My two cents to this discussion.
I consider both of these macros (write_can_lock and
read_can_lock) brain-damaged as they do not guarantee anything.
That's why I haven't copied the read_can_lock macro - it is not
used in the sparc kernel and let it be this way.
The write_can_lock is used in one place only - in ptrace.c file
when it is used to add a little efficiency into a waiting for two
spinlocks to be free. I suppose this call can be removed but I
afraid changing it. A whole idea there is simple: obtain one
lock, try to obtain another one and if it is not successful use
the write_can_lock macro to wait before repeating former steps. I
think that without this macro the waiting may become less
efficient - more trials of obtaining lock will be made. See the
code below:
repeat:
/*
* Nasty, nasty.
*
* We want to hold both the task-lock and the
* tasklist_lock for writing at the same time.
* But that's against the rules (tasklist_lock
* is taken for reading by interrupts on other
* cpu's that may have task_lock).
*/
task_lock(task);
local_irq_disable();
if (!write_trylock(&tasklist_lock)) {
local_irq_enable();
task_unlock(task);
do {
cpu_relax();
} while (!write_can_lock(&tasklist_lock));
goto repeat;
}
There is no need for this macro to be 100% accurate. It is enough
to be conservative as Bob Breuer suggested (it should not spin
waiting either). The other way is to remove the call to
write_can_lock from the code above.
I beg you (sparc32 maintainers) to start pushing smp patches (or
even parts of them) into the kernel tree. I have another patch
which clears sun4m_smp.c a little (replaced loops with
for_each_online_cpu, removed one unused global variable) but this
will stop Bob Breuer's patch from applying so I wait sitting on it.
I am able to boot a dual SM81 in SS20 with all the smp patches
from this list, but the kernel hangs on first module load. It is
something I will investigate.
Regards,
Krzysztof
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