Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd

[Usama Arif <usama.arif@bytedance.com>]

On 03/02/2022 17:56, Jens Axboe wrote:
> On 2/3/22 10:41 AM, Usama Arif wrote:
>> @@ -1726,13 +1732,24 @@ static inline struct io_uring_cqe *io_get_cqe(struct io_ring_ctx *ctx)
>>   	return &rings->cqes[tail & mask];
>>   }
>>   
>> -static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
>> +static void io_eventfd_signal(struct io_ring_ctx *ctx)
>>   {
>> -	if (likely(!ctx->cq_ev_fd))
>> -		return false;
>> +	struct io_ev_fd *ev_fd;
>> +
>> +	rcu_read_lock();
>> +	/* rcu_dereference ctx->io_ev_fd once and use it for both for checking and eventfd_signal */
>> +	ev_fd = rcu_dereference(ctx->io_ev_fd);
>> +
>> +	if (likely(!ev_fd))
>> +		goto out;
>>   	if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
>> -		return false;
>> -	return !ctx->eventfd_async || io_wq_current_is_worker();
>> +		goto out;
>> +
>> +	if (!ctx->eventfd_async || io_wq_current_is_worker())
>> +		eventfd_signal(ev_fd->cq_ev_fd, 1);
>> +
>> +out:
>> +	rcu_read_unlock();
>>   }
> 
> Like Pavel pointed out, we still need the fast path (of not having an
> event fd registered at all) to just do the cheap check and not need rcu
> lock/unlock. Outside of that, I think this looks fine.
> 

Hmm, maybe i didn't understand you and Pavel correctly. Are you 
suggesting to do the below diff over patch 3? I dont think that would be 
correct, as it is possible that just after checking if ctx->io_ev_fd is 
present unregister can be called by another thread and set ctx->io_ev_fd 
to NULL that would cause a NULL pointer exception later? In the current 
patch, the check of whether ev_fd exists happens as the first thing 
after rcu_read_lock and the rcu_read_lock are extremely cheap i believe.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 25ed86533910..0cf282fba14d 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1736,12 +1736,13 @@ static void io_eventfd_signal(struct io_ring_ctx 
*ctx)
  {
         struct io_ev_fd *ev_fd;

+       if (likely(!ctx->io_ev_fd))
+               return;
+
         rcu_read_lock();
         /* rcu_dereference ctx->io_ev_fd once and use it for both for 
checking and eventfd_signal */
         ev_fd = rcu_dereference(ctx->io_ev_fd);

-       if (likely(!ev_fd))
-               goto out;
         if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
                 goto out;




>>   static int io_eventfd_unregister(struct io_ring_ctx *ctx)
>>   {
>> -	if (ctx->cq_ev_fd) {
>> -		eventfd_ctx_put(ctx->cq_ev_fd);
>> -		ctx->cq_ev_fd = NULL;
>> -		return 0;
>> +	struct io_ev_fd *ev_fd;
>> +	int ret;
>> +
>> +	mutex_lock(&ctx->ev_fd_lock);
>> +	ev_fd = rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock));
>> +	if (!ev_fd) {
>> +		ret = -ENXIO;
>> +		goto out;
>>   	}
>> +	synchronize_rcu();
>> +	eventfd_ctx_put(ev_fd->cq_ev_fd);
>> +	kfree(ev_fd);
>> +	rcu_assign_pointer(ctx->io_ev_fd, NULL);
>> +	ret = 0;
>>   
>> -	return -ENXIO;
>> +out:
>> +	mutex_unlock(&ctx->ev_fd_lock);
>> +	return ret;
>>   }
> 
> synchronize_rcu() can take a long time, and I think this is in the wrong
> spot. It should be on the register side, IFF we need to expedite the
> completion of a previous event fd unregistration. If we do it that way,
> at least it'll only happen if it's necessary. What do you think?
> 


How about the approach in v4? so switching back to call_rcu as in v2 and 
if ctx->io_ev_fd is NULL then we call rcu_barrier to make sure all rcu 
callbacks are finished and check for NULL again.

Thanks!
Usama