[libvirt] [PATCH RFC v2] qemu: fix deadlock when waiting in non async jobs
Nikolay Shirokovskiy
nshirokovskiy at virtuozzo.com
Tue Oct 16 07:22:09 UTC 2018
On 16.10.2018 03:00, John Ferlan wrote:
>
>
> On 10/8/18 4:10 AM, Nikolay Shirokovskiy wrote:
>> Block job abort operation can not handle properly qemu crashes when waiting for
>> abort/pivot completion. Deadlock scenario is next:
>>
>> - qemuDomainBlockJobAbort waits for pivot/abort completion
>> - qemu crashes, then qemuProcessBeginStopJob broadcasts for VM condition and
>> then waits for job condition (taken by qemuDomainBlockJobAbort)
>> - qemuDomainBlockJobAbort awakes but nothing really changed, VM is still
>> active (vm->def->id != -1) so thread starts waiting for completion again.
>> Now two threads are in deadlock.
>>
>> First let's remove broadcast in qemuProcessBeginStopJob. It is simply wrong
>> because it is not set any condition before broadcast so that awaked threads can
>> not detect any changes. Crashing domain during async job will continue to be
>> handled properly because destroy job can run concurrently with async job and
>> destroy job calls qemuProcessStop which sets vm->def->id to -1 and broadcasts.
>
> Hmm... Although blockjobs are not my area of expertise, I do seem to
> have a knack for reading and commenting on patches with these edge
> conditions.
>
> At first, taken alone this made it seem like separate patches are
> required, but maybe not depending on the relationship described above.
> As an aside, for this paragraph hunk you could call out commit 4d0c535a3
> where this is/was introduced. Beyond the refactor, the broadcast was
> added; however, it seems it was done so on purpose since the broadcast
> would seemingly allowing something to be awoken.
>
> Beyond that - take away the scenario you describing where QEMU crashes.
> In the normal path, if you remove the broadcast, then do things work
> properly?
As far as I can see. In all jobs where we we wait on vm condition we
check misc state variables after that so if state is not changed than
broadcasting will not help. (The only exception is migration and derivatives
with qemu not supporting events but in this case we use sleeps and
do not wait).
>
> Since a block job would set @priv->blockjob when a block job starts and
> is cleared during qemuBlockJobEventProcess processing when status is
> VIR_DOMAIN_BLOCK_JOB_COMPLETED, VIR_DOMAIN_BLOCK_JOB_FAILED, or
> VIR_DOMAIN_BLOCK_JOB_CANCELED.
>
> What about setting a @priv->blockjobAbort when the abort starts. Then
> perhaps processMonitorEOFEvent or qemuProcessHandleMonitorEOF can handle
> that properly so that we don't deadlock.
But how we can handle it the other way? I see no other option now besides
setting some state variable and signalling after that to help non async
job to finish and then let EOF handler proceed. (However check suggestions after --- )
Nikolay
>
> Perhaps or hopefully, Jirka or Peter will comment too with this bump.
>
> John
>
>>
>> Second let's introduce flag that EOF is received and broadcast after that.
>> Now non async jobs can check this flag in wait loop.
>>
>> Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy at virtuozzo.com>
>>
>> ---
>>
>> Diff from v1:
>>
>> - patches 1 and 2 are already merged
>> - don't bother with reporting monitor EOF reason to user as most of
>> time it is simply "unexpected eof" (this implies dropping patch 3)
>> - drop patch 5 as we now always report "domain is being stopped"
>> in qemuDomainObjWait
>> - don't signal on monitor error for simplicity (otherwise we need to report
>> something more elaborate that "domain is being stopped" as we don't
>> kill domain on monitor errors. On the other hand I guess monitor
>> error is rare case to handle it right now)
>> - keep virDomainObjWait for async jobs
>>
>> It's a bit uneven that for async jobs domain is destroyed concurrently and for
>> non async jobs it will be actually destroyed after job get completed. Also if
>> non async job needs issuing commands to qemu on cleanup then we will send these
>> commands in vain polluting logs etc because qemu process in not running at this
>> moment but typical check (virDomainObjIsActive) will think it is still running.
