Fei Li <lifei1...@126.com> writes:
> 在 2019/1/9 上午1:29, Markus Armbruster 写道:
>> fei <lifei1...@126.com> writes:
>>
>>>> 在 2019年1月8日，01:55，Markus Armbruster <arm...@redhat.com> 写道：
>>>>
>>>> Fei Li <f...@suse.com> writes:
>>>>
>>>>> To avoid the segmentation fault in qemu_thread_join(), just directly
>>>>> return when the QemuThread *thread failed to be created in either
>>>>> qemu-thread-posix.c or qemu-thread-win32.c.
>>>>>
>>>>> Cc: Stefan Weil <s...@weilnetz.de>
>>>>> Signed-off-by: Fei Li <f...@suse.com>
>>>>> Reviewed-by: Fam Zheng <f...@redhat.com>
>>>>> ---
>>>>> util/qemu-thread-posix.c | 3 +++
>>>>> util/qemu-thread-win32.c | 2 +-
>>>>> 2 files changed, 4 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
>>>>> index 39834b0551..3548935dac 100644
>>>>> --- a/util/qemu-thread-posix.c
>>>>> +++ b/util/qemu-thread-posix.c
>>>>> @@ -571,6 +571,9 @@ void *qemu_thread_join(QemuThread *thread)
>>>>> int err;
>>>>> void *ret;
>>>>>
>>>>> + if (!thread->thread) {
>>>>> + return NULL;
>>>>> + }
>>>> How can this happen?
>>> I think I have answered this earlier, please check the following link to
>>> see whether it helps:
>>> http://lists.nongnu.org/archive/html/qemu-devel/2018-11/msg06554.html
>> Thanks for the pointer. Unfortunately, I don't understand your
>> explanation. You also wrote there "I will remove this patch in next
>> version"; looks like you've since changed your mind.
> Emm, issues left over from history.. The background is I was hurry to
> make those five
> Reviewed-by patches be merged, including this v9 16/16 patch but not
> the real
> qemu_thread_create() modification. But actually this patch is to fix
> the segmentation
> fault after we modified qemu_thread_create() related functions
> although it has got a
> Reviewed-by earlier. :) Thus to not make troube, I wrote the
> "remove..." sentence
> to separate it from those 5 Reviewed-by patches, and were plan to send
> only four patches.
> But later I got a message that these five patches are not that urgent
> to catch qemu v3.1,
> thus I joined the earlier 5 R-b patches into the later v8 & v9 to have
> a better review.
>
> Sorry for the trouble, I need to explain it without involving too much
> background..
>
> Back at the farm: in our current qemu code, some cleanups use a loop
> to join()
> the total number of threads if caller fails. This is not a problem
> until applying the
> qemu_thread_create() modification. E.g. when compress_threads_save_setup()
> fails while trying to create the last do_data_compress thread,
> segmentation fault
> will occur when join() is called (sadly there's not enough condition
> to filter this
> unsuccessful created thread) as this thread is actually not be created.
>
> Hope the above makes it clear. :)
Alright, let's have a look at compress_threads_save_setup():
static int compress_threads_save_setup(void)
{
int i, thread_count;
if (!migrate_use_compression()) {
return 0;
}
thread_count = migrate_compress_threads();
compress_threads = g_new0(QemuThread, thread_count);
comp_param = g_new0(CompressParam, thread_count);
qemu_cond_init(&comp_done_cond);
qemu_mutex_init(&comp_done_lock);
for (i = 0; i < thread_count; i++) {
comp_param[i].originbuf = g_try_malloc(TARGET_PAGE_SIZE);
if (!comp_param[i].originbuf) {
goto exit;
}
if (deflateInit(&comp_param[i].stream,
migrate_compress_level()) != Z_OK) {
g_free(comp_param[i].originbuf);
goto exit;
}
/* comp_param[i].file is just used as a dummy buffer to save data,
* set its ops to empty.
*/
comp_param[i].file = qemu_fopen_ops(NULL, &empty_ops);
comp_param[i].done = true;
comp_param[i].quit = false;
qemu_mutex_init(&comp_param[i].mutex);
qemu_cond_init(&comp_param[i].cond);
qemu_thread_create(compress_threads + i, "compress",
do_data_compress, comp_param + i,
QEMU_THREAD_JOINABLE);
}
return 0;
exit:
compress_threads_save_cleanup();
return -1;
}
At label exit, we have @i threads, all fully initialized. That's an
invariant.
compress_threads_save_cleanup() finds the threads to clean up by
checking comp_param[i].file:
static void compress_threads_save_cleanup(void)
{
int i, thread_count;
if (!migrate_use_compression() || !comp_param) {
return;
}
thread_count = migrate_compress_threads();
for (i = 0; i < thread_count; i++) {
/*
* we use it as a indicator which shows if the thread is
* properly init'd or not
*/
---> if (!comp_param[i].file) {
---> break;
---> }
qemu_mutex_lock(&comp_param[i].mutex);
comp_param[i].quit = true;
qemu_cond_signal(&comp_param[i].cond);
qemu_mutex_unlock(&comp_param[i].mutex);
qemu_thread_join(compress_threads + i);
qemu_mutex_destroy(&comp_param[i].mutex);
qemu_cond_destroy(&comp_param[i].cond);
deflateEnd(&comp_param[i].stream);
g_free(comp_param[i].originbuf);
qemu_fclose(comp_param[i].file);
comp_param[i].file = NULL;
}
qemu_mutex_destroy(&comp_done_lock);
qemu_cond_destroy(&comp_done_cond);
g_free(compress_threads);
g_free(comp_param);
compress_threads = NULL;
comp_param = NULL;
}
Due to the invariant, a comp_param[i] with a null .file doesn't need
*any* cleanup.
To maintain the invariant, compress_threads_save_setup() carefully
cleans up any partial initializations itself before a goto exit. Since
the code is arranged smartly, the only such cleanup is the
g_free(comp_param[i].originbuf) before the second goto exit.
Your PATCH 13 adds a third goto exit, but neglects to clean up partial
initializations. Breaks the invariant.
I see two sane solutions:
1. compress_threads_save_setup() carefully cleans up partial
initializations itself. compress_threads_save_cleanup() copes only
with fully initialized comp_param[i]. This is how things work before
your series.
2. compress_threads_save_cleanup() copes with partially initialized
comp_param[i], i.e. does the right thing for each goto exit in
compress_threads_save_setup(). compress_threads_save_setup() doesn't
clean up partial initializations.
Your PATCH 13 together with the fixup PATCH 16 does
3. A confusing mix of the two.
Don't.
> Have a nice day
> Fei
>>
>> What exactly breaks if we omit this patch? Assuming something does
>> break: imagine we did omit this patch, then forgot we ever saw it, and
>> now you've discovered the breakage. Write us the bug report, complete
>> with reproducer.
>>
>> [...]
