On Wed, Dec 11, 2002 at 10:13:33PM +0000, Alan Cox wrote:
> On Wed, 2002-12-11 at 20:27, John Levon wrote:
> > There are notifiers being used that sleep inside the called notifiers.
> >
> > You could easily make a __notifier_call_chain that is lockless and
> > another one that readlocks the notifier_lock ...
>
> The notifier chains assume the users will do the locking needed for
> them. It might be possible to do cool things there with RCU
>
Hmm. If the called notifiers sleep, RCU can't prevent the notifiers
from being unregistered while they are executing. I suppose then that
RCU is not suitable for generic notifiers. It may still be useful
for managing notifiers in very fast paths where acquiring a
read lock may be undesirable (like the oprofile's NMI handler).
Instead of a lockless __notifier_call_chain, I would rather add
a notifier_call_chain_safe() to avoid changing the existing
handlers. Something like the patch below.
The safe version should be used for the trap handlers: we know
that the called notifiers will not sleep and the smp-safeness
makes the handlers simpler.
Thanks,
Vamsi.
-- Vamsi Krishna S. Linux Technology Center, IBM Software Lab, Bangalore. Ph: +91 80 5044959 Internet: vamsi@in.ibm.com -- diff -urN -X /home/vamsi/.dontdiff 51-pure/include/linux/notifier.h 51-notifier/include/linux/notifier.h --- 51-pure/include/linux/notifier.h 2002-12-10 08:15:43.000000000 +0530 +++ 51-notifier/include/linux/notifier.h 2002-12-12 11:53:53.000000000 +0530 @@ -24,6 +24,7 @@ extern int notifier_chain_register(struct notifier_block **list, struct notifier_block *n); extern int notifier_chain_unregister(struct notifier_block **nl, struct notifier_block *n); extern int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v); +extern int notifier_call_chain_safe(struct notifier_block **n, unsigned long val, void *v); #define NOTIFY_DONE 0x0000 /* Don't care */ #define NOTIFY_OK 0x0001 /* Suits me */ diff -urN -X /home/vamsi/.dontdiff 51-pure/kernel/sys.c 51-notifier/kernel/sys.c --- 51-pure/kernel/sys.c 2002-12-10 08:15:43.000000000 +0530 +++ 51-notifier/kernel/sys.c 2002-12-12 11:52:45.000000000 +0530 @@ -166,6 +166,29 @@ } /** + * notifier_call_chain_safe - Call functions in a notifier chain + * @n: Pointer to root pointer of notifier chain + * @val: Value passed unmodified to notifier function + * @v: Pointer passed unmodified to notifier function + * + * Calls each function in a notifier chain in turn while ensuring + * that a notifier cannot be unregistered while it is being + * executed. Because a read_lock is taken, the called notifiers + * must not sleep. + */ + +int notifier_call_chain_safe(struct notifier_block **n, unsigned long val, void *v) +{ + int ret; + + read_lock(¬ifier_lock); + ret = notifier_call_chain(n, val, v); + read_unlock(¬ifier_lock); + + return ret; +} + +/** * register_reboot_notifier - Register function to be called at reboot time * @nb: Info about notifier function to be called * - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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