On 07/09/2015 04:35 AM, Paolo Bonzini wrote:
>
>
> On 08/07/2015 21:13, Kevin Wolf wrote:
>> Am 07.07.2015 um 15:09 hat Juan Quintela geschrieben:
>>> From: Li Zhijian <lizhij...@cn.fujitsu.com>
>>>
>>> Signed-off-by: Li Zhijian <lizhij...@cn.fujitsu.com>
>>> Signed-off-by: Wen Congyang <we...@cn.fujitsu.com>
>>> Signed-off-by: Juan Quintela <quint...@redhat.com>
>>> ---
>>> migration/ram.c | 23 +++++++++++++++++------
>>> 1 file changed, 17 insertions(+), 6 deletions(-)
>>
>> In current master, HMP 'savevm' is broken (looks like a deadlock in RCU
>> code, it just hangs indefinitely). git bisect points to this patch.
>
> This looks like synchronize_rcu() is being called within
> rcu_read_lock()/rcu_read_unlock().
>
> The easiest fix is to somehow use call_rcu, but I haven't looked at the
> code very well.
Yes, why migration doesn't trigger this problem? We will fix it soon.
Thanks
Wen Congyang
>
> I found another embarrassing bug in the RCU code, but it's been there
> forever and can wait for after -rc0 (and it wasn't really a problem
> until BQL-less MMIO was merged a couple days ago).
>
> Paolo
>
>> The stack trace looks like this:
>>
>> (gdb) thread apply all bt
>>
>> Thread 3 (Thread 0x7f06febfe700 (LWP 5717)):
>> #0 0x00007f070e749f7d in __lll_lock_wait () from /lib64/libpthread.so.0
>> #1 0x00007f070e745d32 in _L_lock_791 () from /lib64/libpthread.so.0
>> #2 0x00007f070e745c38 in pthread_mutex_lock () from /lib64/libpthread.so.0
>> #3 0x00007f070fed8bc9 in qemu_mutex_lock (mutex=mutex@entry=0x7f07107e6700
>> <rcu_gp_lock>) at util/qemu-thread-posix.c:73
>> #4 0x00007f070fee7631 in synchronize_rcu () at util/rcu.c:129
>> #5 0x00007f070fee77d9 in call_rcu_thread (opaque=<optimized out>) at
>> util/rcu.c:240
>> #6 0x00007f070e743df5 in start_thread () from /lib64/libpthread.so.0
>> #7 0x00007f07066ab1ad in clone () from /lib64/libc.so.6
>>
>> Thread 2 (Thread 0x7f06f940f700 (LWP 5719)):
>> #0 0x00007f070e749f7d in __lll_lock_wait () from /lib64/libpthread.so.0
>> #1 0x00007f070e74c4ec in _L_cond_lock_792 () from /lib64/libpthread.so.0
>> #2 0x00007f070e74c3c8 in __pthread_mutex_cond_lock () from
>> /lib64/libpthread.so.0
>> #3 0x00007f070e747795 in pthread_cond_wait@@GLIBC_2.3.2 () from
>> /lib64/libpthread.so.0
>> #4 0x00007f070fed8ca9 in qemu_cond_wait (cond=<optimized out>,
>> mutex=mutex@entry=0x7f07103b0400 <qemu_global_mutex>) at
>> util/qemu-thread-posix.c:132
>> #5 0x00007f070fc4daab in qemu_tcg_cpu_thread_fn (arg=<optimized out>) at
>> /home/kwolf/source/qemu/cpus.c:1050
>> #6 0x00007f070e743df5 in start_thread () from /lib64/libpthread.so.0
>> #7 0x00007f07066ab1ad in clone () from /lib64/libc.so.6
>>
>> Thread 1 (Thread 0x7f070fb20bc0 (LWP 5716)):
>> #0 0x00007f07066a5949 in syscall () from /lib64/libc.so.6
>> #1 0x00007f070fed8fa2 in futex_wait (val=4294967295, ev=0x7f07107e66c0
>> <rcu_gp_event>) at util/qemu-thread-posix.c:301
>> #2 qemu_event_wait (ev=ev@entry=0x7f07107e66c0 <rcu_gp_event>) at
>> util/qemu-thread-posix.c:399
>> #3 0x00007f070fee7713 in wait_for_readers () at util/rcu.c:120
>> #4 synchronize_rcu () at util/rcu.c:149
>> #5 0x00007f070fc6e0c2 in migration_end () at
>> /home/kwolf/source/qemu/migration/ram.c:1033
>> #6 0x00007f070fc6ef23 in ram_save_complete (f=0x7f07122f9aa0,
>> opaque=<optimized out>) at /home/kwolf/source/qemu/migration/ram.c:1241
>> #7 0x00007f070fc71d75 in qemu_savevm_state_complete
>> (f=f@entry=0x7f07122f9aa0) at /home/kwolf/source/qemu/migration/savevm.c:836
>> #8 0x00007f070fc7298e in qemu_savevm_state (errp=0x7ffe2a081ff8,
>> f=0x7f07122f9aa0) at /home/kwolf/source/qemu/migration/savevm.c:945
>> #9 hmp_savevm (mon=0x7f071233b500, qdict=<optimized out>) at
