[libvirt-users] error : virHashForEach:597 : Hash operation not allowed during iteration

[Michal Privoznik]

On 04/04/2018 02:59 PM, Vincent Bernat wrote:
>  ❦  4 avril 2018 11:17 +0200, Michal Privoznik <mprivozn at redhat.com> :
> 
>> Dunno, this is the first time I hear about this issue. Maybe you can try
>> to set a break point on virHashIterationError() and when it's hit get
>> stacktrace of all threads 't a a bt'. That might shed more light into
>> the issue. Smells like we are not locking somewhere properly.
> 
> So, we have two threads iterating. Both of them originates from
> virNetServerProgramDispatchCall. So, it seems there is a lock missing?

No. As suspected, both threads are reading internal list of domains
(actually hash table). They are not modifying the list so they grab a
read lock.

> 
> Thread 10 (Thread 0x7f931f814700 (LWP 126453)):
> #0  virHashForEach (table=0x7f92fc69a480, iter=iter at entry=0x7f932ea8fbf0 <virDomainObjListCollectIterator>, data=data at entry=0x7f931f813a20) at ../../../src/util/virhash.c:597
> #1  0x00007f932ea911c3 in virDomainObjListCollect (domlist=0x7f92fc82dd50, conn=conn at entry=0x7f92e0000f80, vms=vms at entry=0x7f931f813a80, nvms=nvms at entry=0x7f931f813a90, filter=0x7f932ebdc6b0 <virConnectListAllDomainsCheckACL>, flags=48)
>     at ../../../src/conf/virdomainobjlist.c:935
> #2  0x00007f932ea91482 in virDomainObjListExport (domlist=<optimized out>, conn=0x7f92e0000f80, domains=0x7f931f813af0, filter=<optimized out>, flags=<optimized out>) at ../../../src/conf/virdomainobjlist.c:1019
> #3  0x00007f932eaf3b75 in virConnectListAllDomains (conn=0x7f92e0000f80, domains=0x7f931f813af0, flags=48) at ../../../src/libvirt-domain.c:6491
> #4  0x0000559f219f6dca in remoteDispatchConnectListAllDomains (server=0x559f23795530, msg=0x559f237b78c0, ret=0x7f92d0000920, args=0x7f92d0000ad0, rerr=0x7f931f813bc0, client=<optimized out>) at ../../../daemon/remote_dispatch.h:1244
> #5  remoteDispatchConnectListAllDomainsHelper (server=0x559f23795530, client=<optimized out>, msg=0x559f237b78c0, rerr=0x7f931f813bc0, args=0x7f92d0000ad0, ret=0x7f92d0000920) at ../../../daemon/remote_dispatch.h:1220
> #6  0x00007f932eb5c5b9 in virNetServerProgramDispatchCall (msg=0x559f237b78c0, client=0x559f237b7bd0, server=0x559f23795530, prog=0x559f237b2a50) at ../../../src/rpc/virnetserverprogram.c:437
> #7  virNetServerProgramDispatch (prog=0x559f237b2a50, server=server at entry=0x559f23795530, client=0x559f237b7bd0, msg=0x559f237b78c0) at ../../../src/rpc/virnetserverprogram.c:307
> #8  0x0000559f21a20268 in virNetServerProcessMsg (msg=<optimized out>, prog=<optimized out>, client=<optimized out>, srv=0x559f23795530) at ../../../src/rpc/virnetserver.c:148
> #9  virNetServerHandleJob (jobOpaque=<optimized out>, opaque=0x559f23795530) at ../../../src/rpc/virnetserver.c:169
> #10 0x00007f932ea3871b in virThreadPoolWorker (opaque=opaque at entry=0x559f2377da20) at ../../../src/util/virthreadpool.c:167
> #11 0x00007f932ea37ac8 in virThreadHelper (data=<optimized out>) at ../../../src/util/virthread.c:206
> #12 0x00007f932e0ed6ba in start_thread (arg=0x7f931f814700) at pthread_create.c:333
