Re: [PULL 11/33] scsi: only access SCSIDevice->requests from one thread

From: Hanna Czenczek <hreitz@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-block@nongnu.org, stefanha@redhat.com, qemu-devel@nongnu.org
Subject: Re: [PULL 11/33] scsi: only access SCSIDevice->requests from one thread
Date: Fri, 26 Jan 2024 16:24:49 +0100	[thread overview]
Message-ID: <4c4173f2-b8fc-4c6f-88e1-8c31c4411837@redhat.com> (raw)
In-Reply-To: <ZbOxI9Ar-YDn51Z0@redhat.com>

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On 26.01.24 14:18, Kevin Wolf wrote:
> Am 25.01.2024 um 18:32 hat Hanna Czenczek geschrieben:
>> On 23.01.24 18:10, Kevin Wolf wrote:
>>> Am 23.01.2024 um 17:40 hat Hanna Czenczek geschrieben:
>>>> On 21.12.23 22:23, Kevin Wolf wrote:
>>>>> From: Stefan Hajnoczi<stefanha@redhat.com>
>>>>>
>>>>> Stop depending on the AioContext lock and instead access
>>>>> SCSIDevice->requests from only one thread at a time:
>>>>> - When the VM is running only the BlockBackend's AioContext may access
>>>>>      the requests list.
>>>>> - When the VM is stopped only the main loop may access the requests
>>>>>      list.
>>>>>
>>>>> These constraints protect the requests list without the need for locking
>>>>> in the I/O code path.
>>>>>
>>>>> Note that multiple IOThreads are not supported yet because the code
>>>>> assumes all SCSIRequests are executed from a single AioContext. Leave
>>>>> that as future work.
>>>>>
>>>>> Signed-off-by: Stefan Hajnoczi<stefanha@redhat.com>
>>>>> Reviewed-by: Eric Blake<eblake@redhat.com>
>>>>> Message-ID:<20231204164259.1515217-2-stefanha@redhat.com>
>>>>> Signed-off-by: Kevin Wolf<kwolf@redhat.com>
>>>>> ---
>>>>>     include/hw/scsi/scsi.h |   7 +-
>>>>>     hw/scsi/scsi-bus.c     | 181 ++++++++++++++++++++++++++++-------------
>>>>>     2 files changed, 131 insertions(+), 57 deletions(-)
>>>> My reproducer forhttps://issues.redhat.com/browse/RHEL-3934  now breaks more
>>>> often because of this commit than because of the original bug, i.e. when
>>>> repeatedly hot-plugging and unplugging a virtio-scsi and a scsi-hd device,
>>>> this tends to happen when unplugging the scsi-hd:

Note: We (on issues.redhat.com) have a separate report that seems to be 
concerning this very problem: https://issues.redhat.com/browse/RHEL-19381

>>>> {"execute":"device_del","arguments":{"id":"stg0"}}
>>>> {"return": {}}
>>>> qemu-system-x86_64: ../block/block-backend.c:2429: blk_get_aio_context:
>>>> Assertion `ctx == blk->ctx' failed.
>> [...]
>>
>>>> I don’t know anything about the problem yet, but as usual, I like
>>>> speculation and discovering how wrong I was later on, so one thing I came
>>>> across that’s funny about virtio-scsi is that requests can happen even while
>>>> a disk is being attached or detached.  That is, Linux seems to probe all
>>>> LUNs when a new virtio-scsi device is being attached, and it won’t stop just
>>>> because a disk is being attached or removed.  So maybe that’s part of the
>>>> problem, that we get a request while the BB is being detached, and
>>>> temporarily in an inconsistent state (BDS context differs from BB context).
>>> I don't know anything about the problem either, but since you already
>>> speculated about the cause, let me speculate about the solution:
>>> Can we hold the graph writer lock for the tran_commit() call in
>>> bdrv_try_change_aio_context()? And of course take the reader lock for
>>> blk_get_aio_context(), but that should be completely unproblematic.
>> Actually, now that completely unproblematic part is giving me trouble.  I
>> wanted to just put a graph lock into blk_get_aio_context() (making it a
>> coroutine with a wrapper)
> Which is the first thing I neglected and already not great. We have
> calls of blk_get_aio_context() in the SCSI I/O path, and creating a
> coroutine and doing at least two context switches simply for this call
> is a lot of overhead...
>
>> but callers of blk_get_aio_context() generally assume the context is
>> going to stay the BB’s context for as long as their AioContext *
>> variable is in scope.
> I'm not so sure about that. And taking another step back, I'm actually
> also not sure how much it still matters now that they can submit I/O
> from any thread.

That’s my impression, too, but “not sure” doesn’t feel great. :) 
scsi_device_for_each_req_async_bh() specifically double-checks whether 
it’s still in the right context before invoking the specified function, 
so it seems there was some intention to continue to run in the context 
associated with the BB.

(Not judging whether that intent makes sense or not, yet.)

