| Message ID | 20230906190141.1286893-1-stefanha@redhat.com |
|---|---|
| Headers | show |
| Series | qmp: make qmp_device_add() a coroutine | expand |
On 9/6/23 21:01, Stefan Hajnoczi wrote: > It is not safe to call drain_call_rcu() from qmp_device_add() because > some call stacks are not prepared for drain_call_rcu() to drop the Big > QEMU Lock (BQL). > > For example, device emulation code is protected by the BQL but when it > calls aio_poll() -> ... -> qmp_device_add() -> drain_call_rcu() then the > BQL is dropped. See https://bugzilla.redhat.com/show_bug.cgi?id=2215192 for a > concrete bug of this type. > > Another limitation of drain_call_rcu() is that it cannot be invoked within an > RCU read-side critical section since the reclamation phase cannot complete > until the end of the critical section. Unfortunately, call stacks have been > seen where this happens (see > https://bugzilla.redhat.com/show_bug.cgi?id=2214985). I think the root cause here is that do_qmp_dispatch_bh is called on the wrong context, namely qemu_get_aio_context() instead of iohandler_get_aio_context(). This is what causes it to move to the vCPU thread. Auditing all subsystems that use iohandler_get_aio_context(), for example via qemu_set_fd_handler(), together with bottom halves, would be a bit daunting. I don't have any objection to this patch series actually, but I would like to see if using the right AioContext also fixes the bug---and then treat these changes as more of a cleanup. Coroutines are pretty pervasive in QEMU and are not going away which, as you say in the updated docs, makes drain_call_rcu_co() preferrable to drain_call_rcu(). Paolo > This patch series introduces drain_call_rcu_co(), which does the same thing as > drain_call_rcu() but asynchronously. By yielding back to the event loop we can > wait until the caller drops the BQL and leaves its RCU read-side critical > section. > > Patch 1 changes HMP so that coroutine monitor commands yield back to the event > loop instead of running inside a nested event loop. > > Patch 2 introduces the new drain_call_rcu_co() API. > > Patch 3 converts qmp_device_add() into a coroutine monitor command and uses > drain_call_rcu_co(). > > I'm sending this as an RFC because I don't have confirmation yet that the bugs > mentioned above are fixed by this patch series. > > Stefan Hajnoczi (3): > hmp: avoid the nested event loop in handle_hmp_command() > rcu: add drain_call_rcu_co() API > qmp: make qmp_device_add() a coroutine > > MAINTAINERS | 2 ++ > docs/devel/rcu.txt | 21 ++++++++++++++++ > qapi/qdev.json | 1 + > include/monitor/qdev.h | 3 ++- > include/qemu/rcu.h | 1 + > util/rcu-internal.h | 8 ++++++ > monitor/hmp.c | 28 +++++++++++---------- > monitor/qmp-cmds.c | 2 +- > softmmu/qdev-monitor.c | 34 +++++++++++++++++++++++--- > util/rcu-co.c | 55 ++++++++++++++++++++++++++++++++++++++++++ > util/rcu.c | 3 ++- > hmp-commands.hx | 1 + > util/meson.build | 2 +- > 13 files changed, 140 insertions(+), 21 deletions(-) > create mode 100644 util/rcu-internal.h > create mode 100644 util/rcu-co.c >
On Thu, Sep 07, 2023 at 01:28:55PM +0200, Paolo Bonzini wrote: > On 9/6/23 21:01, Stefan Hajnoczi wrote: > > It is not safe to call drain_call_rcu() from qmp_device_add() because > > some call stacks are not prepared for drain_call_rcu() to drop the Big > > QEMU Lock (BQL). > > > > For example, device emulation code is protected by the BQL but when it > > calls aio_poll() -> ... -> qmp_device_add() -> drain_call_rcu() then the > > BQL is dropped. See https://bugzilla.redhat.com/show_bug.cgi?id=2215192 for a > > concrete bug of this type. > > > > Another limitation of drain_call_rcu() is that it cannot be invoked within an > > RCU read-side critical section since the reclamation phase cannot complete > > until the end of the critical section. Unfortunately, call stacks have been > > seen where this happens (see > > https://bugzilla.redhat.com/show_bug.cgi?id=2214985). > > I