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[209.85.128.48]) by smtp.gmail.com with ESMTPSA id fg16-20020a1709069c5000b006fe8d8c54a7sm986578ejc.87.2022.06.01.11.53.11 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 01 Jun 2022 11:53:12 -0700 (PDT) Received: by mail-wm1-f48.google.com with SMTP id f7-20020a1c3807000000b0039c1a10507fso1592322wma.1 for ; Wed, 01 Jun 2022 11:53:11 -0700 (PDT) X-Received: by 2002:a7b:cb91:0:b0:397:3225:244 with SMTP id m17-20020a7bcb91000000b0039732250244mr29945960wmi.68.1654109591504; Wed, 01 Jun 2022 11:53:11 -0700 (PDT) MIME-Version: 1.0 References: <20220326122838.19d7193f@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> <9a932cc6-2cb7-7447-769f-3898b576a479@kernel.dk> <20220326130615.2d3c6c85@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> <234e3155-e8b1-5c08-cfa3-730cc72c642c@kernel.dk> <20220326143049.671b463c@kernel.org> <78d9a5e2eaad11058f54b1392662099549aa925f.camel@trillion01.com> <32c3c699-3e3a-d85e-d717-05d1557c17b9@kernel.dk> In-Reply-To: From: Linus Torvalds Date: Wed, 1 Jun 2022 11:52:55 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [GIT PULL] io_uring updates for 5.18-rc1 To: Jens Axboe Cc: Olivier Langlois , Jakub Kicinski , io-uring Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On Wed, Jun 1, 2022 at 11:34 AM Jens Axboe wrote: > > But as a first step, let's just mark it deprecated with a pr_warn() for > 5.20 and then plan to kill it off whenever a suitable amount of relases > have passed since that addition. I'd love to, but it's not actually realistic as things stand now. epoll() is used in a *lot* of random libraries. A "pr_warn()" would just be senseless noise, I bet. No, there's a reason that EPOLL is still there, still 'default y', even though I dislike it and think it was a mistake, and we've had several nasty bugs related to it over the years. It really can be a very useful system call, it's just that it really doesn't work the way the actual ->poll() interface was designed, and it kind of hijacks it in ways that mostly work, but the have subtle lifetime issues that you don't see with a regular select/poll because those will always tear down the wait queues. Realistically, the proper fix to epoll is likely to make it explicit, and make files and drivers that want to support it have to actually opt in. Because a lot of the problems have been due to epoll() looking *exactly* like a regular poll/select to a driver or a filesystem, but having those very subtle extended requirements. (And no, the extended requirements aren't generally onerous, and regular ->poll() works fine for 99% of all cases. It's just that occasionally, special users are then fooled about special contexts). In other words, it's a bit like our bad old days when "splice()" ended up falling back to regular ->read()/->write() implementations with set_fs(KERNEL_DS). Yes, that worked fine for 99% of all cases, and we did it for years, but it also caused several really nasty issues for when the read/write actor did something slightly unusual. So I may dislike epoll quite intensely, but I don't think we can *really* get rid of it. But we might be able to make it a bit more controlled. But so far every time it has caused issues, we've worked around it by fixing it up in the particular driver or whatever that ended up being triggered by epoll semantics. Linus