Hello Peter!
Sorry, I missed the question onf the CONFIG_TRACE_IRQFLAGS - I think
that config option is meant for tracing the IRQ flow in the event of
debugging. Correct?
O.K. So it's only a help for debugging...
I use semaphores now (down() and up()). And the interface doesn't seem
to be slower. Is that also O.K.?
I don't know mutex_lock. What's the difference between mutex_locks and
semaphores? Is it better to use mutex_locks here?
2 diff things. semap are sync mechanism used - to sync among
CPUs/processes/tasks etc. Just a counting mechanism. It DOES NOT
lock the other CPU while another CPU is using the same resources. but
this counting must be atomic....different arch has different instruction
set to do it.
I thought that because there is an 'init_MUTEX(struct semaphore *)'
macro, there would be no difference between semaphores and mutex_locks.
So, did I understand it right:
semaphores are only local (for one CPU)?
mutex_locks also lock the other CPUs?
sometime the kernel guarantee the atomicity by putting a
spin lock around it.
Take a look at the fs/super.c code - mutex is
often used TOGETHER with the semaphore.
static int grab_super(struct super_block *s) __releases(sb_lock)
{
s->s_count++;
spin_unlock(&sb_lock);
down_write(&s->s_umount);
if (s->s_root) {
Or this:
void unlock_super(struct super_block * sb)
{
put_fs_excl();
mutex_unlock(&sb->s_lock);
}
I see.
(in general all the get_xxx and put_xxx functions u see in the source
code are semaphore - sorry if this generalization is wrong :-)). They
goes in pair to ensure balance in counting. but u used semaphore ONLY
WHEN u want to have multiple user sharing the same resources. eg, one
data, many reader, but writer u can only have one - so semaphore cannot
be used here - either spin lock or mutex solely.
O.K. but is there something I can use if I have for example one writer
and many readers on _different_ CPUs?
have u heard of lockfree/waitfree algorithm (check wiki)? or
non-blocking synchronization? (linux call it RCU - check wiki, and
under Documentation/RCU directory in source). It is a large topic
itself....i cannot justify to describe it here.....type "lockfree" in
google video and u can listen to a seminar on this
algorithm.....wow...google video has lots of open source seminar btw.
I haven't heard about these algorithms. I've read the wiki entry but I
will look for more information about this topic.
Many Thanks again!
Regards,
Lukas
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