7/31/2019 Reasons for Deadlock
1/2
A deadlock situation can arise if and only if all of the
following conditions hold simultaneously
in a system
1. Mutual Exclusion: At least one resource must be
non-shareable.Only one processcan use the resource at any given
instant of time.
2.
Hold and Wait or Resource Holding: A process is currently
holding at least oneresource and requesting additional resources
which are being held by other processes.
3. NoPreemption: The operating system must not de-allocate
resources once they havebeen allocated; they must be released by
the holding process voluntarily.
4. Circular Wait:A process must be waiting for a resource which
is being held byanother process, which in turn is waiting for the
first process to release the resource.In general, there is asetof
waiting processes, P = {P1, P2, ..., PN}, such that P1 is
waiting for a resource held by P2, P2 is waiting for a resource
held by P3 and so on till
PN is waiting for a resource held by P1.
5. A limited number of a particular resource. In the case of a
monitor in C# (what youuse when you employ the lock keyword), this
limited number is one, since a monitor
is a mutual-exclusion lock (meaning only one thread can own a
monitor at a time).6. The ability to hold one resource and request
another. In C#, this is akin to locking onone object and then
locking on another before releasing the first lock, for
example:
lock(a)
{
...
lock(b)
{
...
}
}
No preemption capability. In C#, this means that one thread
can't force another thread torelease a lock.
A circular wait condition. This means that there is a cycle of
threads, each of which iswaiting for the next to release a resource
before it can continue.
He goes on to explain that the way to avoid deadlocks is to
avoid (or thwart) condition four.
Joe Duffy discusses several techniquesfor avoiding and detecting
deadlocks, including one
known as lock leveling. In lock leveling, locks are assigned
numerical values, and threads must
only acquire locks that have higher numbers than locks they have
already acquired. This preventsthe possibility of a cycle. It's
also frequently difficult to do well in a typical software
application
today, and a failure to follow lock leveling on every lock
acquisition invites deadlock.
http://en.wikipedia.org/wiki/Mutually_exclusive_eventshttp://en.wikipedia.org/wiki/Mutually_exclusive_eventshttp://en.wikipedia.org/wiki/Preemption_%28computing%29http://en.wikipedia.org/wiki/Preemption_%28computing%29http://en.wikipedia.org/wiki/Preemption_%28computing%29http://en.wikipedia.org/wiki/Circular_referencehttp://en.wikipedia.org/wiki/Circular_referencehttp://en.wikipedia.org/wiki/Sethttp://en.wikipedia.org/wiki/Sethttp://en.wikipedia.org/wiki/Sethttp://msdn.microsoft.com/msdnmag/issues/06/04/Deadlockshttp://msdn.microsoft.com/msdnmag/issues/06/04/Deadlockshttp://msdn.microsoft.com/msdnmag/issues/06/04/Deadlockshttp://en.wikipedia.org/wiki/Sethttp://en.wikipedia.org/wiki/Circular_referencehttp://en.wikipedia.org/wiki/Preemption_%28computing%29http://en.wikipedia.org/wiki/Mutually_exclusive_events
