On 23.01.2018 19:26, Stephen Hemminger wrote:
> On Mon, 22 Jan 2018 12:41:41 +0300
> Kirill Tkhai <ktk...@virtuozzo.com> wrote:
>
>> Commit be3fc413da9e "net: use synchronize_rcu_expedited()" introducing
>> synchronize_net() says:
>>
>> >When we hold RTNL mutex, we would like to spend some cpu cycles but not
>> >block too long other processes waiting for this mutex.
>> >We also want to setup/dismantle network features as fast as possible at
>> >boot/shutdown time.
>> >This patch makes synchronize_net() call the expedited version if RTNL is
>> >locked.
>>
>> At the time of the commit (May 23 2011) there was no possible to differ,
>> who is the actual owner of the mutex. Only the fact that it's locked
>> by someone at the moment. So (I guess) this is the only reason the generic
>> primitive mutex_is_locked() was used.
>>
>> But now mutex owner is available outside the locking subsystem and
>> __mutex_owner() may be used instead (there is an example in
>> audit_log_start()).
>> So, let's make expensive synchronize_rcu_expedited() be used only
>> when a caller really owns rtnl_mutex().
>>
>> There are several possibilities to fix that. The first one is
>> to fix synchronize_net(), the second is to change rtnl_is_locked().
>>
>> I prefer the second, as it seems it's more intuitive for people
>> to think that rtnl_is_locked() is about current process, not
>> about the fact mutex is locked in general. Grep over kernel
>> sources just proves this fact:
>>
>> drivers/staging/rtl8723bs/os_dep/osdep_service.c:297
>> drivers/staging/rtl8723bs/os_dep/osdep_service.c:316
>>
>> if (!rtnl_is_locked())
>> ret = register_netdev(pnetdev);
>> else
>> ret = register_netdevice(pnetdev);
>
> Conditional locking like this especially in the registration path
> is wrong and broken. The driver should just be removed for this.
>
>> drivers/staging/wilc1000/linux_mon.c:310
>>
>> if (rtnl_is_locked()) {
>> rtnl_unlock();
>> rollback_lock = true;
>> }
>
>
> This driver like most of staging is sh*t.
Yeah, but the history of in-tree drivers shows they are not the only
drivers had the same mistake. My work tree is 3.10 and buggy caif_serial.c
is there.
>> Side effect of this patch is three BUGs in above examples
>> become fixed.
>>
>> Signed-off-by: Kirill Tkhai <ktk...@virtuozzo.com>
>> ---
>> net/core/rtnetlink.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
>> index 16d644a4f974..a5ddf373ffa9 100644
>> --- a/net/core/rtnetlink.c
>> +++ b/net/core/rtnetlink.c
>> @@ -117,7 +117,7 @@ EXPORT_SYMBOL(rtnl_trylock);
>>
>> int rtnl_is_locked(void)
>> {
>> - return mutex_is_locked(&rtnl_mutex);
>> + return __mutex_owner(&rtnl_mutex) == current;
>> }
>> EXPORT_SYMBOL(rtnl_is_locked);
>
> No. You are breaking the concept of is_locked. It means the mutex
> is locked anywhere, not just in the calling process. For example,
> rtnl_is_locked is used to break AB BA lock deadlock in sysfs.
Then, it could be moved to synchronize_net():
diff --git a/net/core/dev.c b/net/core/dev.c
index 94435cd09072..2dfcbde8f2fa 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -8372,7 +8372,7 @@ EXPORT_SYMBOL(free_netdev);
void synchronize_net(void)
{
might_sleep();
- if (rtnl_is_locked())
+ if (__mutex_owner(&rtnl_mutex) == current)
synchronize_rcu_expedited();
else
synchronize_rcu();
The same effect.
Kirill
