Re: [PATCH v3 03/22] netconsole: Introduce 'enabled' state-machine

[Mike Waychison]

2010/12/14 MichaÅ MirosÅaw <mirqus@xxxxxxxxx>:
> 2010/12/14 Mike Waychison <mikew@xxxxxxxxxx>:
>> Representing the internal state within netconsole isn't really a boolean
>> value, but rather a state machine with transitions.
> [...]
>> diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
>> index 6e16888..288a025 100644
>> --- a/drivers/net/netconsole.c
>> +++ b/drivers/net/netconsole.c
> [...]
>> @@ -350,9 +362,9 @@ static ssize_t store_enabled(struct netconsole_target *nt,
>> Â Â Â Â Â Â Â Âerr = netpoll_setup(&nt->np);
>> Â Â Â Â Â Â Â Âspin_lock_irqsave(&target_list_lock, flags);
>> Â Â Â Â Â Â Â Âif (err)
>> - Â Â Â Â Â Â Â Â Â Â Â nt->enabled = 0;
>> + Â Â Â Â Â Â Â Â Â Â Â nt->np_state = NETPOLL_DISABLED;
>> Â Â Â Â Â Â Â Âelse
>> - Â Â Â Â Â Â Â Â Â Â Â nt->enabled = 1;
>> + Â Â Â Â Â Â Â Â Â Â Â nt->np_state = NETPOLL_ENABLED;
>> Â Â Â Â Â Â Â Âspin_unlock_irqrestore(&target_list_lock, flags);
>> Â Â Â Â Â Â Â Âif (err)
>> Â Â Â Â Â Â Â Â Â Â Â Âreturn err;
> [...]
>
> Since the spinlock protects only nt->np_state setting, you might be
> able to remove it altogether and use cmpxchg() where nt->np_state
> transitions from enabled or disabled state.
>
> Maybe the locking scheme just needs more thought altogether?

The target_list_lock protects the list, as well as the state
transitions.  This makes iterating through the list and getting a
consistent view of the targets a lot easier when it comes time to
transmitting packets we are guaranteed that nobody is changing the
target state underneath us if nt->np_state == NETPOLL_ENABLED while we
hold the lock.
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