asked on

mutex generic question

A and B are mutexes, and are initialized
Whats the proper way to handle mutex A And B

void Task1(void)
{
while (1)
{
mutex_acquire(A);
do_task1_A_stuff();

mutex_acquire(B);
do_task1_AB_stuff();

mutex_release(A);
do_task1_B_stuff();

mutex_release(B);
}
}

void Task2(void)
{
while (1)
{
mutex_acquire(B);
do_task2_B_stuff();

mutex_acquire(A);
do_task2_AB_stuff();

mutex_release(B);
do_task2_A_stuff();

mutex_release(A);
}
}

ASKER CERTIFIED SOLUTION

sunnycoder

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sunnycoder

One possible way would be

void Task1(void)
{
while (1)
{
mutex_acquire(A);
do_task1_A_stuff();

mutex_acquire(B);
do_task1_AB_stuff();

do_task1_B_stuff();

mutex_release(B);
mutex_release(A);
}
}

void Task2(void)
{
while (1)
{
mutex_acquire(A);
mutex_acquire(B);
do_task2_B_stuff();

do_task2_AB_stuff();

mutex_release(B);
do_task2_A_stuff();

mutex_release(A);
}
}

Make sure that resources/mutexes are acquired in same fixed order and released in the reverse order. It is similar to prioritizing the resources and making sure that no lower priority resource is requested if you have a larger priority resource already acquired.

balder

how about

void Task1(void)
{
while (1)
{
mutex_acquire(A);
do_task1_A_stuff();
mutex_release(A); /* can be removed, but might interleave better */

mutex_acquire(A); /* can be removed ... */
mutex_acquire(B);
do_task1_AB_stuff();
mutex_release(B); /* avoid deadlock with task 2 */
mutex_release(A); /* release task 2 if blocked */

mutex_acquire(B);
do_task1_B_stuff();
mutex_release(B);
}
}

void Task2(void)
{
while (1)
{
mutex_acquire(B);
do_task2_B_stuff();
mutex_release(B); /* release task 1 if blocked */

mutex_acquire(A); /* wait for task 1 */
mutex_acquire(B);
do_task2_AB_stuff();
mutex_release(B);
mutex_release(A); /* can be removed ... */

mutex_acquire(A); /* can be removed ... */
do_task2_A_stuff();
mutex_release(A);
}
}

Kelvin_King

Are you allowed to reorder the P() and V() calls ? or is this some kind of exercise you need to solve in that specific order ?