From 2.6.24 to 2.6.25-rc2, CPU can not do hotplug when one task set affinity to it

[youquan_song@linux.intel.com]

In SMP sever platform, with kernel 2.6.24-rc2 and kernel 2.6.25-rc2,
Set one task affinity to one CPU core, then set offline the CPU core.
After that we can not set online the CPU core again.


1.  run the test.sh script:   test.sh 1  ( 1 is the logical CPU )
test.sh script:

#!/bin/bash

main(){

    typeset -i CPU=$1
    ./task.sh > /dev/null&
    PID=$!

    if [ `cat /sys/devices/system/cpu/cpu${CPU}/online` = "0" ]; then
        echo "1" > /sys/devices/system/cpu/cpu${CPU}/online
    fi

    MASK=$((1<<${CPU}))

    `taskset -p ${MASK} ${PID} > /dev/null 2>&1`

    echo "0" > /sys/devices/system/cpu/cpu${CPU}/online

    echo "1" > /sys/devices/system/cpu/cpu${CPU}/online

    kill -9 $PID > /dev/null 2>&1

    echo "PASS\n"

}

typeset -i TEST_CPU=$1
main $TEST_CPU

2. task.sh script as following

#!/bin/bash

while :
do
  NOOP=1
done

Actual Result:
The test.sh will block at set online the CPU ( echo "1" >
/sys/devices/system/cpu/cpu${CPU}/online ).

we check the test.sh has dead by 'ps -aux'.