On 23.01.2018 18:12, Eric Dumazet wrote: > On Tue, Jan 23, 2018 at 6:41 AM, Kirill Tkhai <ktk...@virtuozzo.com> wrote: >> Hi, Eric, >> >> thanks for your review. >> >> On 22.01.2018 20:15, Eric Dumazet wrote: >>> On Mon, 2018-01-22 at 12:41 +0300, Kirill Tkhai 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); >>>> >>>> drivers/staging/wilc1000/linux_mon.c:310 >>>> >>>> if (rtnl_is_locked()) { >>>> rtnl_unlock(); >>>> rollback_lock = true; >>>> } >>>> >>>> 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); >>>> >>>> >>> >>> Seems good to me, but this looks a net-next candidate to me. >> >> No objections. What for this may be need for net tree?! Only to fix >> the staging drivers above. But AFAIR, staging drivers guarantees, which >> the kernel gives, are that they may be compiled. If so, we do not need >> this in net tree. >> >>> Note that this does not catch illegal uses from BH, where current is >>> not related to our context of execution. >> >> It's true, but the patch is about reducing of synchronize_rcu_expedited() >> calls. > > You have not touched only this path, but all paths using ASSERT_RTNL() > > This is why I think your patch would target net-next, not net tree. > >> There was no an objective to limit area of the places, where >> rtnl_is_locked() can be used. For me it looks like another logical change. >> If we really need that, one more patch on top of this may be submitted. >> But honestly, I can't imagine someone really needs that check. > > I believe you missed ASSERT_RTNL(), used all over the place.
Not missed. I grepped all over the kernel source, and this is how BUGs in staging drivers were found. I just can't believe we really need this check. Ok, then how about something like this: int rtnl_is_locked(void) { #ifdef CONFIG_DEBUG_KERNEL BUG_ON(!in_task()); #endif return __mutex_owner(&rtnl_mutex) == current; } CONFIG_DEBUG_KERNEL is because of rtnl_is_locked() is used widely, and the check has only the debug purpose. Kirill