You may need to write code that acquires more than one lock.
This opens up the possibility of deadlock. Consider the following
piece of code:
Lock *l1, *l2;
void p() {
l1->Acquire();
l2->Acquire();
code that manipulates data that l1 and l2 protect
l2->Release();
l1->Release();
}
void q() {
l2->Acquire();
l1->Acquire();
code that manipulates data that l1 and l2 protect
l1->Release();
l2->Release();
}
If p and q execute concurrently, consider what may
happen. First, p acquires l1 and q acquires
l2. Then, p waits to acquire l2 and
q waits to acquire l1. How long will they wait? Forever.
This case is called deadlock.
What are conditions for deadlock?
Mutual Exclusion: Only one thread can hold lock at a time.
Hold and Wait: At least one thread holds a lock and is waiting
for another process to release a lock.
No preemption: Only the process holding the lock can release it.
Circular Wait: There is a set t1, ..., tn such that
t1 is waiting for a lock held by t2, ..., tn is
waiting for a lock held by t1.
How can p and q avoid deadlock? Order the locks,
and always acquire the locks in that order. Eliminates the circular
wait condition.
Occasionally you may need to write code that needs to acquire
locks in different orders. Here is what to do in this situation.
First, most locking abstractions offer an operation that tries to
acquire the lock but returns if it cannot. We will call this
operation TryAcquire.
Use this operation to
try to acquire the lock that you need to acquire out of order.
If the operation succeeds, fine. Once you've got the lock,
there is no problem.
If the operation fails, your code will need to release all
locks that come after the lock you are trying to acquire. Make
sure the associated data structures are in a state where you can
release the locks without crashing the system.
Release all of the locks that come after the lock you are
trying to acquire, then reacquire all of the locks in the right
order. When the code resumes, bear in mind that the data structures
might have changed between the time when you released and reacquired
the lock.
Here is an example.
int d1, d2;
// The standard acquisition order for these two locks
// is l1, l2.
Lock *l1, // protects d1
*l2; // protects d2
// Decrements d2, and if the result is 0, increments d1
void increment() {
l2->Acquire();
int t = d2;
t--;
if (t == 0) {
if (l1->TryAcquire()) {
d1++;
} else {
// Any modifications to d2 go here - in this case none
l2->Release();
l1->Acquire();
l2->Acquire();
t = d2;
t--;
// some other thread may have changed d2 - must recheck it
if (t == 0) {
d1++;
}
}
l1->Release();
}
d2 = t;
l2->Release();
}
This example is somewhat contrived, but you will recognize the
situation when it occurs.
There is a generalization of the deadlock problem to situations
in which processes need multiple resources, and the hardware may
have multiple kinds of each resource - two printers, etc.
Seems kind of like a batch model - processes request
resources, then system schedules process to run when resources
are available.
In this model, processes issue requests to OS for resources, and
OS decides who gets which resource when. A lot of theory built up
to handle this situation.
Process first requests a resource, the OS issues it and the
process uses the resource, then the process releases the resource
back to the OS.
Reason about resource allocation using resource allocation
graph. Each resource is represented with a box, each process with
a circle, and the individual instances of the resources with
dots in the boxes. Arrows go from processes to resource boxes
if the process is waiting for the resource. Arrows go from
dots in resource box to processes if the process holds that instance
of the resource. See Fig. 7.1.
If graph contains no cycles, is no deadlock. If has a cycle,
may or may not have deadlock. See Fig. 7.2, 7.3.
System can either
Restrict the way in which processes will request resources
to prevent deadlock.
Require processes to give advance information about which
resources they will require, then use algorithms that schedule
the processes in a way that avoids deadlock.
Detect and eliminate deadlock when it occurs.
First consider prevention. Look at the deadlock conditions
listed above.
Mutual Exclusion -
To eliminate mutual exclusion, allow everybody to use the resource immediately
if they want to. Unrealistic in general - do you want your
printer output
interleaved with someone elses?
Hold and Wait. To prevent hold and wait, ensure that when a
process requests resources, does not hold
any other resources. Either asks
for all resources before executes, or dynamically asks for resources
in chunks as needed,
then releases all resources before asking for more.
Two problems - processes may hold but not use
resources for a long time because they will eventually hold them.
Also, may have starvation. If a process asks for lots of resources,
may never run because other processes always hold some subset of
the resources.
Circular Wait. To prevent circular wait, order resources and
require processes to request resources in that order.
