From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 4868340B0E9 for ; Fri, 28 Aug 2026 10:41:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787913697; cv=none; b=mrJKsyhq7IIqdrIuyqAeO6xGTxKSOeQIficbG6H6b32hRM+/hx0iboMYQqOyXo9DnXUMWmbb5mI3DNzAKrAqp+uV8BNlrF8TxiWH/nQQsINS1RKF1SYl69Me/7PtrekKT/GHwaMcZ+PtDTeRmaXnS/r0j1ksYOWILJkHoGV/Tys= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787913697; c=relaxed/simple; bh=dFrvw7iTZKmBzwV15Z42JzPl17IL7wCOJV+mM/6ClzE=; h=Message-ID:Date:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=msky4EsGohylGr6KqlGULtzLTLobCLTD8KUSCXS+u7UjdZqlyieRNjss/K8uUapXe0wIDa0oEsBN3aS7QN/l3x5PPKoQPOWyNidxqzHJ5H4ClIcdH1UC3c1sj3tdGZa+beOOnUgw5uO83wveaLO0FwaU52TU5gcZAG+Wo1wQGfE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=pass smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=vlVJ5sm9; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="vlVJ5sm9" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=Content-Type:MIME-Version:References: Subject:Cc:To:From:Date:Message-ID:Sender:Reply-To:Content-Transfer-Encoding: Content-ID:Content-Description:In-Reply-To; bh=gTNkLy3rzRLHf/Y7wVAVEU5GaJWvM4O0ubkGWvEOWH0=; b=vlVJ5sm9iNp72Nr+n07hW3uG3l GLQVkPhOkoCFkd/zjrNB9Kygwc0bxp83QEOlggthTQEp0R1G3lT6DQ2/ZHYxE50dnE/5L+kR9FHLQ VY9vz5lbNvpjoyHW5xTG76Yte0UAtGYmbxriCzZs2d8I6KNo7qK41loRYVng5d/l8DsbYTaYQa8h3 oUx7su7K71eKqJrypbo53c4A0aneIGuSG5WkGyEFwKB2My5wgJbKQ9Fft59xymd6qFO+XCsHIn9M8 cvZFtzv+a+ykODKR78VCg7eEzLDQr+srhMb0i8shBAwlEC76vpLs8tCTNw3blE8cs6lT7pAaHb2YX bCN6JCNg==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.99.1 #2 (Red Hat Linux)) id 1wzu1S-0000000EWAf-43jM; Fri, 28 Aug 2026 10:41:23 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 0) id D31FC30116A; Fri, 28 Aug 2026 12:41:21 +0200 (CEST) Message-ID: <20260828104018.378378994@infradead.org> User-Agent: quilt/0.68 Date: Fri, 28 Aug 2026 12:17:00 +0200 From: Peter Zijlstra To: mingo@kernel.org Cc: peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, linux-kernel@vger.kernel.org, tj@kernel.org Subject: [PATCH 1/7] sched/core: Fix pick_next_task() self recursion References: <20260828101659.812011872@infradead.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 It is possible for another sibling to end up in pick_next_task() when: pick_next_task() pick_task() sched_class::pick_task() drops the core wide rq->lock. In this case they end up trampling the core wide task selection state, possibly leading to NULL derefs. Detect this case by keeping a local copy of core_task_seq, a value that is incremented on {en,de}queue and schedule. Since RETRY_TASK is only possible when a higher priority task gets enqueued during the lock break, this must mean core_task_seq will also be incremented and is thus completely covered by the seq number mismatch. XXX words on forward progress go here Signed-off-by: Peter Zijlstra (Intel) --- kernel/sched/core.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-) --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -6232,14 +6232,14 @@ static struct task_struct * pick_next_task(struct rq *rq, struct rq_flags *rf) __must_hold(__rq_lockp(rq)) { + bool core_clock_updated = (rq == rq->core); struct task_struct *next, *p, *max; const struct cpumask *smt_mask; + int i, cpu, seq, occ = 0; bool fi_before = false; - bool core_clock_updated = (rq == rq->core); + bool need_sync = false; unsigned long cookie; - int i, cpu, occ = 0; struct rq *rq_i; - bool need_sync = false; if (!sched_core_enabled(rq)) return __pick_next_task(rq, rf); @@ -6314,7 +6314,7 @@ pick_next_task(struct rq *rq, struct rq_ * However, preemptions can cause multiple picks on the same task set. * 'Fix' this by also increasing @task_seq for every pick. */ - rq->core->core_task_seq++; + seq = ++rq->core->core_task_seq; /* * Optimize for common case where this CPU has no cookies @@ -6362,7 +6362,8 @@ pick_next_task(struct rq *rq, struct rq_ update_rq_clock(rq_i); p = pick_task(rq_i, rf); - if (unlikely(p == RETRY_TASK)) { + if (unlikely(seq != rq->core->core_task_seq || + WARN_ON_ONCE(p == RETRY_TASK))) { /* rq lock may have been dropped, clocks invalidated */ core_clock_updated = false; if (!(rq->clock_update_flags & RQCF_UPDATED)) @@ -6392,7 +6393,7 @@ pick_next_task(struct rq *rq, struct rq_ if (cookie) p = sched_core_find(rq_i, cookie); if (!p) - p = idle_sched_class.pick_task(rq_i, rf); + p = idle_sched_class.pick_task(rq_i, NULL); } rq_i->core_pick = p;