> #13 0x00007f932de2341d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
> 
> Thread 6 (Thread 0x7f9321818700 (LWP 126449)):
> #0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
> #1  0x00007f932e0efdbd in __GI___pthread_mutex_lock (mutex=mutex at entry=0x7f92ec011c80) at ../nptl/pthread_mutex_lock.c:80
> #2  0x00007f932ea37c45 in virMutexLock (m=m at entry=0x7f92ec011c80) at ../../../src/util/virthread.c:89
> #3  0x00007f932ea180ca in virObjectLock (anyobj=anyobj at entry=0x7f92ec011c70) at ../../../src/util/virobject.c:388
> #4  0x00007f932ea8fd81 in virDomainObjListCopyActiveIDs (payload=0x7f92ec011c70, name=<optimized out>, opaque=0x7f9321817aa0) at ../../../src/conf/virdomainobjlist.c:679
> #5  0x00007f932e9ee6cd in virHashForEach (table=0x7f92fc69a480, iter=iter at entry=0x7f932ea8fd70 <virDomainObjListCopyActiveIDs>, data=data at entry=0x7f9321817aa0) at ../../../src/util/virhash.c:606
> #6  0x00007f932ea90fba in virDomainObjListGetActiveIDs (doms=0x7f92fc82dd50, ids=<optimized out>, maxids=<optimized out>, filter=<optimized out>, conn=<optimized out>) at ../../../src/conf/virdomainobjlist.c:701
> #7  0x00007f932eae1c6e in virConnectListDomains (conn=0x7f92e40018b0, ids=0x7f92f4015ae0, maxids=14) at ../../../src/libvirt-domain.c:66
> #8  0x0000559f21a0d3c1 in remoteDispatchConnectListDomains (server=<optimized out>, msg=0x559f237beea0, ret=0x7f92f4009eb0, args=0x7f92f4016290, rerr=0x7f9321817bc0, client=<optimized out>) at ../../../daemon/remote_dispatch.h:2108
> #9  remoteDispatchConnectListDomainsHelper (server=<optimized out>, client=<optimized out>, msg=0x559f237beea0, rerr=0x7f9321817bc0, args=0x7f92f4016290, ret=0x7f92f4009eb0) at ../../../daemon/remote_dispatch.h:2076
> #10 0x00007f932eb5c5b9 in virNetServerProgramDispatchCall (msg=0x559f237beea0, client=0x559f237c0ce0, server=0x559f23795530, prog=0x559f237b2a50) at ../../../src/rpc/virnetserverprogram.c:437
> #11 virNetServerProgramDispatch (prog=0x559f237b2a50, server=server at entry=0x559f23795530, client=0x559f237c0ce0, msg=0x559f237beea0) at ../../../src/rpc/virnetserverprogram.c:307
> #12 0x0000559f21a20268 in virNetServerProcessMsg (msg=<optimized out>, prog=<optimized out>, client=<optimized out>, srv=0x559f23795530) at ../../../src/rpc/virnetserver.c:148
> #13 virNetServerHandleJob (jobOpaque=<optimized out>, opaque=0x559f23795530) at ../../../src/rpc/virnetserver.c:169
> #14 0x00007f932ea3871b in virThreadPoolWorker (opaque=opaque at entry=0x559f2377d860) at ../../../src/util/virthreadpool.c:167
> #15 0x00007f932ea37ac8 in virThreadHelper (data=<optimized out>) at ../../../src/util/virthread.c:206
> #16 0x00007f932e0ed6ba in start_thread (arg=0x7f9321818700) at pthread_create.c:333
> #17 0x00007f932de2341d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

Both threads call virHashForEach(table=0x7f92fc69a480). Thread 6 was
first so it starts iterating and sets table->iterating so later when
thread 10 enters the function an error is reported.

I guess we can go with what Dan suggested and after some rework we can
just drop ->iterating completely.

Michal