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<stefa...@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<stefa...@redhat.com>
Reviewed-by: Eric Blake<ebl...@redhat.com>
Message-ID:<20231204164259.1515217-2-stefa...@redhat.com>
Signed-off-by: Kevin Wolf<kw...@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
