[{"id":3687689,"web_url":"http://patchwork.ozlabs.org/comment/3687689/","msgid":"<87lddvy2dl.fsf@jogness.linutronix.de>","list_archive_url":null,"date":"2026-05-07T09:50:46","subject":"Re: [PATCH tty v4 2/6] serial: Replace driver usage of UPF_CONS_FLOW","submitter":{"id":207,"url":"http://patchwork.ozlabs.org/api/people/207/","name":"John Ogness","email":"john.ogness@linutronix.de"},"content":"Hi,\n\n(This email is particularly directed at the Samsung folks.)\n\nResponding to Sashiko:\n\nhttps://sashiko.dev/#/patchset/20260506121606.5805-1-john.ogness%40linutronix.de\n\nOn 2026-05-06, Sashiko wrote:\n>> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c\n>> index e27806bf2cf3e..2f94fc798cffb 100644\n>> --- a/drivers/tty/serial/samsung_tty.c\n>> +++ b/drivers/tty/serial/samsung_tty.c\n>> @@ -319,7 +319,7 @@ static void s3c24xx_serial_stop_tx(struct uart_port *port)\n>> \tourport->tx_enabled = 0;\n>> \tourport->tx_in_progress = 0;\n>> \n>> -\tif (port->flags & UPF_CONS_FLOW)\n>> +\tif (uart_cons_flow_enabled(port))\n>> \t\ts3c24xx_serial_rx_enable(port);\n>\n> This isn't a new bug introduced by this patch, but does this code\n> result in a recursive spinlock acquisition and deadlock when console\n> flow control is enabled?\n>\n> The serial core framework calls port->ops->stop_tx() with the\n> port->lock spinlock already held. When uart_cons_flow_enabled(port)\n> evaluates to true, s3c24xx_serial_stop_tx() invokes\n> s3c24xx_serial_rx_enable().\n>\n> Looking at s3c24xx_serial_rx_enable(), it unconditionally attempts to\n> acquire the exact same lock again:\n>\n> static void s3c24xx_serial_rx_enable(struct uart_port *port)\n> {\n> \tstruct s3c24xx_uart_port *ourport = to_ourport(port);\n> \tunsigned long flags;\n> \t...\n> \tuart_port_lock_irqsave(port, &flags);\n> \t...\n> }\n>\n> Because spinlocks are not recursive in the kernel, it appears this\n> sequence would cause an immediate deadlock on the same CPU.\n\nAlthough out of scope for my series, the reported issue is real. Perhaps\nsomething for Krzysztof or Alim to look into.\n\n>> @@ -493,7 +493,7 @@ static void s3c24xx_serial_start_tx(struct uart_port *port)\n>> \tstruct tty_port *tport = &port->state->port;\n>> \n>> \tif (!ourport->tx_enabled) {\n>> -\t\tif (port->flags & UPF_CONS_FLOW)\n>> +\t\tif (uart_cons_flow_enabled(port))\n>> \t\t\ts3c24xx_serial_rx_disable(port);\n>\n> Similarly here, s3c24xx_serial_start_tx() is also called with\n> port->lock held, and s3c24xx_serial_rx_disable() will attempt to\n> acquire port->lock again:\n>\n> static void s3c24xx_serial_rx_disable(struct uart_port *port)\n> {\n> \tstruct s3c24xx_uart_port *ourport = to_ourport(port);\n> \tunsigned long flags;\n> \t...\n> \tuart_port_lock_irqsave(port, &flags);\n> \t...\n> }\n>\n> Could this pre-existing locking issue in the samsung_tty driver be\n> addressed so that the rx enable/disable helpers do not try to take the\n> port lock when it is already held by the caller?\n\nAlso legitimate. But out of scope for my series.\n\nJohn Ogness","headers":{"Return-Path":"\n ","X-Original-To":["incoming@patchwork.ozlabs.org","sparclinux@vger.kernel.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","patchwork-incoming@ozlabs.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n secure) header.d=linutronix.de header.i=@linutronix.de header.a=rsa-sha256\n header.s=2020 header.b=leWMdPdy;\n\tdkim=pass header.d=linutronix.de header.i=@linutronix.de\n header.a=ed25519-sha256 header.s=2020e header.b=Tq2nphDt;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=ozlabs.org\n (client-ip=2404:9400:2221:ea00::3; helo=mail.ozlabs.org;\n envelope-from=srs0=mo0k=de=vger.kernel.org=sparclinux+bounces-6763-patchwork-incoming=ozlabs.org@ozlabs.org;\n receiver=patchwork.ozlabs.org)","gandalf.ozlabs.org;\n arc=pass smtp.remote-ip=\"2600:3c15:e001:75::12fc:5321\"\n arc.chain=subspace.kernel.org","gandalf.ozlabs.org;\n dmarc=pass (p=none dis=none) header.from=linutronix.de","gandalf.ozlabs.org;\n\tdkim=pass (2048-bit key;\n secure) header.d=linutronix.de header.i=@linutronix.de header.a=rsa-sha256\n header.s=2020 header.b=leWMdPdy;\n\tdkim=pass header.d=linutronix.de header.i=@linutronix.de\n header.a=ed25519-sha256 header.s=2020e header.b=Tq2nphDt;\n\tdkim-atps=neutral","gandalf.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=vger.kernel.org\n (client-ip=2600:3c15:e001:75::12fc:5321; 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Miller\" ,\n Ilpo =?utf-8?b?SsOkcnZpbmVu?= ,\n Andy Shevchenko ,\n Thomas Fourier , Kees Cook ,\n linux-serial@vger.kernel.org, linux-arm-kernel@lists.infradead.org,\n linux-samsung-soc@vger.kernel.org, sparclinux@vger.kernel.org","Subject":"Re: [PATCH tty v4 2/6] serial: Replace driver usage of UPF_CONS_FLOW","In-Reply-To":"<20260506121606.5805-3-john.ogness@linutronix.de>","References":"<20260506121606.5805-1-john.ogness@linutronix.de>\n <20260506121606.5805-3-john.ogness@linutronix.de>","Date":"Thu, 07 May 2026 11:56:46 +0206","Message-ID":"<87lddvy2dl.fsf@jogness.linutronix.de>","Precedence":"bulk","X-Mailing-List":"sparclinux@vger.kernel.org","List-Id":"","List-Subscribe":"","List-Unsubscribe":"","MIME-Version":"1.0","Content-Type":"text/plain","X-Spam-Status":"No, score=-0.6 required=5.0 tests=ARC_SIGNED,ARC_VALID,\n\tDKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DMARC_PASS,\n\tHEADER_FROM_DIFFERENT_DOMAINS,INVALID_DATE_TZ_ABSURD,\n\tMAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=disabled\n\tversion=4.0.1","X-Spam-Checker-Version":"SpamAssassin 4.0.1 (2024-03-25) on gandalf.ozlabs.org"}}]