Re: [PATCH v3 2/4] block-backend: fix edge case in bdrv_next() where BDS associated to BB changes

[Fiona Ebner]

Am 04.06.24 um 17:28 schrieb Kevin Wolf:
> Am 04.06.2024 um 09:58 hat Fiona Ebner geschrieben:
>> Am 03.06.24 um 18:21 schrieb Kevin Wolf:
>>> Am 03.06.2024 um 16:17 hat Fiona Ebner geschrieben:
>>>> Am 26.03.24 um 13:44 schrieb Kevin Wolf:
>>>>>
>>>>> The fix for bdrv_flush_all() is probably to make it bdrv_co_flush_all()
>>>>> with a coroutine wrapper so that the graph lock is held for the whole
>>>>> function. Then calling bdrv_co_flush() while iterating the list is safe
>>>>> and doesn't allow concurrent graph modifications.
>>>>
>>>> The second is that iotest 255 ran into an assertion failure upon QMP 
>>>> 'quit':
>>>>
>>>>> ../block/graph-lock.c:113: bdrv_graph_wrlock: Assertion 
>>>>> `!qemu_in_coroutine()' failed.
>>>>
>>>> Looking at the backtrace:
>>>>
>>>>> #5  0x0000762a90cc3eb2 in __GI___assert_fail
>>>>>     (assertion=0x5afb07991e7d "!qemu_in_coroutine()", file=0x5afb07991e00 
>>>>> "../block/graph-lock.c", line=113, function=0x5afb07991f20 
>>>>> <__PRETTY_FUNCTION__.4> "bdrv_graph_wrlock")
>>>>>     at ./assert/assert.c:101
>>>>> #6  0x00005afb07585311 in bdrv_graph_wrlock () at 
>>>>> ../block/graph-lock.c:113
>>>>> #7  0x00005afb07573a36 in blk_remove_bs (blk=0x5afb0af99420) at 
>>>>> ../block/block-backend.c:901
>>>>> #8  0x00005afb075729a7 in blk_delete (blk=0x5afb0af99420) at 
>>>>> ../block/block-backend.c:487
>>>>> #9  0x00005afb07572d88 in blk_unref (blk=0x5afb0af99420) at 
>>>>> ../block/block-backend.c:547
>>>>> #10 0x00005afb07572fe8 in bdrv_next (it=0x762a852fef00) at 
>>>>> ../block/block-backend.c:618
>>>>> #11 0x00005afb0758cd65 in bdrv_co_flush_all () at ../block/io.c:2347
>>>>> #12 0x00005afb0753ba37 in bdrv_co_flush_all_entry (opaque=0x7ffff12c6050) 
>>>>> at block/block-gen.c:1391
>>>>> #13 0x00005afb0773bf41 in coroutine_trampoline (i0=168365184, i1=23291)
>>>>
>>>> So I guess calling bdrv_next() is not safe from a coroutine, because
>>>> the function doing the iteration could end up being the last thing to
>>>> have a reference for the BB.
>>>
>>> Does your bdrv_co_flush_all() take the graph (reader) lock? If so, this
>>> is surprising, because while we hold the graph lock, no reference should
>>> be able to go away - you need the writer lock for that and you won't get
>>> it as long as bdrv_co_flush_all() locks the graph. So whatever had a
>>> reference before the bdrv_next() loop must still have it now. Do you
>>> know where it gets dropped?
>>>
>>
>> AFAICT, yes, it does hold the graph reader lock. The generated code is:
>>
>>> static void coroutine_fn bdrv_co_flush_all_entry(void *opaque)
>>> {
>>>     BdrvFlushAll *s = opaque;
>>>
>>>     bdrv_graph_co_rdlock();
>>>     s->ret = bdrv_co_flush_all();
>>>     bdrv_graph_co_rdunlock();
>>>     s->poll_state.in_progress = false;
>>>
>>>     aio_wait_kick();
>>> }
>>
>> Apparently when the mirror job is aborted/exits, which can happen during
>> the polling for bdrv_co_flush_all_entry(), a reference can go away
>> without the write lock (at least my breakpoints didn't trigger) being held:
>>
>>> #0  blk_unref (blk=0x5cdefe943d20) at ../block/block-backend.c:537
>>> #1  0x00005cdefb26697e in mirror_exit_common (job=0x5cdefeb53000) at 
>>> ../block/mirror.c:710
>>> #2  0x00005cdefb263575 in mirror_abort (job=0x5cdefeb53000) at 
>>> ../block/mirror.c:823
>>> #3  0x00005cdefb2248a6 in job_abort (job=0x5cdefeb53000) at ../job.c:825
>>> #4  0x00005cdefb2245f2 in job_finalize_single_locked (job=0x5cdefeb53000) 
>>> at ../job.c:855
>>> #5  0x00005cdefb223852 in job_completed_txn_abort_locked 
>>> (job=0x5cdefeb53000) at ../job.c:958
>>> #6  0x00005cdefb223714 in job_completed_locked (job=0x5cdefeb53000) at 
>>> ../job.c:1065
>>> #7  0x00005cdefb224a8b in job_exit (opaque=0x5cdefeb53000) at ../job.c:1088
>>> #8  0x00005cdefb4134fc in aio_bh_call (bh=0x5cdefe7487c0) at 
>>> ../util/async.c:171
>>> #9  0x00005cdefb4136ce in aio_bh_poll (ctx=0x5cdefd9cd750) at 
>>> ../util/async.c:218
>>> #10 0x00005cdefb3efdfd in aio_poll (ctx=0x5cdefd9cd750, blocking=true) at 
>>> ../util/aio-posix.c:722
>>> #11 0x00005cdefb20435e in bdrv_poll_co (s=0x7ffe491621d8) at 
>>> ../block/block-gen.h:43
>>> #12 0x00005cdefb206a33 in bdrv_flush_all () at block/block-gen.c:1410
>>> #13 0x00005cdefae5c8ed in do_vm_stop (state=RUN_STATE_SHUTDOWN, 
>>> send_stop=false)
>>>     at ../system/cpus.c:297
>>> #14 0x00005cdefae5c850 in vm_shutdown () at ../system/cpus.c:308
>>> #15 0x00005cdefae6d892 in qemu_cleanup (status=0) at 
>>> ../system/runstate.c:871
>>> #16 0x00005cdefb1a7e78 in qemu_default_main () at ../system/main.c:38
>>> #17 0x00005cdefb1a7eb8 in main (argc=34, argv=0x7ffe491623a8) at 
>>> ../system/main.c:48
>>
>> Looking at the code in mirror_exit_common(), it doesn't seem to acquire
>> a write lock:
>>
>>>     bdrv_graph_rdunlock_main_loop();
>>>
>>>     /*
>>>      * Remove target parent that still uses BLK_PERM_WRITE/RESIZE before
>>>      * inserting target_bs at s->to_replace, where we might not be able to 
>>> get
>>>      * these permissions.
>>>      */
>>>     blk_unref(s->target);
>>>     s->target = NULL;
>>
>> The write lock is taken in blk_remove_bs() when the refcount drops to 0
>> and the BB is actually removed:
>>
>>>     bdrv_graph_wrlock();
>>>     bdrv_root_unref_child(root);
>>>     bdrv_graph_wrunlock();
> 
> Ah, so it _would_ take the writer lock (and wait for bdrv_flush_all() to
> finish) if it were the last reference, but because bdrv_next() took
> another reference, it can just decrease the refcount without modifying
> the graph.
> 
> Does this mean that if we just remove the whole blk/bdrv_ref/unref()
> code from bdrv_next(), which is what the final state should look like
> anyway, it would actually work?

