SQL Server scheduler/worker is dsigned such that they are Cooperative and the scheduler expects that the workers yield cooperative. However, the API calls such as External modules, Extended procedures etc. may never yield and as a result the worker will be stuck doing an activity.

Error 17883 is reported if the Worker does not yield over 60 seconds (the trace flag –T1262 can ne used to alter this frequency

When all the schedulers encounter 17883 error or all workers are blocked on a critical resource or all workers are executing long running queiries, SQL Server reports scheduler detect Deadlock conditions with the error number 17884,

for further information on this

http://technet.microsoft.com/en-us/library/cc917684.aspx#_17883