Deadlock avoidance. Simplest algorithm - each process
tells max number of resources it will ever need. As process
runs, it requests resources but never exceeds max number
of resources. System schedules processes and allocates
resoures in a way that ensures that no
deadlock results.
Example: system has 12 tape drives.
System currently running
P0 needs max 10 has 5, P1 needs max 4 has 2, P2 needs max 9 has
2.
Can system prevent deadlock even if all processes request
the max? Well, right now system has 3 free tape drives. If
P1 runs first and completes, it will have 5 free tape drives.
P0 can run to completion with those 5 free tape drives even if it
requests max. Then P2 can complete. So, this schedule will
execute without deadlock.
If P2 requests two more tape drives, can system give it the
drives? No, because cannot be sure it can run all jobs to completion
with only 1 free drive. So, system must not give P2 2 more tape
drives until P1 finishes. If P2 asks for 2 tape drives, system
suspends P2 until P1 finishes.
Concept: Safe Sequence. Is an ordering of processes such that
all processes can execute to completion in that order even if all
request maximum resources. Concept: Safe State - a state in which there
exists a safe sequence. Deadlock avoidance algorithms always ensure
that system stays in a safe state.
How can you figure out if a system is in a safe state?
Given the current and maximum allocation, find a safe sequence.
System must maintain some information about the resources and
how they are used. See OSC 7.5.3.
Avail[j] = number of resource j available
Max[i,j] = max number of resource j that process i will use
Alloc[i,j] = number of resource j that process i currently has
Need[i,j] = Max[i,j] - Alloc[i,j]
Notation: A<=B if for all processes i,
A[i]<=B[i].
Safety Algorithm: will try to find a safe sequence.
Simulate evolution of system over time under worst case
assumptions of resource demands.
1: Work = Avail;
Finish[i] = False for all i;
2: Find i such that Finish[i] = False and Need[i] <= Work
If no such i exists, goto 4
3: Work = Work + Alloc[i]; Finish[i] = True; goto 2
4: If Finish[i] = True for all i, system is in a safe state
Now, can use safety algorithm to determine if we can
satisfy a given resource demand. When a process demands
additional resources, see if can give them to process and
remain in a safe state. If not, suspend process until system
can allocate resources and remain in a safe state. Need
an additional data structure:
Request[i,j] = number of j resources that process i requests
Here is algorithm. Assume process i has
just requested additional resources.
1: If Request[i] <= Need[i] goto 2. Otherwise, process has
violated its maximum resource claim.
2: If Request[i] <= Avail goto 3. Otherwise, i must wait
because resources are not available.
3: Pretend to allocate resources as follows:
Avail = Avail - Request[i]
Alloc[i] = Alloc[i] + Request[i]
Need[i] = Need[i] - Request[i]
If this is a safe state, give the process the resources. Otherwise,
suspend the process and restore the old state.
When to check if a suspended process should be given the
resources and resumed? Obvious choice - when some other process
relinquishes its resources. Obvious problem - process starves
because other processes with lower resource requirements are
always taking freed resources.
See Example in Section 7.5.3.3.
Third alternative: deadlock detection and elimination. Just
let deadlock happen. Detect when it does, and eliminate the deadlock
by preempting resources.
Here is deadlock detection algorithm. Is very similar to
safe state detection algorithm.
1: Work = Avail;
Finish[i] = False for all i;
2: Find i such that Finish[i] = False and Request[i] <= Work
If no such i exists, goto 4
3: Work = Work + Alloc[i]; Finish[i] = True; goto 2
4: If Finish[i] = False for some i, system is deadlocked.
Moreover, Finish[i] = False implies that process i is deadlocked.
When to run deadlock detection algorithm? Obvious time: whenever
a process requests more resources and suspends. If deadlock detection
takes too much time, maybe run it less frequently.
OK, now you've found a deadlock. What do you do? Must free
up some resources so that some processes can run. So, preempt
resources - take them away from processes. Several different
preemption cases:
Can preempt some
resources without killing job - for example, main memory. Can
just swap out to disk and resume job later.
If job provides rollback points, can roll job back to
point before acquired resources. At a later time, restart
job from rollback point. Default rollback point - start of job.
For some resources must just kill job. All resources
are then free. Can either kill processes one by one until your
system is no longer deadlocked. Or, just go ahead and kill all
deadlocked processes.
In a real system, typically use different deadlock strategies
for different situations based on resource characteristics.
This whole topic has a sort of 60's and 70's batch mainframe feel
to it. How come these topics never seem to arise in modern Unix
systems?