think the root cause here is that do_qmp_dispatch_bh is called on the > wrong context, namely qemu_get_aio_context() instead of > iohandler_get_aio_context(). This is what causes it to move to the vCPU > thread. > > Auditing all subsystems that use iohandler_get_aio_context(), for example > via qemu_set_fd_handler(), together with bottom halves, would be a bit > daunting. > > I don't have any objection to this patch series actually, but I would like > to see if using the right AioContext also fixes the bug---and then treat > these changes as more of a cleanup. Coroutines are pretty pervasive in QEMU > and are not going away which, as you say in the updated docs, makes > drain_call_rcu_co() preferrable to drain_call_rcu(). While I agree that the issue would not happen if monitor commands only ran in the iohandler AioContext, I don't think we can change that. When Kevin implemented coroutine commands in commit 9ce44e2ce267 ("qmp: Move dispatcher to a coroutine"), he used qemu_get_aio_context() deliberately so that AIO_WAIT_WHILE() can make progress. I'm not clear on the exact scenario though, because coroutines shouldn't call AIO_WAIT_WHILE(). Kevin? There is only one coroutine monitor command that calls the QEMU block layer: qmp_block_resize(). If we're going to change how the AioContext works then now is the time to do it before there are more commands that need to be audited/refactored. Stefan > > Paolo > > > > This patch series introduces drain_call_rcu_co(), which does the same thing as > > drain_call_rcu() but asynchronously. By yielding back to the event loop we can > > wait until the caller drops the BQL and leaves its RCU read-side critical > > section. > > > > Patch 1 changes HMP so that coroutine monitor commands yield back to the event > > loop instead of running inside a nested event loop. > > > > Patch 2 introduces the new drain_call_rcu_co() API. > > > > Patch 3 converts qmp_device_add() into a coroutine monitor command and uses > > drain_call_rcu_co(). > > > > I'm sending this as an RFC because I don't have confirmation yet that the bugs > > mentioned above are fixed by this patch series. > > > > Stefan Hajnoczi (3): > > hmp: avoid the nested event loop in handle_hmp_command() > > rcu: add drain_call_rcu_co() API > > qmp: make qmp_device_add() a coroutine > > > > MAINTAINERS | 2 ++ > > docs/devel/rcu.txt | 21 ++++++++++++++++ > > qapi/qdev.json | 1 + > > include/monitor/qdev.h | 3 ++- > > include/qemu/rcu.h | 1 + > > util/rcu-internal.h | 8 ++++++ > > monitor/hmp.c | 28 +++++++++++---------- > > monitor/qmp-cmds.c | 2 +- > > softmmu/qdev-monitor.c | 34 +++++++++++++++++++++++--- > > util/rcu-co.c | 55 ++++++++++++++++++++++++++++++++++++++++++ > > util/rcu.c | 3 ++- > > hmp-commands.hx | 1 + > > util/meson.build | 2 +- > > 13 files changed, 140 insertions(+), 21 deletions(-) > > create mode 100644 util/rcu-internal.h > > create mode 100644 util/rcu-co.c > > >
On Thu, Sep 7, 2023 at 4:00 PM Stefan Hajnoczi <stefanha@redhat.com> wrote: > While I agree that the issue would not happen if monitor commands only > ran in the iohandler AioContext, I don't think we can change that. > When Kevin implemented coroutine commands in commit 9ce44e2ce267 ("qmp: > Move dispatcher to a coroutine"), he used qemu_get_aio_context() > deliberately so that AIO_WAIT_WHILE() can make progress. Ah, you are referring to + /* + * Move the coroutine from iohandler_ctx to qemu_aio_context for + * executing the command handler so that it can make progress if it + * involves an AIO_WAIT_WHILE(). + */ + aio_co_schedule(qemu_get_aio_context(), qmp_dispatcher_co); + qemu_coroutine_yield(); > I'm not clear on the exact scenario though, because coroutines shouldn't > call AIO_WAIT_WHILE(). I think he meant "so that an AIO_WAIT_WHILE() invoked through a bottom half will make progress on the coroutine as well". However I am not sure the comment applies here, because do_qmp_dispatch_bh() only applies to non-coroutine commands; that commit allowed monitor commands to run in vCPU threads when they previously weren't. Thinking more about it, I don't like that the if (!!