RE: [patch] printk subsystems

[Tom Zanussi]

Hi,

Perez-Gonzalez, Inaky writes:
 > > 
 > > relayfs is there to solve the data transfer problems for the most
 > > demanding of applications. Sending a few messages here and there
 > > isn't really a problem. Sending messages/events/what-you-want-to-call-it
 > > by the thousand every second, while using as little locking as possible
 > > (lockless-logging is implemented in the case of relayfs' buffer handling
 > > routines), and providing per-cpu buffering requires a different beast.
 > 
 > Well, you are doing an IRQ lock (relay_lock_channel()), so it is not
 > lockless. Or am I missing anything here? Please let me know, I am
 > really interested on how to reduce locking in for logging to the 
 > minimal.

relayfs actually uses 2 mutually-exclusive schemes internally -
'lockless' and 'locking', depending on the availability of a cmpxchg
instruction (lockless needs cmpxchg).  If the lockless scheme is being
used, relay_lock_channel() does no locking or irq disabling of any
kind i.e. it's basically a no-op in that case.  It's only when the
'locking' scheme is in use that relay_lock_channel() does locking/irq
disabling.  Normally the lockless scheme would be in use - the locking
scheme is there mainly as a fallback, so normally relay_lock_channel()
would indeed cause no locking.

-- 
Regards,

Tom Zanussi <[e-mail]>
IBM Linux Technology Center/RAS