>>
>> Domain is destroyed (qemuProcessStop) in a job due to patches [1] and [2].
>> However AFAIU it is not neccessary. If qemuProcessStop does not drop VM lock
>> then we don't need extra job to make qemuProcessStop and main job not
>> interleave. And we can drop the lock now only in qemuDomainObjBeginNestedJob in
>> qemuProcessStop which is introduced in [2]. AFAIU we can fix issues mentioned in
>> [2] the other way for example like it is done for qemu agent - we save agent
>> monitor reference on stack for entering/exiting agent monitor.
>>
>> So I wonder can we instead of this fix remove job for qemuProcessStop and run
>> destroying domain cuncurrently for non async jobs too.
>>
>> [1]
>> commit 8c9ff9960b29d4703a99efdd1cadcf6f48799cc0
>> Author: Jiri Denemark <jdenemar at redhat.com>
>> Date: Thu Feb 11 15:32:48 2016 +0100
>>
>> qemu: Process monitor EOF in a job
>>
>> [2]
>> commit 81f50cb92d16643bcd749e3ab5b404b8b7cec643
>> Author: Jiri Denemark <jdenemar at redhat.com>
>> Date: Thu Feb 11 11:20:28 2016 +0100
>>
>> qemu: Avoid calling qemuProcessStop without a job
>>
>> src/qemu/qemu_domain.c | 39 +++++++++++++++++++++++++++++++++++++++
>> src/qemu/qemu_domain.h | 4 ++++
>> src/qemu/qemu_driver.c | 2 +-
>> src/qemu/qemu_hotplug.c | 4 ++--
>> src/qemu/qemu_process.c | 9 +++++----
>> 5 files changed, 51 insertions(+), 7 deletions(-)
>>
>> diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
>> index 939b2a3..aead72b 100644
>> --- a/src/qemu/qemu_domain.c
>> +++ b/src/qemu/qemu_domain.c
>> @@ -13534,3 +13534,42 @@ qemuDomainRunningReasonToResumeEvent(virDomainRunningReason reason)
>>
>> return VIR_DOMAIN_EVENT_RESUMED_UNPAUSED;
>> }
>> +
>> +
>> +/**
>> + * Waits for domain condition to be triggered for a specific period of time.
>> + * if @until is 0 then waits indefinetely.
>> + *
>> + * Returns:
>> + * -1 on error
>> + * 0 on success
>> + * 1 on timeout
>> + */
>> +int
>> +qemuDomainObjWait(virDomainObjPtr vm, unsigned long long until)
>> +{
>> + qemuDomainObjPrivatePtr priv = vm->privateData;
>> + int rc;
>> +
>> + if (until)
>> + rc = virCondWaitUntil(&vm->cond, &vm->parent.lock, until);
>> + else
>> + rc = virCondWait(&vm->cond, &vm->parent.lock);
>> +
>> + if (rc < 0) {
>> + if (until && errno == ETIMEDOUT)
>> + return 1;
>> +
>> + virReportSystemError(errno, "%s",
>> + _("failed to wait for domain condition"));
>> + return -1;
>> + }
>> +
>> + if (priv->monEOF) {
>> + virReportError(VIR_ERR_OPERATION_FAILED, "%s",
>> + _("domain is being stopped"));
>> + return -1;
>> + }
>> +
>> + return 0;
>> +}
>> diff --git a/src/qemu/qemu_domain.h b/src/qemu/qemu_domain.h
>> index 2f8a1bf..36ab294 100644
>> --- a/src/qemu/qemu_domain.h
>> +++ b/src/qemu/qemu_domain.h
>> @@ -281,6 +281,7 @@ struct _qemuDomainObjPrivate {
>> virDomainChrSourceDefPtr monConfig;
>> bool monJSON;
>> bool monError;