(cmd->options & QCO_COROUTINE) == qemu_in_coroutine()) { } check is in qmp_dispatch() rather than monitor_qmp_dispatch(). Any caller of qmp_dispatch() knows if it is in a coroutine or not. qemu-ga uses neither a coroutine dispatcher nor coroutine commands. QEMU uses non-coroutine dispatch for out-of-band commands (and we can forbid coroutine + allow-oob at the same time), and coroutine dispatch for the others. So, moving out of coroutine context (through a bottom half) should be done by monitor_qmp_dispatch(), and likewise moving temporarily out of the iohandler context in the case of coroutine commands. In the case of !req_obj->req you don't need to do either of those. qmp_dispatch() can still assert that the coroutine-ness of the command matches the context in which qmp_dispatch() is called. Once this is done, I think moving out of coroutine context can use a BH that runs in the iohandler context. Paolo
On Thu, 7 Sept 2023 at 10:26, Paolo Bonzini <pbonzini@redhat.com> wrote: > > On Thu, Sep 7, 2023 at 4:00 PM Stefan Hajnoczi <stefanha@redhat.com> wrote: > > While I agree that the issue would not happen if monitor commands only > > ran in the iohandler AioContext, I don't think we can change that. > > When Kevin implemented coroutine commands in commit 9ce44e2ce267 ("qmp: > > Move dispatcher to a coroutine"), he used qemu_get_aio_context() > > deliberately so that AIO_WAIT_WHILE() can make progress. > > Ah, you are referring to > > + /* > + * Move the coroutine from iohandler_ctx to qemu_aio_context for > + * executing the command handler so that it can make progress if it > + * involves an AIO_WAIT_WHILE(). > + */ > + aio_co_schedule(qemu_get_aio_context(), qmp_dispatcher_co); > + qemu_coroutine_yield(); > > > I'm not clear on the exact scenario though, because coroutines shouldn't > > call AIO_WAIT_WHILE(). > > I think he meant "so that an AIO_WAIT_WHILE() invoked through a bottom > half will make progress on the coroutine as well". > > However I am not sure the comment applies here, because > do_qmp_dispatch_bh() only applies to non-coroutine commands; that > commit allowed monitor commands to run in vCPU threads when they > previously weren't. > > Thinking more about it, I don't like that the > > if (!!(cmd->options & QCO_COROUTINE) == qemu_in_coroutine()) { > } > > check is in qmp_dispatch() rather than monitor_qmp_dispatch(). > > Any caller of qmp_dispatch() knows if it is in a coroutine or not. > qemu-ga uses neither a coroutine dispatcher nor coroutine commands. > QEMU uses non-coroutine dispatch for out-of-band commands (and we can > forbid coroutine + allow-oob at the same time), and coroutine dispatch > for the others. > > So, moving out of coroutine context (through a bottom half) should be > done by monitor_qmp_dispatch(), and likewise moving temporarily out of > the iohandler context in the case of coroutine commands. In the case > of !req_obj->req you don't need to do either of those. qmp_dispatch() > can still assert that the coroutine-ness of the command matches the > context in which qmp_dispatch() is called. > > Once this is done, I think moving out of coroutine context can use a > BH that runs in the iohandler context. I'll wait for Kevin's input and will then revisit the patches based on the conclusion we come to. Stefan
