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 3BBB68F40 for ; Sun, 16 Jul 2023 20:32:06 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B08EAC433C8; Sun, 16 Jul 2023 20:32:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1689539526; bh=4rTdc/T2DNXWxcPuwMno5T5ZJrQ9yC3prMiN+PNcexQ=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=C61IgJ2RPi/alOgMUvq/Pwp81CMRTALG9zzyxE98/R3/25AHbmzGQyNKgNg3caS3r L8sNDZeUegaUSnEyyj8E9HRTLY5i+GVOHoleEJSlXDeSb9230m3hBJ6KYwmoemGQCK sf1XpBqF40JesCjBK0jgJt7A5NnqNiindHIvkXeA= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Thomas Gleixner , Frederic Weisbecker , Sasha Levin Subject: [PATCH 6.1 038/591] posix-timers: Prevent RT livelock in itimer_delete() Date: Sun, 16 Jul 2023 21:42:57 +0200 Message-ID: <20230716194924.866071599@linuxfoundation.org> X-Mailer: git-send-email 2.41.0 In-Reply-To: <20230716194923.861634455@linuxfoundation.org> References: <20230716194923.861634455@linuxfoundation.org> User-Agent: quilt/0.67 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Thomas Gleixner [ Upstream commit 9d9e522010eb5685d8b53e8a24320653d9d4cbbf ] itimer_delete() has a retry loop when the timer is concurrently expired. On non-RT kernels this just spin-waits until the timer callback has completed, except for posix CPU timers which have HAVE_POSIX_CPU_TIMERS_TASK_WORK enabled. In that case and on RT kernels the existing task could live lock when preempting the task which does the timer delivery. Replace spin_unlock() with an invocation of timer_wait_running() to handle it the same way as the other retry loops in the posix timer code. Fixes: ec8f954a40da ("posix-timers: Use a callback for cancel synchronization on PREEMPT_RT") Signed-off-by: Thomas Gleixner Reviewed-by: Frederic Weisbecker Link: https://lore.kernel.org/r/87v8g7c50d.ffs@tglx Signed-off-by: Sasha Levin --- kernel/time/posix-timers.c | 43 +++++++++++++++++++++++++++++++------- 1 file changed, 35 insertions(+), 8 deletions(-) diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c index 808a247205a9a..ed3c4a9543982 100644 --- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -1037,27 +1037,52 @@ SYSCALL_DEFINE1(timer_delete, timer_t, timer_id) } /* - * return timer owned by the process, used by exit_itimers + * Delete a timer if it is armed, remove it from the hash and schedule it + * for RCU freeing. */ static void itimer_delete(struct k_itimer *timer) { -retry_delete: - spin_lock_irq(&timer->it_lock); + unsigned long flags; + + /* + * irqsave is required to make timer_wait_running() work. + */ + spin_lock_irqsave(&timer->it_lock, flags); +retry_delete: + /* + * Even if the timer is not longer accessible from other tasks + * it still might be armed and queued in the underlying timer + * mechanism. Worse, that timer mechanism might run the expiry + * function concurrently. + */ if (timer_delete_hook(timer) == TIMER_RETRY) { - spin_unlock_irq(&timer->it_lock); + /* + * Timer is expired concurrently, prevent livelocks + * and pointless spinning on RT. + * + * timer_wait_running() drops timer::it_lock, which opens + * the possibility for another task to delete the timer. + * + * That's not possible here because this is invoked from + * do_exit() only for the last thread of the thread group. + * So no other task can access and delete that timer. + */ + if (WARN_ON_ONCE(timer_wait_running(timer, &flags) != timer)) + return; + goto retry_delete; } list_del(&timer->list); - spin_unlock_irq(&timer->it_lock); + spin_unlock_irqrestore(&timer->it_lock, flags); release_posix_timer(timer, IT_ID_SET); } /* - * This is called by do_exit or de_thread, only when nobody else can - * modify the signal->posix_timers list. Yet we need sighand->siglock - * to prevent the race with /proc/pid/timers. + * Invoked from do_exit() when the last thread of a thread group exits. + * At that point no other task can access the timers of the dying + * task anymore. */ void exit_itimers(struct task_struct *tsk) { @@ -1067,10 +1092,12 @@ void exit_itimers(struct task_struct *tsk) if (list_empty(&tsk->signal->posix_timers)) return; + /* Protect against concurrent read via /proc/$PID/timers */ spin_lock_irq(&tsk->sighand->siglock); list_replace_init(&tsk->signal->posix_timers, &timers); spin_unlock_irq(&tsk->sighand->siglock); + /* The timers are not longer accessible via tsk::signal */ while (!list_empty(&timers)) { tmr = list_first_entry(&timers, struct k_itimer, list); itimer_delete(tmr); -- 2.39.2