From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 13D3A364959; Tue, 12 May 2026 18:13:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778609618; cv=none; b=a377ecyhnRJsgSnBI7zae5PF6S1gjI3wVDHmKQk1njzQxuyAkJfBbI2hC7ujR6o7DzPwofUUl3TysN1yb1Z61SEXy1l/qrK4NfZa57yoTpXk2JfGCFadsAstubkvLSb/ehZWKi4xnJm/6gbuzG/2NYHkjBEgkiarRJTXlUidouU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778609618; c=relaxed/simple; bh=TEh8kxenU2OWLIglqZHAErCa133EZhFh48f4n+j0IwI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=I9H4YJjL6NV6h3CeLLwHnc75URf03g3WsMToOsFcJ3Mj37yqOSN3wyE3Jx2mxpilR59Mk8T5AKRDagGE3w82L8cLEEO04Wi7sf6M35mBqOaSEL8M0CdXT5/Hws1dRAEjToKROAftO4l0NPGwnw5AKoAb2MpiV9+deTd8Z+Lq3yE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=wyE9CB8E; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="wyE9CB8E" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 96E60C2BCB0; Tue, 12 May 2026 18:13:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1778609617; bh=TEh8kxenU2OWLIglqZHAErCa133EZhFh48f4n+j0IwI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=wyE9CB8E+haKSCf5xhVWlKVk8UGcEj5wbCAs/x02Pn+yxvVMAZ/3RmfLPfPCZZVrp W5RaoBJMF/h0ZoAmMZfl/uzxGwNuO33yMeIfBhqxqDHO2WolskSCwSGHHZScq33trO WDtA9XI7R12X60oBXEZ5NQZu1gambiIK7B4J1MnI= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Chris Mason , Tejun Heo , Andrea Righi , Ryan Newton Subject: [PATCH 7.0 248/307] sched_ext: Use dsq->first_task instead of list_empty() in dispatch_enqueue() FIFO-tail Date: Tue, 12 May 2026 19:40:43 +0200 Message-ID: <20260512173945.360002090@linuxfoundation.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260512173940.117428952@linuxfoundation.org> References: <20260512173940.117428952@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 7.0-stable review patch. If anyone has any objections, please let me know. ------------------ From: Tejun Heo commit 2f2ea77092660b53bfcbc4acc590b57ce9ab5dce upstream. dispatch_enqueue()'s FIFO-tail path used list_empty(&dsq->list) to decide whether to set dsq->first_task on enqueue. dsq->list can contain parked BPF iterator cursors (SCX_DSQ_LNODE_ITER_CURSOR), so list_empty() is not a reliable "no real task" check. If the last real task is unlinked while a cursor is parked, first_task becomes NULL; the next FIFO-tail enqueue then sees list_empty() == false and skips the first_task update, leaving scx_bpf_dsq_peek() returning NULL for a non-empty DSQ. Test dsq->first_task directly, which already tracks only real tasks and is maintained under dsq->lock. Fixes: 44f5c8ec5b9a ("sched_ext: Add lockless peek operation for DSQs") Cc: stable@vger.kernel.org # v6.19+ Reported-by: Chris Mason Signed-off-by: Tejun Heo Reviewed-by: Andrea Righi Cc: Ryan Newton Signed-off-by: Greg Kroah-Hartman --- kernel/sched/ext.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -1093,11 +1093,13 @@ static void dispatch_enqueue(struct scx_ if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN)) rcu_assign_pointer(dsq->first_task, p); } else { - bool was_empty; - - was_empty = list_empty(&dsq->list); + /* + * dsq->list can contain parked BPF iterator cursors, so + * list_empty() here isn't a reliable proxy for "no real + * task in the DSQ". Test dsq->first_task directly. + */ list_add_tail(&p->scx.dsq_list.node, &dsq->list); - if (was_empty && !(dsq->id & SCX_DSQ_FLAG_BUILTIN)) + if (!dsq->first_task && !(dsq->id & SCX_DSQ_FLAG_BUILTIN)) rcu_assign_pointer(dsq->first_task, p); } }