>> + bool monEOF;
>> unsigned long long monStart;
>>
>> qemuAgentPtr agent;
>> @@ -1085,4 +1086,7 @@ void qemuDomainStorageIdReset(qemuDomainObjPrivatePtr priv);
>> virDomainEventResumedDetailType
>> qemuDomainRunningReasonToResumeEvent(virDomainRunningReason reason);
>>
>> +int
>> +qemuDomainObjWait(virDomainObjPtr vm, unsigned long long until);
>> +
>> #endif /* __QEMU_DOMAIN_H__ */
>> diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
>> index b238309..f4250da 100644
>> --- a/src/qemu/qemu_driver.c
>> +++ b/src/qemu/qemu_driver.c
>> @@ -17142,7 +17142,7 @@ qemuDomainBlockJobAbort(virDomainPtr dom,
>> qemuDomainDiskPrivatePtr diskPriv = QEMU_DOMAIN_DISK_PRIVATE(disk);
>> qemuBlockJobUpdate(vm, QEMU_ASYNC_JOB_NONE, disk, NULL);
>> while (diskPriv->blockjob) {
>> - if (virDomainObjWait(vm) < 0) {
>> + if (qemuDomainObjWait(vm, 0) < 0) {
>> ret = -1;
>> goto endjob;
>> }
>> diff --git a/src/qemu/qemu_hotplug.c b/src/qemu/qemu_hotplug.c
>> index 4558a3c..8189629 100644
>> --- a/src/qemu/qemu_hotplug.c
>> +++ b/src/qemu/qemu_hotplug.c
>> @@ -165,7 +165,7 @@ qemuHotplugWaitForTrayEject(virDomainObjPtr vm,
>> return -1;
>>
>> while (disk->tray_status != VIR_DOMAIN_DISK_TRAY_OPEN) {
>> - if ((rc = virDomainObjWaitUntil(vm, now + CHANGE_MEDIA_TIMEOUT)) < 0)
>> + if ((rc = qemuDomainObjWait(vm, now + CHANGE_MEDIA_TIMEOUT)) < 0)
>> return -1;
>>
>> if (rc > 0) {
>> @@ -5002,7 +5002,7 @@ qemuDomainWaitForDeviceRemoval(virDomainObjPtr vm)
>> until += qemuDomainRemoveDeviceWaitTime;
>>
>> while (priv->unplug.alias) {
>> - if ((rc = virDomainObjWaitUntil(vm, until)) == 1)
>> + if ((rc = qemuDomainObjWait(vm, until)) == 1)
>> return 0;
>>
>> if (rc < 0) {
>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
>> index 29b0ba1..dd03269 100644
>> --- a/src/qemu/qemu_process.c
>> +++ b/src/qemu/qemu_process.c
>> @@ -290,9 +290,12 @@ qemuProcessHandleMonitorEOF(qemuMonitorPtr mon,
>>
>> virObjectLock(vm);
>>
>> + priv = vm->privateData;
>> + priv->monEOF = true;
>> + virDomainObjBroadcast(vm);
>> +
>> VIR_DEBUG("Received EOF on %p '%s'", vm, vm->def->name);
>>
>> - priv = vm->privateData;
>> if (priv->beingDestroyed) {
>> VIR_DEBUG("Domain is being destroyed, EOF is expected");
>> goto cleanup;
>> @@ -5996,6 +5999,7 @@ qemuProcessPrepareDomain(virQEMUDriverPtr driver,
>>
>> priv->monJSON = true;
>> priv->monError = false;
>> + priv->monEOF = false;
>> priv->monStart = 0;
>> priv->gotShutdown = false;
>> priv->runningReason = VIR_DOMAIN_RUNNING_UNKNOWN;
>> @@ -6965,9 +6969,6 @@ qemuProcessBeginStopJob(virQEMUDriverPtr driver,
>> if (qemuProcessKill(vm, killFlags) < 0)
>> goto cleanup;
>>
>> - /* Wake up anything waiting on domain condition */
>> - virDomainObjBroadcast(vm);
>> -
>> if (qemuDomainObjBeginJob(driver, vm, job) < 0)
>> goto cleanup;
>>
>>
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