Completely perplexed-Operation would block

[Michael Barber]

I can not figure out for the life of me why suddenly my secondary quit
slaving against the master.  I have a terteriary internal dns that is
slaving just fine.  Everything is on the same network so how could there be
a network problem.  The log has these messages in it.

zone transfer timeout for "yourdomain.com"; pid 524 kill failed Errcode:
10035: Operation would block

What does Operation would block mean?

Mike