Re: [PATCH 04/33] Hold threads

[Janani Venkataraman]

On 03/21/2014 12:31 AM, Pavel Emelyanov wrote:
> On 03/20/2014 01:39 PM, Janani Venkataraman wrote:
> > Getting number of threads and their respective IDs through /proc/pid/stat and
> > /proc/pid/task.
> >
> > The threads are then seized and interrupted. After the dump is taken they are
> > detached.
> >
> > Signed-off-by: Janani Venkataraman <jananive@xxxxxxxxxxxxxxxxxx>
> > ---
> > +/* Gets the Thread IDS and siezes them */
> > +int seize_threads(int pid)
> > +{
> > +	char filename[40];
> > +	DIR *dir;
> > +	int ct = 0, ret = 0, tmp_tid;
> > +	struct dirent *entry;
> > +	char state;
> > +
> > +	ret = get_thread_count(pid);
> > +	if (ret == -1)
> > +		return -1;
> > +
> > +	cp.thread_count = ret;
> > +	cp.t_id = calloc(cp.thread_count, sizeof(int));
> > +	if (!cp.t_id) {
> > +		status = errno;
> > +		gencore_log("Could not allocate memory for thread_ids.\n");
> > +		return -1;
> > +	}
> > +
> > +	snprintf(filename, 40, "/proc/%d/task", pid);
> > +	dir = opendir(filename);
> > +
> > +	while ((entry = readdir(dir))) {
> This simple loop is not enough -- threads may appear and disappear while
> you do the readdir and seize, so you should scan it several times to
> make sure you caught all the threads.
I will look into this.
> You can look at how this is done in CRIU in cr-dump,c:collect_threads().
Yes I will look into that function. Thanks.

Thanks.
Janani
> > +		if (entry->d_type == DT_DIR && entry->d_name[0] != '.') {
> > +			tmp_tid = atoi(entry->d_name);
> > +			ret = ptrace(PTRACE_SEIZE, tmp_tid, 0, 0);
> > +			if (ret) {
> > +				state = get_thread_status(tmp_tid);
> > +				if (state == 'Z')
> > +					goto assign;
> > +				status = errno;
> > +				gencore_log("Could not seize thread: %d\n",
> > +								tmp_tid);
> > +				break;
> > +			}
> > +			ret = ptrace(PTRACE_INTERRUPT, tmp_tid, 0, 0);
> > +			if (ret) {
> > +				state = get_thread_status(tmp_tid);
> > +				if (state == 'Z')
> > +					goto assign;
> > +				status = errno;
> > +				gencore_log("Could not interrupt thread: %d\n",
> > +								tmp_tid);
> > +				break;
> > +			}
> > +assign:
> > +			/* If a new thread, is created after we fetch the thread_count,
> > +			 * we may encounter a buffer overflow situation in the cp_tid.
> > +			 * Hence we check this case and re-allocate memory if required.
> > +			 */
> > +			cp.t_id[ct++] = tmp_tid;
> > +		}
> > +	}
> > +
> > +	/* Reassigning based on successful seizes */
> > +	cp.thread_count = ct;
> > +
> > +	closedir(dir);
> > +
> > +	/* Successful seize and interrupt on all threads makes ret = 0 */
> > +	return ret;
> > +}

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