Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Sat, Nov 18, 2006 at 09:46:24PM +0300, Oleg Nesterov wrote:
> On 11/17, Paul E. McKenney wrote:
> >
> > Oleg, any thoughts about Jens's optimization?  He would code something
> > like:
> >
> > 	if (srcu_readers_active(&my_srcu))
> > 		synchronize_srcu();
> > 	else
> > 		smp_mb();
> 
> Well, this is clearly racy, no? I am not sure, but may be we can do
> 
> 	smp_mb();
> 	if (srcu_readers_active(&my_srcu))
> 		synchronize_srcu();
> 
> in this case we also need to add 'smp_mb()' into srcu_read_lock() after
> 'atomic_inc(&sp->hardluckref)'.
> 
> > However, he is doing ordered I/O requests rather than protecting data
> > structures.
> 
> Probably this makes a difference, but I don't understand this.

OK, one hypothesis here...

	The I/Os must be somehow explicitly ordered to qualify
	for I/O-barrier separation.  If two independent processes
	issue I/Os concurrently with a third process doing an
	I/O barrier, the I/O barrier is free to separate the
	two concurrent I/Os or not, on its whim.

Jens, is the above correct?  If so, what would the two processes
need to do in order to ensure that their I/O was considered to be
ordered with respect to the I/O barrier?  Here are some possibilities:

1.	I/O barriers apply only to preceding and following I/Os from
	the process issuing the I/O barrier.

2.	As for #1 above, but restricted to task rather than process.

3.	I/O system calls that have completed are ordered by the
	barrier to precede I/O system calls that have not yet
	started, but I/O system calls still in flight could legally
	land on either side of the concurrently executing I/O
	barrier.

4.	Something else entirely?

Given some restriction like one of the above, it is entirely possible
that we don't even need the memory barrier...

						Thanx, Paul
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux