User-space developers working with highly threaded applications would often
like to be able to use spinlocks to protect shared data structures from
concurrent access. There is a fundamental problem with user-space
spinlocks, though: there is no way to prevent a thread from being
preempted. Various ways of working around this problem have been explored,
but this
patch from Steven Rostedt questions the premise on which much of that
work is based: what if it were possible to prevent preemption, for a
short period at least?

Source: LWN.net – [$] Deferred scheduling for user-space critical sections