Re: [PATCH 2/3] qcow2: Assert that qcow2_cache_get() callers hold s->lock

[Kevin Wolf <kwolf@redhat.com>]

Am 24.10.2019 um 15:03 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 24.10.2019 13:57, Kevin Wolf wrote:
> > Am 24.10.2019 um 12:01 hat Denis Lunev geschrieben:
> >> On 10/23/19 6:26 PM, Kevin Wolf wrote:
> >>> qcow2_cache_do_get() requires that s->lock is locked because it can
> >>> yield between picking a cache entry and actually taking ownership of it
> >>> by setting offset and increasing the reference count.
> >>>
> >>> Add an assertion to make sure the caller really holds the lock. The
> >>> function can be called outside of coroutine context, where bdrv_pread
> >>> and flushes become synchronous operations. The lock cannot and need not
> >>> be taken in this case.
> >>>
> >>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> >>> ---
> >>>   block/qcow2-cache.c | 5 +++++
> >>>   1 file changed, 5 insertions(+)
> >>>
> >>> diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
> >>> index d29b038a67..75b13dad99 100644
> >>> --- a/block/qcow2-cache.c
> >>> +++ b/block/qcow2-cache.c
> >>> @@ -327,6 +327,9 @@ static int qcow2_cache_do_get(BlockDriverState *bs, Qcow2Cache *c,
> >>>       int min_lru_index = -1;
> >>>   
> >>>       assert(offset != 0);
> >>> +    if (qemu_in_coroutine()) {
> >>> +        qemu_co_mutex_assert_locked(&s->lock);
> >>> +    }
> >>
> >> that is looking not good to me. If this is really requires lock, we should
> >> check for the lock always. In the other hand we could face missed
> >> lock out of coroutine.
> > 
> > As the commit message explains, outside of coroutine context, we can't
> > yield and bdrv_pread and bdrv_flush become synchronous operations
> > instead, so there is nothing else that we need to protect against.
> > 
> 
> Recently we discussed similar problems about block-dirty-bitmap-* qmp
> commands, which wanted to update qcow2 metadata about bitmaps from
> non-coroutine context.  "qcow2 lock"
> <135df452-397a-30bb-7518-2184fa5971aa@virtuozzo.com>
> https://lists.gnu.org/archive/html/qemu-devel/2019-09/msg01419.html

Hm, right, I already forgot about the nested event loop again...

> And, as I understand, the correct way is to convert to coroutine and
> lock mutex appropriately. Or we want to lock aio context and to be in
> drained section to avoid parallel requests accessing critical section.
> Should we assert such conditions in case of !qemu_in_coroutine() ?

The AioContext lock must be held anyway, so I don't think this would be
a new requirement. As for draining, I'll have to see.

I'm currently still auditing all the callers of qcow2_cache_do_get().
The synchronous callers I already know are the snapshot functions. I
think these happen to be in a drained section anyway (or should be at
least), so AioContext lock + drain seems like a very reasonable option
for them.

For other synchronous callers, if any, maybe conversion to a coroutine
would make more sense.

Kevin