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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 1040
SEC. 12.3
IMPLEMENTATION
1009
Reentrancy
Reentrancy refers to the ability of code to be executed two or more times si-
multaneously.
On a multiprocessor, there is always the danger than while one CPU
is executing some procedure, another CPU will start executing it as well, before the
first one has finished. In this case, two (or more) threads on different CPUs might
be executing the same code at the same time. This situation must be protected
against by using mutexes or some other means to protect critical regions.
However, the problem also exists on a uniprocessor.
In particular, most of any
operating system runs with interrupts enabled.
To do otherwise would lose many
interrupts and make the system unreliable. While the operating system is busy ex-
ecuting some procedure,
P
, it is entirely possible that an interrupt occurs and that
the interrupt handler also calls
P
.
If the data structures of
P
were in an inconsistent
state at the time of the interrupt, the handler will see them in an inconsistent state
and fail.
An obvious example where this can happen is if
P
is the scheduler. Suppose
that some process has used up its quantum and the operating system is moving it to
the end of its queue. Partway through the list manipulation, the interrupt occurs,
makes some process ready, and runs the scheduler. With the queues in an inconsis-
tent state, the system will probably crash.
As a consequence even on a uniproc-
essor, it is best that most of the operating system is reentrant, critical data struc-
tures are protected by mutexes, and interrupts are disabled at moments when they
cannot be tolerated.
Brute Force
Using brute-force to solve a problem has acquired a bad name over the years,
but it is often the way to go in the name of simplicity. Every operating system has
many procedures that are rarely called or operate with so few data that optimizing
them is not worthwhile. For example, it is frequently necessary to search various
tables and arrays within the system. The brute force algorithm is to just leave the
table in the order the entries are made and search it linearly when something has to
be looked up.
If the number of entries is small (say, under 1000), the gain from
sorting the table or hashing it is small, but the code is far more complicated and
more likely to have bugs in it.
Sorting or hashing the mount table (which keeps
track of mounted file systems in UNIX systems) really is not a good idea.
Of course, for functions that are on the critical path, say, context switching,
everything should be done to make them very fast, possibly even writing them in
(heaven forbid) assembly language. But large parts of the system are not on the
critical path.
For example, many system calls are rarely invoked. If there is one
fork
every second, and it takes 1 msec to carry out, then even optimizing it to 0
wins only 0.1%.
If the optimized code is bigger and buggier, a case can be made
not to bother with the optimization.
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