From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6CCD540756E for ; Mon, 3 Aug 2026 12:46:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785761215; cv=none; b=DABcySnVkhaEvO7P/lmeQ2eN0SYP4dDK9mcrP4gSAqPuj6T4mHqz9XeVL4CmeNMclX6A5lmVbrMyfxswwpRnDVOofdX8fZL8I5N62mmXypBr+RwTln075UhN63OtG7WfKYwvNtyxZzTCaUVKu0iuj+ukLUkzXp2uISNf0qTvxhM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785761215; c=relaxed/simple; bh=5TCgU5zbyBLDg+DnZg8eCxnpYXT4Vv2vkwIQjgy7L6g=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition; b=SFpHh4sg8R2LEA1sgSLICwuTtM805Bo5AMCDwqUl5zV8ej18c76PB6EgfgVocN/eJyRmpgD4CcRbZicmyjMW+PmaeIaS2FZOM8cMc4v1AlZU9EBRGuDNocT7847hxuiM4fS9VVS9OOtsdX4qa2Hi5bVL2s7WJ6Jk91z6XOFqaWA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=EO0Ii5p0; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="EO0Ii5p0" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1785761213; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type; bh=lQ5xQ35lK31BPQRUsnqePjDzH7l92VhsEt/1x8cX/6g=; b=EO0Ii5p0GlO/D9wl4FTsN4Ypp44KwiyYeRSnKmC2IQJ+t52RjyNBnzI1cPRktuni6qPk1F zGCmuwKroD6WVd+enLSK4CvZu/0egwhH4yuJJWBJ3CH8kOonVvZxA5AVt4fSCpZl1dw+Cx YuQTPY41LQU6zSi1tFDl4Qquhky9hCY= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-589-NdRnfQLPNs6h-q9o-NN78g-1; Mon, 03 Aug 2026 08:46:34 -0400 X-MC-Unique: NdRnfQLPNs6h-q9o-NN78g-1 X-Mimecast-MFC-AGG-ID: NdRnfQLPNs6h-q9o-NN78g_1785761192 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 78C74195608D; Mon, 3 Aug 2026 12:46:31 +0000 (UTC) Received: from fedora (unknown [10.44.32.54]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with SMTP id BEDC41956087; Mon, 3 Aug 2026 12:46:26 +0000 (UTC) Received: by fedora (nbSMTP-1.00) for uid 1000 oleg@redhat.com; Mon, 3 Aug 2026 14:46:31 +0200 (CEST) Date: Mon, 3 Aug 2026 14:46:25 +0200 From: Oleg Nesterov To: Breno Leitao , Christian Brauner , Jens Axboe , Linus Torvalds , Mateusz Guzik , Pavel Begunkov Cc: Alexander Viro , Jan Kara , Alexey Gladkov , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org Subject: [PATCH v4] pipe: only enable the extra wake_up(rd_wait) for EPOLLET consumers Message-ID: Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 pipe_poll() unconditionally sets ->poll_usage on the first call, forcing anon_pipe_write() to wake up readers on every write even if the pipe was not empty. The reason is that some legacy epoll(EPOLLET) users depend on historical per-write wakeups, see commit 3a34b13a88ca ("pipe: make pipe writes always wake up readers"). Test-case: #include #include #include int main(void) { int pfd[2], efd; struct epoll_event evt = { .events = EPOLLIN | EPOLLET }; pipe(pfd); efd = epoll_create1(0); epoll_ctl(efd, EPOLL_CTL_ADD, pfd[0], &evt); for (int i = 0; i < 2; ++i) { write(pfd[1], "", 1); assert(epoll_wait(efd, &evt, 1, 0) == 1); } return 0; } it fails if WRITE_ONCE(poll_usage, true) is removed from pipe_poll(). However, without EPOLLET in .events, it does not need the extra wakeup and succeeds even if write() is called only once before the main loop. Currently io_uring without (unsupported) IORING_POLL_ADD_LEVEL always sets EPOLLET, and in IORING_POLL_ADD_MULTI mode it depends on per-write wakeups the same way: #include #include #include #include #include #include int main(void) { struct io_uring_params p = {}; int fd, pfd[2]; pipe(pfd); fd = syscall(SYS_io_uring_setup, 2, &p); assert(fd >= 0); void *ring = mmap(0, p.cq_off.cqes + p.cq_entries * sizeof(struct io_uring_cqe), PROT_READ | PROT_WRITE, MAP_SHARED, fd, IORING_OFF_SQ_RING); assert(ring != MAP_FAILED); *(unsigned *)(ring + p.sq_off.tail) = 1; struct io_uring_sqe *sqes = mmap(0, p.sq_entries * sizeof(*sqes), PROT_READ | PROT_WRITE, MAP_SHARED, fd, IORING_OFF_SQES); assert(sqes != MAP_FAILED); sqes[0].opcode = IORING_OP_POLL_ADD; sqes[0].fd = pfd[0]; sqes[0].len = IORING_POLL_ADD_MULTI; sqes[0].poll32_events = EPOLLIN; syscall(SYS_io_uring_enter, fd, 1, 0, 0, 0, 0); unsigned *cq_head = ring + p.cq_off.head; unsigned *cq_tail = ring + p.cq_off.tail; for (int i = 0; i < 2; ++i) { write(pfd[1], "", 1); syscall(SYS_io_uring_enter, fd, 0, 0, IORING_ENTER_GETEVENTS, 0, 0); assert(*cq_tail == ++*cq_head); } return 0; } the 2nd assert() in the main loop fails without ->poll_usage == true. Rename ->poll_usage to ->pseudo_edgetrigger to make the purpose clearer, update the comments, and change pipe_poll() to set ->pseudo_edgetrigger only if wait->_key & EPOLLET is true. This check should catch both users, and this way poll/select and epoll without EPOLLET users will not pay for the extra wakeup. Signed-off-by: Oleg Nesterov --- fs/pipe.c | 20 +++++++++++++------- include/linux/pipe_fs_i.h | 4 ++-- 2 files changed, 15 insertions(+), 9 deletions(-) diff --git a/fs/pipe.c b/fs/pipe.c index 429b0714ec57..84a81db39f8a 100644 --- a/fs/pipe.c +++ b/fs/pipe.c @@ -686,10 +686,9 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from) * how (for example) the GNU make jobserver uses small writes to * wake up pending jobs * - * Epoll nonsensically wants a wakeup whether the pipe - * was already empty or not. + * ->pseudo_edgetrigger enables per-write wakeups, see pipe_poll() */ - if (was_empty || pipe->poll_usage) + if (was_empty || READ_ONCE(pipe->pseudo_edgetrigger)) wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM); kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); if (wake_next_writer) @@ -752,7 +751,6 @@ static long pipe_ioctl(struct file *filp, unsigned int cmd, unsigned long arg) } } -/* No kernel lock held - fine */ static __poll_t pipe_poll(struct file *filp, poll_table *wait) { @@ -760,9 +758,17 @@ pipe_poll(struct file *filp, poll_table *wait) struct pipe_inode_info *pipe = filp->private_data; union pipe_index idx; - /* Epoll has some historical nasty semantics, this enables them */ - if (unlikely(!READ_ONCE(pipe->poll_usage))) - WRITE_ONCE(pipe->poll_usage, true); + /* + * Legacy epoll(EPOLLET) users depend on historical per-write wakeups, + * see 3a34b13a88ca ("pipe: make pipe writes always wake up readers") + * and the ->pseudo_edgetrigger check in anon_pipe_write(). + * Currently io_uring sets EPOLLET for multishot polls, so it gets the + * same behaviour. + */ + if ((filp->f_mode & FMODE_READ) && + wait && (wait->_key & EPOLLET) && + unlikely(!READ_ONCE(pipe->pseudo_edgetrigger))) + WRITE_ONCE(pipe->pseudo_edgetrigger, true); /* * Reading pipe state only -- no need for acquiring the semaphore. diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h index 7f6a92ac9704..1acc76581af5 100644 --- a/include/linux/pipe_fs_i.h +++ b/include/linux/pipe_fs_i.h @@ -74,7 +74,7 @@ union pipe_index { * @files: number of struct file referring this pipe (protected by ->i_lock) * @r_counter: reader counter * @w_counter: writer counter - * @poll_usage: is this pipe used for epoll, which has crazy wakeups? + * @pseudo_edgetrigger: has an EPOLLET consumer, enable per-write wakeups * @fasync_readers: reader side fasync * @fasync_writers: writer side fasync * @bufs: the circular array of pipe buffers @@ -95,7 +95,7 @@ struct pipe_inode_info { unsigned int files; unsigned int r_counter; unsigned int w_counter; - bool poll_usage; + bool pseudo_edgetrigger; #ifdef CONFIG_WATCH_QUEUE bool note_loss; #endif -- 2.52.0