3 ms·

Can/do supervisors poll for this situation? Seems like that should be possible.

by btown 7y ago

xenadu02 7y ago
Also curious why the kernel can't abandon the thread since the process is dead anyway; set a bit in kernel space so on the walk back from the syscall it bails out instead of trying to return to userspace. That doesn't solve the kernel thread being stuck forever problem. I'm not sure what the fix is there... is it bad drivers with no timeout mechanism? I don't know how they'd do that unless the kernel IO is all async anyway. Just tearing down the kernel thread seems likely to leave the locks abandoned in bad states.
- cryptonector 7y ago
  The kernel can't abandon that thread entirely, but it could "detach" it from the process it was part of and arrange for it to clean itself up and disappear if ever it wakes. And so the parent process could get told of the child's death.
jerf 7y ago
This becomes a tarpit. Whatever metric you think to measure for this may itself become a source of false positives in the field. Whatever things you choose to try to avoid that becomes another source of falseness in the system. This tends to cascade as people apply patch upon patch until you have a system that basically randomly fails and nobody can tell you why. This is not a theoretical report; in theory this approach can work, if you get it perfect enough. This is a practical report, both of systems I've built myself and of many other such cases I've witnessed, including some things around process management and supervision. In the end, you really have little choice but to trust what the OS is telling you, and if the OS is lying for whatever reason, to fix the OS. As hard as that last bit may be as a solution, it still turns out to be the easiest, most-effective fix. And it may not be a perfect fix, either... but it'll still be the best.
phkahler 7y ago
>> Can/do supervisors poll for this situation? Sounds like a hack.
- jrockway 7y ago
  Typically supervisors are monitoring readiness/liveness of the managed process for other reasons; you don't have the number of replicas you've promised if one of the replicas is un-live (perhaps hitting this bug, perhaps internally deadlocked, perhaps internally waiting forever on something the OS doesn't care about, like a certain network request), and you shouldn't be routing network traffic to a service that isn't ready for it. So while I agree with the other posts that a supervisor can't recover from this state, it can be aware that it's happened. And it's quite common for supervisors to check for readiness and liveness beyond "well the OS says it's running, sounds good to me."
sigjuice 7y ago
Sure supervisors can be made to poll. But a supervisor won’t be able to do anything to recover from this.

Can/do supervisors poll for this situation? Seems like that should be possible.

by btown 7y ago

Can/do supervisors poll for this situation? Seems like that should be possible.

Also curious why the kernel can't abandon the thread since the process is dead anyway; set a bit in kernel space so on the walk back from the syscall it bails out instead of trying to return to userspace. That doesn't solve the kernel thread being stuck forever problem. I'm not sure what the fix is there... is it bad drivers with no timeout mechanism? I don't know how they'd do that unless the kernel IO is all async anyway. Just tearing down the kernel thread seems likely to leave the locks abandoned in bad states.

cryptonector 7y ago
The kernel can't abandon that thread entirely, but it could "detach" it from the process it was part of and arrange for it to clean itself up and disappear if ever it wakes. And so the parent process could get told of the child's death.

jerf 7y ago

This becomes a tarpit. Whatever metric you think to measure for this may itself become a source of false positives in the field. Whatever things you choose to try to avoid that becomes another source of falseness in the system. This tends to cascade as people apply patch upon patch until you have a system that basically randomly fails and nobody can tell you why. This is not a theoretical report; in theory this approach can work, if you get it perfect enough. This is a practical report, both of systems I've built myself and of many other such cases I've witnessed, including some things around process management and supervision. In the end, you really have little choice but to trust what the OS is telling you, and if the OS is lying for whatever reason, to fix the OS. As hard as that last bit may be as a solution, it still turns out to be the easiest, most-effective fix. And it may not be a perfect fix, either... but it'll still be the best.

phkahler 7y ago

>> Can/do supervisors poll for this situation? Sounds like a hack.

jrockway 7y ago
Typically supervisors are monitoring readiness/liveness of the managed process for other reasons; you don't have the number of replicas you've promised if one of the replicas is un-live (perhaps hitting this bug, perhaps internally deadlocked, perhaps internally waiting forever on something the OS doesn't care about, like a certain network request), and you shouldn't be routing network traffic to a service that isn't ready for it. So while I agree with the other posts that a supervisor can't recover from this state, it can be aware that it's happened. And it's quite common for supervisors to check for readiness and liveness beyond "well the OS says it's running, sounds good to me."

sigjuice 7y ago

Sure supervisors can be made to poll. But a supervisor won’t be able to do anything to recover from this.