Re: [libvirt] an AB deadlock or libvirtd crash problem in virsh console and virsh destroy

The reason of this problem is that fdstream abort event or close event occured at the same time, libvirtd doesn't deal with the synchronousness well enough. the flows about fdStream is bellow 1、qemuDomainDefineXMLFlags -> virDomainObjListAdd -> qemuDomainObjPrivateAlloc -> virChrdevAlloc -> virHashCreate 2、qemuDomainOpenConsole -> virChrdevOpen -> virHashAddEntry(devs->hash, path, st) 3、virDomainObjDispose -> privateDataFreeFunc (qemuDomainObjPrivateFree) - > virChrdevFree(*dev locked*) -> virChrdevFreeClearCallbacks - > virFDStreamSetInternalCloseCb(*fdst locked*) 4、virFDStreamCloseInt (*fdst locked*) -> icbFreeOpaque(virChrdevFDStreamCloseCb (*dev locked*)) -> virHashRemoveEntry

The AB lock problem is obviouse: in step 3, it locks chardev before fdst, and in step 4, it's the opsite way. The reason of libvirtd crash is that: in virFDStreamCloseInt function we set fdst to NULL, while in virFDStreamSetInternalCloseCb we use fdst->lock, note that fdst has already been freed. another crash problem occurs because that when virChrdevFree was earlier finished, dev->hash freed and all date of hash is freed, but fdstream event flow use fdStream after hash free. Ahha~~， libvirtd coredump. All of those problem is because clear vm flow and fdStream flow concurs synchronously. then I fix this problem by modify virChrdevFree() void virChrdevFree(virChrdevsPtr devs) { if (!devs || !devs->hash) return; for (;;) { virMutexLock(&devs->lock); if (0 == virHashSize(devs->hash)) { virMutexUnlock(&devs->lock); break; } virMutexUnlock(&devs->lock); usleep(10 * 1000); } virMutexLock(&devs->lock); virHashFree(devs->hash); virMutexUnlock(&devs->lock); virMutexDestroy(&devs->lock); VIR_FREE(devs);

} If the chardev is removed by fdStream close or fdStream abort when vm destroy or shutdown, the modification works well. But I'm not sure is that all chardev would be removed when we clear vm, if not, it would always sleep here. Another solution is as follows: virMutexLock(vm); virChrdevFree(); virMutexUnlock(vm); virMutexLock(vm); virFDStreamCloseInt(); virMutexUnlock(vm);