Michael S. Tsirkin wrote: > On Tue, Jun 16, 2009 at 10:11:08AM -0400, Gregory Haskins wrote: > >> Michael S. Tsirkin wrote: >> >>> On Mon, Jun 15, 2009 at 10:29:56PM -0400, Gregory Haskins wrote: >>> >>> >>>> irqfd and its underlying implementation, eventfd, currently utilize >>>> the embedded wait-queue in eventfd for signal notification. The nice thing >>>> about this design decision is that it re-uses the existing >>>> eventfd/wait-queue code and it generally works well....with several >>>> limitations. >>>> >>>> One of the limitations is that notification callbacks are always called >>>> inside a spin_lock_irqsave critical section. Another limitation is >>>> that it is very difficult to build a system that can recieve release >>>> notification without being racy. >>>> >>>> Therefore, we introduce a new registration interface that is SRCU based >>>> instead of wait-queue based, and implement the internal wait-queue >>>> infrastructure in terms of this new interface. We then convert irqfd >>>> to use this new interface instead of the existing wait-queue code. >>>> >>>> The end result is that we now have the opportunity to run the interrupt >>>> injection code serially to the callback (when the signal is raised from >>>> process-context, at least) instead of always deferring the injection to a >>>> work-queue. >>>> >>>> Signed-off-by: Gregory Haskins <[email protected]> >>>> CC: Paul E. McKenney <[email protected]> >>>> CC: Davide Libenzi <[email protected]> >>>> --- >>>> >>>> fs/eventfd.c | 115 >>>> +++++++++++++++++++++++++++++++++++++++++++---- >>>> include/linux/eventfd.h | 30 ++++++++++++ >>>> virt/kvm/eventfd.c | 114 >>>> +++++++++++++++++++++-------------------------- >>>> 3 files changed, 188 insertions(+), 71 deletions(-) >>>> >>>> diff --git a/fs/eventfd.c b/fs/eventfd.c >>>> index 72f5f8d..505d5de 100644 >>>> --- a/fs/eventfd.c >>>> +++ b/fs/eventfd.c >>>> @@ -30,8 +30,47 @@ struct eventfd_ctx { >>>> */ >>>> __u64 count; >>>> unsigned int flags; >>>> + struct srcu_struct srcu; >>>> + struct list_head nh; >>>> + struct eventfd_notifier notifier; >>>> }; >>>> >>>> +static void _eventfd_wqh_notify(struct eventfd_notifier *en) >>>> +{ >>>> + struct eventfd_ctx *ctx = container_of(en, >>>> + struct eventfd_ctx, >>>> + notifier); >>>> + >>>> + if (waitqueue_active(&ctx->wqh)) >>>> + wake_up_poll(&ctx->wqh, POLLIN); >>>> +} >>>> + >>>> +static void _eventfd_notify(struct eventfd_ctx *ctx) >>>> +{ >>>> + struct eventfd_notifier *en; >>>> + int idx; >>>> + >>>> + idx = srcu_read_lock(&ctx->srcu); >>>> + >>>> + /* >>>> + * The goal here is to allow the notification to be preemptible >>>> + * as often as possible. We cannot achieve this with the basic >>>> + * wqh mechanism because it requires the wqh->lock. Therefore >>>> + * we have an internal srcu list mechanism of which the wqh is >>>> + * a client. >>>> + * >>>> + * Not all paths will invoke this function in process context. >>>> + * Callers should check for suitable state before assuming they >>>> + * can sleep (such as with preemptible()). Paul McKenney assures >>>> + * me that srcu_read_lock is compatible with in-atomic, as long as >>>> + * the code within the critical section is also compatible. >>>> + */ >>>> + list_for_each_entry_rcu(en, &ctx->nh, list) >>>> + en->ops->signal(en); >>>> + >>>> + srcu_read_unlock(&ctx->srcu, idx); >>>> +} >>>> + >>>> /* >>>> * Adds "n" to the eventfd counter "count". Returns "n" in case of >>>> * success, or a value lower then "n" in case of coutner overflow. >>>> >>>> >>> This is ugly, isn't it? With CONFIG_PREEMPT=no preemptible() is always >>> false. >>> >>> Further, to do useful things it might not be enough that you can sleep: >>> with iofd you also want to access current task with e.g. copy from user. >>> >>> Here's an idea: let's pass a flag to ->signal, along the lines of >>> signal_is_task, that tells us that it is safe to use current, and add >>> eventfd_signal_task() which is the same as eventfd_signal but lets everyone >>> know that it's safe to both sleep and use current->mm. >>> >>> Makes sense? >>> >>> >> It does make sense, yes. What I am not clear on is how would eventfd >> detect this state such as to populate such flags, and why cant the >> ->signal() CB do the same? >> >> Thanks Michael, >> -Greg >> >> > > eventfd can't detect this state. But the callers know in what context they > are. > So the *caller* of eventfd_signal_task makes sure of this: if you are in a > task, > you can call eventfd_signal_task() if not, you must call eventfd_signal. > > > Hmm, this is an interesting idea, but I think it would be problematic in real-world applications for the long-term. For instance, the -rt tree and irq-threads .config option in the process of merging into mainline changes context types for established code. Therefore, what might be "hardirq/softirq" logic today may execute in a kthread tomorrow. I think its dangerous to try to solve the problem with caller provided info: the caller may be ignorant of its true state. IMO, the ideal solution needs to be something we can detect at run-time.
Thanks Michael, -Greg
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