Am 07.09.2023 um 16:25 hat Paolo Bonzini geschrieben: > On Thu, Sep 7, 2023 at 4:00 PM Stefan Hajnoczi <stefanha@redhat.com> wrote: > > While I agree that the issue would not happen if monitor commands only > > ran in the iohandler AioContext, I don't think we can change that. > > When Kevin implemented coroutine commands in commit 9ce44e2ce267 ("qmp: > > Move dispatcher to a coroutine"), he used qemu_get_aio_context() > > deliberately so that AIO_WAIT_WHILE() can make progress. > > Ah, you are referring to > > + /* > + * Move the coroutine from iohandler_ctx to qemu_aio_context for > + * executing the command handler so that it can make progress if it > + * involves an AIO_WAIT_WHILE(). > + */ > + aio_co_schedule(qemu_get_aio_context(), qmp_dispatcher_co); > + qemu_coroutine_yield(); > > > I'm not clear on the exact scenario though, because coroutines shouldn't > > call AIO_WAIT_WHILE(). > > I think he meant "so that an AIO_WAIT_WHILE() invoked through a bottom > half will make progress on the coroutine as well". It's been a while, but I think I may have meant an AIO_WAIT_WHILE() that is executed by someone else and that depends on the coroutine. For example, I imagine this is what I could have seen: 1. The QMP command handler does some I/O and yields for it (like updating the qcow2 header for block_resize) with increased bs->in_flight 2. Something else calls drain, which polls qemu_aio_context, but not iohandler_ctx, until the request completes. 3. Nothing will ever resume the coroutine -> deadlock > However I am not sure the comment applies here, because > do_qmp_dispatch_bh() only applies to non-coroutine commands; that > commit allowed monitor commands to run in vCPU threads when they > previously weren't. > > Thinking more about it, I don't like that the > > if (!!(cmd->options & QCO_COROUTINE) == qemu_in_coroutine()) { > } > > check is in qmp_dispatch() rather than monitor_qmp_dispatch(). > > Any caller of qmp_dispatch() knows if it is in a coroutine or not. > qemu-ga uses neither a coroutine dispatcher nor coroutine commands. > QEMU uses non-coroutine dispatch for out-of-band commands (and we can > forbid coroutine + allow-oob at the same time), and coroutine dispatch > for the others. > > So, moving out of coroutine context (through a bottom half) should be > done by monitor_qmp_dispatch(), and likewise moving temporarily out of > the iohandler context in the case of coroutine commands. In the case > of !req_obj->req you don't need to do either of those. qmp_dispatch() > can still assert that the coroutine-ness of the command matches the > context in which qmp_dispatch() is called. > > Once this is done, I think moving out of coroutine context can use a > BH that runs in the iohandler context. Non-coroutine handlers could probably stay in iothread_ctx, but I don't think we can avoid switching to a different for coroutine handlers. So maybe we can just move the rescheduling down to the coroutine case in qmp_dispatch(). Kevin
On Tue, Sep 12, 2023 at 7:08 PM Kevin Wolf <kwolf@redhat.com> wrote: > > Any caller of qmp_dispatch() knows if it is in a coroutine or not. > > qemu-ga uses neither a coroutine dispatcher nor coroutine commands. > > QEMU uses non-coroutine dispatch for out-of-band commands (and we can > > forbid coroutine + allow-oob at the same time), and coroutine dispatch > > for the others. > > > > So, moving out of coroutine context (through a bottom half) should be > > done by monitor_qmp_dispatch(), and likewise moving temporarily out of > > the iohandler context in the case of coroutine commands. In the case > > of !req_obj->req you don't need to do either of those. qmp_dispatch() > > can still assert that the coroutine-ness of the command matches the > > context in which qmp_dispatch() is called. > > > > Once this is done, I think moving out of coroutine context can use a > > BH that runs in the iohandler context. > > Non-coroutine handlers could probably stay in iothread_ctx, but I don't > think we can avoid switching to a different for coroutine handlers. Agreed. > So maybe we can just move the rescheduling down to the coroutine case in > qmp_dispatch(). Not sure about qmp_dispatch (see above: any caller of the function knows if it is in a coroutine or not, and qemu-ga need not know about coroutines at all). But what you said also applies if the rescheduling is only pushed to monitor_qmp_dispatch(), which would be my first option. Thanks! Paolo