> Maybe the correct solution is to remove the assertion from
> blk_get_aio_context() and just always return blk->ctx. If it's in the
> middle of a change, you'll either get the old one or the new one. Either
> one is fine to submit I/O from, and if you care about changes for other
> reasons (like SCSI does), then you need explicit code to protect it
> anyway (which SCSI apparently has, but it doesn't work).

I think most callers do just assume the BB stays in the context they got 
(without any proof, admittedly), but I agree that under re-evaluation, 
it probably doesn’t actually matter to them, really. And yes, basically, 
if the caller doesn’t need to take a lock because it doesn’t really 
matter whether blk->ctx changes while its still using the old value, 
blk_get_aio_context() in turn doesn’t need to double-check blk->ctx 
against the root node’s context either, and nobody needs a lock.

So I agree, it’s on the caller to protect against a potentially changing 
context, blk_get_aio_context() should just return blk->ctx and not check 
against the root node.

(On a tangent: blk_drain() is a caller of blk_get_aio_context(), and it 
polls that context.  Why does it need to poll that context specifically 
when requests may be in any context?  Is it because if there are 
requests in the main thread, we must poll that, but otherwise it’s fine 
to poll any thread, and we can only have requests in the main thread if 
that’s the BB’s context?)

>> I was tempted to think callers know what happens to the BB they pass
>> to blk_get_aio_context(), and it won’t change contexts so easily, but
>> then I remembered this is exactly what happens in this case; we run
>> scsi_device_for_each_req_async_bh() in one thread (which calls
>> blk_get_aio_context()), and in the other, we change the BB’s context.
> Let's think a bit more about scsi_device_for_each_req_async()
> specifically. This is a function that runs in the main thread. Nothing
> will change any AioContext assignment if it doesn't call it. It wants to
> make sure that scsi_device_for_each_req_async_bh() is called in the
> same AioContext where the virtqueue is processed, so it schedules a BH
> and waits for it.

I don’t quite follow, it doesn’t wait for the BH.  It uses 
aio_bh_schedule_oneshot(), not aio_wait_bh_oneshot().  While you’re 
right that if it did wait, the BB context might still change, in 
practice we wouldn’t have the problem at hand because the caller is 
actually the one to change the context, concurrently while the BH is 
running.

> Waiting for it means running a nested event loop that could do anything,
> including changing AioContexts. So this is what needs the locking, not
> the blk_get_aio_context() call in scsi_device_for_each_req_async_bh().
> If we lock before the nested event loop and unlock in the BH, the check
> in the BH can become an assertion. (It is important that we unlock in
> the BH rather than after waiting because if something takes the writer
> lock, we need to unlock during the nested event loop of bdrv_wrlock() to
> avoid a deadlock.)
>
> And spawning a coroutine for this looks a lot more acceptable because
> it's on a slow path anyway.
>
> In fact, we probably don't technically need a coroutine to take the
> reader lock here. We can have a new graph lock function that asserts
> that there is no writer (we know because we're running in the main loop)
> and then atomically increments the reader count. But maybe that already
> complicates things again...

So as far as I understand we can’t just use aio_wait_bh_oneshot() and 
wrap it in bdrv_graph_rd{,un}lock_main_loop(), because that doesn’t 
actually lock the graph.  I feel like adding a new graph lock function 
for this quite highly specific case could be dangerous, because it seems 
easy to use the wrong way.

Just having a trampoline coroutine to call bdrv_graph_co_rd{,un}lock() 
seems simple enough and reasonable here (not a hot path).  Can we have 
that coroutine then use aio_wait_bh_oneshot() with the existing _bh 
function, or should that be made a coroutine, too?

>> It seems like there are very few blk_* functions right now that
>> require taking a graph lock around it, so I’m hesitant to go that
>> route.  But if we’re protecting a BB’s context via the graph write
>> lock, I can’t think of a way around having to take a read lock
>> whenever reading a BB’s context, and holding it for as long as we
>> assume that context to remain the BB’s context.  It’s also hard to
>> figure out how long that is, case by case; for example, dma_blk_read()
>> schedules an AIO function in the BB context; but we probably don’t
>> care that this context remains the BB’s context until the request is
>> done.  In the case of scsi_device_for_each_req_async_bh(), we already
>> take care to re-schedule it when it turns out the context is outdated,
>> so it does seem quite important here, and we probably want to keep the
>> lock until after the QTAILQ_FOREACH_SAFE() loop.
> Maybe we need to audit all callers. Fortunately, there don't seem to be
> too many. At least not direct ones...
>
>> On a tangent, this TOCTTOU problem makes me wary of other blk_*
>> functions that query information.  For example, fuse_read() (in
>> block/export/fuse.c) truncates requests to the BB length.  But what if
>> the BB length changes concurrently between blk_getlength() and
>> blk_pread()?  While we can justify using the graph lock for a BB’s
>> AioContext, we can’t use it for other metadata like its length.
> Hm... Is "tough luck" an acceptable answer? ;-)

Absolutely, if we do it acknowledgingly (great word).  I’m just a bit 
worried not all of these corner cases have been acknowledged, and some 
of them may be looking for a different answer.

Hanna

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