I tried this and removed those calls. Unfortunately, I wasn't able to
run into the case where blk_unref() via job_exit() waits in
bdrv_graph_wrlock(), because there is a blk_drain() call before
blk_delete() and in my testing, it always waited there, in
bdrv_do_drained_begin()'s

>         BDRV_POLL_WHILE(bs, bdrv_drain_poll_top_level(bs, parent));

> Of course, we then need to audit the callers of bdrv_next() to make sure
> that all of them really keep the graph lock and don't poll during the
> whole iteration like they are supposed to (i.e. nobody else violates the
> rules the same way bdrv_flush_all() does).

Note that below, none of the functions I list at the top level (i.e.
after a number) are coroutines.

1. hmp_info_snapshots() in block/monitor/block-hmp-cmds.c
2. vm_completion() in migration/migration-hmp-cmds.c
both call bdrv_can_snapshot() which calls the coroutine wrapper
bdrv_is_inserted() and thus polls

3. bdrv_flush_all() in block/io.c:
calls the coroutine wrapper bdrv_flush() and thus polls, the one that
led us here ;)

4. bdrv_all_get_snapshot_devices() in block/snapshot.c:
marked as GRAPH_RDLOCK, only calls g_list_append() in the loop body, so
is fine :)

5. bdrv_activate_all() in block.c:
calls bdrv_activate() which can (e.g. after outgoing migration) call the
(manual) coroutine wrappers bdrv_invalidate_cache() and
bdrv_refresh_total_sectors() and thus poll

6. bdrv_inactivate_all() in block.c:
calls bdrv_inactivate_recurse() which for qcow2, calls
qcow2_inactivate() which calls qcow2_cache_flush() which calls the
coroutine wrapper bdrv_flush() and thus polls

My list of code paths leading to polling here is probably not even
complete, but it's already clear that there is some work left before no
callers of bdrv_next() violate the rules.

Best Regards,
Fiona