;
}
+ if (linux_proc_get_tgid (pid) == pid)
+ {
+ DIR *dir;
+ char pathname[128];
+
+ sprintf (pathname, "/proc/%ld/task", pid);
+
+ dir = opendir (pathname);
+
+ if (!dir)
+ {
+ fprintf (stderr, "Could not open /proc/%ld/task.\n", pid);
+ fflush (stderr);
+ }
+ else
+ {
+ /* At this point we attached to the tgid. Scan the task for
+ existing threads. */
+ unsigned long lwp;
+ int new_threads_found;
+ int iterations = 0;
+ struct dirent *dp;
+
+ while (iterations < 2)
+ {
+ new_threads_found = 0;
+ /* Add all the other threads. While we go through the
+ threads, new threads may be spawned. Cycle through
+ the list of threads until we have done two iterations without
+ finding new threads. */
+ while ((dp = readdir (dir)) != NULL)
+ {
+ /* Fetch one lwp. */
+ lwp = strtoul (dp->d_name, NULL, 10);
+
+ /* Is this a new thread? */
+ if (lwp
+ && find_thread_ptid (ptid_build (pid, lwp, 0)) == NULL)
+ {
+ linux_attach_lwp_1 (lwp, 0);
+ new_threads_found++;
+
+ if (debug_threads)
+ fprintf (stderr, "\
+Found and attached to new lwp %ld\n", lwp);
+ }
+ }
+
+ if (!new_threads_found)
+ iterations++;
+ else
+ iterations = 0;
+
+ rewinddir (dir);
+ }
+ closedir (dir);
+ }
+ }
+
return 0;
}
--- .pc/stop_threads.diff/gdb/gdbserver/Makefile.in 2011-08-24 09:19:15.709049000 -0300
+++ gdb/gdbserver/Makefile.in 2011-08-24 09:19:33.981049000 -0300
@@ -354,6 +354,8 @@ linux_ptrace_h = $(srcdir)/../common/lin
gdb_thread_db_h = $(srcdir)/../common/gdb_thread_db.h
+linux_procfs_h = $(srcdir)/../common/linux-procfs.h
+
lynx_low_h = $(srcdir)/lynx-low.h $(srcdir)/server.h
nto_low_h = $(srcdir)/nto-low.h
@@ -405,6 +407,9 @@ gdbreplay.o: gdbreplay.c config.h
signals.o: ../common/signals.c $(server_h) $(signals_def)
$(CC) -c $(CPPFLAGS) $(INTERNAL_CFLAGS) $< -DGDBSERVER
+linux-procfs.o: ../common/linux-procfs.c $(server_h)
+ $(CC) -c $(CPPFLAGS) $(INTERNAL_CFLAGS) $< -DGDBSERVER
+
common-utils.o: ../common/common-utils.c $(server_h)
$(CC) -c $(CPPFLAGS) $(INTERNAL_CFLAGS) $< -DGDBSERVER
@@ -443,7 +448,8 @@ i386-low.o: i386-low.c $(i386_low_h) $(s
i387-fp.o: i387-fp.c $(server_h)
-linux-low.o: linux-low.c $(linux_low_h) $(linux_ptrace_h) $(server_h) $(linux_osdata_h)
+linux-low.o: linux-low.c $(linux_low_h) $(linux_ptrace_h) $(linux_procfs_h) \
+ $(server_h) $(linux_osdata_h)
$(CC) -c $(CPPFLAGS) $(INTERNAL_CFLAGS) $< @USE_THREAD_DB@
linux-arm-low.o: linux-arm-low.c $(linux_low_h) $(server_h) \