public inbox for [email protected]
 help / color / mirror / Atom feed
From: Pavel Begunkov <[email protected]>
To: Jens Axboe <[email protected]>,
	Usama Arif <[email protected]>,
	[email protected], [email protected]
Cc: [email protected]
Subject: Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd
Date: Thu, 3 Feb 2022 19:00:39 +0000	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

On 2/3/22 18:29, Jens Axboe wrote:
> On 2/3/22 11:26 AM, Usama Arif wrote:
>> 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.
> 
> They are cheap, but they are still noticeable at high requests/sec
> rates. So would be best to avoid them.
> 
> And yes it's obviously racy, there's the potential to miss an eventfd
> notification if it races with registering an eventfd descriptor. But
> that's not really a concern, as if you register with inflight IO
> pending, then that always exists just depending on timing. The only
> thing I care about here is that it's always _safe_. Hence something ala
> what you did below is totally fine, as we're re-evaluating under rcu
> protection.

Indeed, the patch doesn't have any formal guarantees for propagation
to already inflight requests, so this extra unsynchronised check
doesn't change anything.

I'm still more сurious why we need RCU and extra complexity when
apparently there is no use case for that. If it's only about
initial initialisation, then as I described there is a much
simpler approach.

-- 
Pavel Begunkov

  reply	other threads:[~2022-02-03 19:05 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-03 17:41 [PATCH v3 0/3] io_uring: avoid ring quiesce in io_uring_register for eventfd opcodes Usama Arif
2022-02-03 17:41 ` [PATCH v3 1/3] io_uring: remove trace for eventfd Usama Arif
2022-02-03 17:41 ` [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd Usama Arif
2022-02-03 17:56   ` Jens Axboe
2022-02-03 18:26     ` [External] " Usama Arif
2022-02-03 18:29       ` Jens Axboe
2022-02-03 19:00         ` Pavel Begunkov [this message]
2022-02-03 19:06           ` Jens Axboe
2022-02-03 19:43             ` Pavel Begunkov
2022-02-03 22:18               ` Jens Axboe
2022-02-03 19:54             ` Usama Arif
2022-02-03 21:47               ` Pavel Begunkov
2022-02-03 22:16                 ` Jens Axboe
2022-02-03 23:21                   ` Pavel Begunkov
2022-02-03 22:02               ` Pavel Begunkov
2022-02-03 17:41 ` [PATCH v3 3/3] io_uring: avoid ring quiesce for IORING_REGISTER_EVENTFD_ASYNC Usama Arif

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    [email protected] \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox