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rraaccee ccoonnddiittiioonn -- Definition


The term  _r_a_c_e _c_o_n_d_i_t_i_o_n refers to  the condition that exists  when the the
outcome of  a sequence of  instructions cannot be  guaranteed.  This occurs
when program has two sections of  code that can run in any order and either
share a  variable or  change the  state of the  machine: the  code executed
first  wins  the  ``race''  and  so  controls  execution  of  the  program.
Obviously, it  is desirable to avoid  this situation; you can  do so if you
can force a certain ordering of the code sections.

Race conditions most often happen in operating system related environments.
If, as in  the case of a device driver,  your program has a main section of
code that manipulates a few variables  and it also has an interrupt handler
that does  the same, your  program must lock out  interrupts during certain
critical times to guarantee that the variables will not be compromised.

Consider, for example, the following pseudo-code:

    set interrupt priority to keep out the gremlins
    while (work is not yet completed)
        v_sleep( &some_variable_in_the_kernel_data_area )
    restore interrupt mask

If an interrupt  were to occur between the wwhhiillee  statement and the call to
vv_sslleeeepp(), the driver would never wake  up because the event it was waiting
for (sleeping  on) will  have already  occurred.  To avoid  this situation,
your  code must  this block  of  code with  calls to  the kernel  functions
sspphhii()/ssppll().  This will  ensure that interrupts  cannot occur  until after
vv_sslleeeepp() has  been called.  The system will  re-enable interrupts when the
driver calls  vv_sslleeeepp(), but  it is guaranteed  to have the  same interrupt
level (mask) when it awakens,  thus preserving the lockout of the interrupt
handler.

In most  cases, drivers lock out interrupts  when manipulating the internal
linked lists associated with tasks to be performed or buffers in use.  This
keeps the interrupt  handler from using stale data or,  worse yet, a linked
list that isn't correctly linked.

_S_e_e _A_l_s_o
ddeevviiccee ddrriivveerrss

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