>> /home/kwolf/source/qemu/migration/savevm.c:1353
>> #10 0x00007f070fc552d0 in handle_hmp_command (mon=mon@entry=0x7f071233b500,
>> cmdline=0x7f0712350197 "foo") at /home/kwolf/source/qemu/monitor.c:4058
>> #11 0x00007f070fc56467 in monitor_command_cb (opaque=0x7f071233b500,
>> cmdline=<optimized out>, readline_opaque=<optimized out>)
>> at /home/kwolf/source/qemu/monitor.c:5081
>> #12 0x00007f070fee6dbf in readline_handle_byte (rs=0x7f0712350190,
>> ch=<optimized out>) at util/readline.c:391
>> #13 0x00007f070fc55387 in monitor_read (opaque=<optimized out>,
>> buf=<optimized out>, size=<optimized out>) at
>> /home/kwolf/source/qemu/monitor.c:5064
>> #14 0x00007f070fd17b21 in qemu_chr_be_write (len=<optimized out>,
>> buf=0x7ffe2a082640 "\n\331\367\325\001\200\377\377\200&\b*\376\177",
>> s=0x7f0712304670)
>> at qemu-char.c:306
>> #15 fd_chr_read (chan=<optimized out>, cond=<optimized out>,
>> opaque=0x7f0712304670) at qemu-char.c:1012
>> #16 0x00007f070e04c9ba in g_main_context_dispatch () from
>> /lib64/libglib-2.0.so.0
>> #17 0x00007f070fe61678 in glib_pollfds_poll () at main-loop.c:199
>> #18 os_host_main_loop_wait (timeout=<optimized out>) at main-loop.c:244
>> #19 main_loop_wait (nonblocking=<optimized out>) at main-loop.c:493
>> #20 0x00007f070fc24e9e in main_loop () at vl.c:1901
>> #21 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>)
>> at vl.c:4649
>>
>> Kevin
>>
>>> diff --git a/migration/ram.c b/migration/ram.c
>>> index 644f52a..9c0bcfe 100644
>>> --- a/migration/ram.c
>>> +++ b/migration/ram.c
>>> @@ -494,6 +494,7 @@ static int save_xbzrle_page(QEMUFile *f, uint8_t
>>> **current_data,
>>> return 1;
>>> }
>>>
>>> +/* Called with rcu_read_lock() to protect migration_bitmap */
>>> static inline
>>> ram_addr_t migration_bitmap_find_and_reset_dirty(MemoryRegion *mr,
>>> ram_addr_t start)
>>> @@ -502,26 +503,31 @@ ram_addr_t
>>> migration_bitmap_find_and_reset_dirty(MemoryRegion *mr,
>>> unsigned long nr = base + (start >> TARGET_PAGE_BITS);
>>> uint64_t mr_size = TARGET_PAGE_ALIGN(memory_region_size(mr));
>>> unsigned long size = base + (mr_size >> TARGET_PAGE_BITS);
>>> + unsigned long *bitmap;
>>>
>>> unsigned long next;
>>>
>>> + bitmap = atomic_rcu_read(&migration_bitmap);
>>> if (ram_bulk_stage && nr > base) {
>>> next = nr + 1;
>>> } else {
>>> - next = find_next_bit(migration_bitmap, size, nr);
>>> + next = find_next_bit(bitmap, size, nr);
>>> }
>>>
>>> if (next < size) {
>>> - clear_bit(next, migration_bitmap);
>>> + clear_bit(next, bitmap);
>>> migration_dirty_pages--;
>>> }
>>> return (next - base) << TARGET_PAGE_BITS;
>>> }
>>>
>>> +/* Called with rcu_read_lock() to protect migration_bitmap */
>>> static void migration_bitmap_sync_range(ram_addr_t start, ram_addr_t
>>> length)
>>> {
>>> + unsigned long *bitmap;
>>> + bitmap = atomic_rcu_read(&migration_bitmap);
>>> migration_dirty_pages +=
>>> - cpu_physical_memory_sync_dirty_bitmap(migration_bitmap, start,
>>> length);
>>> + cpu_physical_memory_sync_dirty_bitmap(bitmap, start, length);
>>> }
>>>
>>>
>>> @@ -1017,10 +1023,15 @@ void free_xbzrle_decoded_buf(void)
>>>
>>> static void migration_end(void)
>>> {
>>> - if (migration_bitmap) {
>>> + /* caller have hold iothread lock or is in a bh, so there is
>>> + * no writing race against this migration_bitmap
>>> + */
>>> + unsigned long *bitmap = migration_bitmap;
>>> + atomic_rcu_set(&migration_bitmap, NULL);
>>> + if (bitmap) {
>>> memory_global_dirty_log_stop();
>>> - g_free(migration_bitmap);
>>> - migration_bitmap = NULL;
>>> + synchronize_rcu();
>>> + g_free(bitmap);
>>> }
>>>
>>> XBZRLE_cache_lock();
>>> --
>>> 2.4.3
>>>
>>>
>>
>>
>
> .
>
