From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail114-241.sinamail.sina.com.cn (mail114-241.sinamail.sina.com.cn [218.30.114.241]) (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 36C18192B7F for ; Fri, 7 Mar 2025 06:08:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=218.30.114.241 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741327733; cv=none; b=gh9S7Jr9uK2Qg/9A3SUDB5TVCOsR29H8ZIEy99/xQtKb+S5BPk98WRTYVYz6lTI3tw+8cNE9DbKlkatJhtsXzc5epMH117yw5Ak2BPWcDeJPQypphvb9svhPzMi30K8T7iA8o6p69DhtHu8oV7zTWROoj+Ce35xMZ30d59bJHZQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741327733; c=relaxed/simple; bh=T2QkG4EZej3NuePHZ+cAjBoD70+eX/KlMVHwBiryygw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=hJ8J5W5SZzRFJN1MYRpL90hXyWYk4dfRBVh+sAxBjM0oOHJXvUQsr9ZcBTBILBQpa878EF4l+Nj/gpDmiWHjC/8ak9Fno3kYtRO5sCKmAfqSVuLbNKjqjjoMF86QIdCmSZzNKFoPs6jKZV0V/wV0XQyKtsBsvxUxI0t/4JS+b7s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=sina.com; spf=pass smtp.mailfrom=sina.com; arc=none smtp.client-ip=218.30.114.241 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=sina.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=sina.com X-SMAIL-HELO: localhost.localdomain Received: from unknown (HELO localhost.localdomain)([113.88.50.28]) by sina.com (10.185.250.23) with ESMTP id 67CA8D64000021D4; Fri, 7 Mar 2025 14:08:38 +0800 (CST) X-Sender: hdanton@sina.com X-Auth-ID: hdanton@sina.com Authentication-Results: sina.com; spf=none smtp.mailfrom=hdanton@sina.com; dkim=none header.i=none; dmarc=none action=none header.from=hdanton@sina.com X-SMAIL-MID: 4079398913364 X-SMAIL-UIID: D44F113D61CE430ABCB960723B7B6BCE-20250307-140838-1 From: Hillf Danton To: Oleg Nesterov Cc: K Prateek Nayak , Mateusz Guzik , "Sapkal, Swapnil" , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full Date: Fri, 7 Mar 2025 14:08:26 +0800 Message-ID: <20250307060827.3083-1-hdanton@sina.com> In-Reply-To: <20250306093021.GA19868@redhat.com> References: <20250227211229.GD25639@redhat.com> <06ae9c0e-ba5c-4f25-a9b9-a34f3290f3fe@amd.com> <20250228143049.GA17761@redhat.com> <20250228163347.GB17761@redhat.com> <20250304050644.2983-1-hdanton@sina.com> <20250304102934.2999-1-hdanton@sina.com> <20250304233501.3019-1-hdanton@sina.com> <20250305045617.3038-1-hdanton@sina.com> <20250305224648.3058-1-hdanton@sina.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Thu, 6 Mar 2025 10:30:21 +0100 Oleg Nesterov > On 03/06, Hillf Danton wrote: > > On Wed, 5 Mar 2025 12:44:34 +0100 Oleg Nesterov > > > On 03/05, Hillf Danton wrote: > > > > See the loop in ___wait_event(), > > > > > > > > for (;;) { > > > > prepare_to_wait_event(); > > > > > > > > // flip > > > > if (condition) > > > > break; > > > > > > > > schedule(); > > > > } > > > > > > > > After wakeup, waiter will sleep again if condition flips false on the waker > > > > side before waiter checks condition, even if condition is atomic, no? > > > > > > Yes, but in this case pipe_full() == true is correct, this writer can > > > safely sleep. > > > > > No, because no reader is woken up before sleep to make pipe not full. > > Why the reader should be woken before this writer sleeps? Why the reader > should be woken at all in this case (when pipe is full again) ? > "to make pipe not full" failed to prevent you asking questions like this one. > We certainly can't understand each other. > > Could your picture the exact scenario/sequence which can hang? > If you think the scenario in commit 3d252160b818 [1] is correct, check the following one. step-00 pipe->head = 36 pipe->tail = 36 after 3d252160b818 step-01 task-118762 writer pipe->head++; wakes up task-118740 and task-118768 step-02 task-118768 writer makes pipe full; sleeps without waking up any reader as pipe was not empty after step-01 step-03 task-118766 new reader makes pipe empty sleeps step-04 task-118740 reader sleeps as pipe is empty [ Tasks 118740 and 118768 can then indefinitely wait on each other. ] [1] https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/pipe.c?id=3d252160b818045f3a152b13756f6f37ca34639d