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 552663B776C; Wed, 13 May 2026 19:50:48 +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=1778701848; cv=none; b=SkKsfu9kF/BUb7GQC+j6ySIJ70nzC+Zi9Bp9yZ8V3rME6xPtUljt0pS9NtwlBxrRJeN4O2ecB9gbXbfKd/a3FvkRZYoSHCkY0/EO0aFpkxbzb1rn4KKlMnww+YNNuhsrNvDI9fguQS/Ox9jxK8w5X4PPgfk4oQPoZH/rEeus1Xc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778701848; c=relaxed/simple; bh=DUyitwcj2pqGB9DvTz8dhmsbk8ZH1JOgKGFr5wFe24s=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=q62eLw3tp6Is35pCB1JVdCN5F9DbIMo30vuW6rlOZMFS+aaFlXaN+BuEnsE+TZ4sJcQp2pxYlGuF9xuIK/myZD7LMnPQCL8Gy/3l2tee+ASJX1hTojCLDtmPwzAvy3l5ISJRkQlUiqprcBz3MiiImQB0hRcDiM/nQtJEfXO1fSY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=W4xkVuoZ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="W4xkVuoZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EFD33C19425; Wed, 13 May 2026 19:50:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778701848; bh=DUyitwcj2pqGB9DvTz8dhmsbk8ZH1JOgKGFr5wFe24s=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=W4xkVuoZtpqwljl1IJw98+ExgePhhSGwfgR5puXpO22uMy+CUH/gGXLVFPWf+QNxC WATz4gIt6B5+d1BhgFI5QKk3KRsGRV5BCPkahkz1OBGhG/TYVE0L3CkEz9l8FnSE++ VC3eHwHtrm/kNEGHSuNLBOqUAzf1B79imGShcRIsbfDLTD/AW+O8Kgs7Q+XROZ3Su0 6E7Gaeu6kJ4iZOeeKwA2tCEKFOUcsoWp8c6wrbJCqYCgvf7iZtGLLEhxBHOx2SJs79 IwyEpaNd3tY97rrvpsVcauOoY0Oy9Nas0+1EwhYqdnQ+po7nTr5sSJxmMV35yReq8a R25xr/k/Lwrqg== Message-ID: <1a5abd2e-e9ab-4a48-94c2-5e082f57adde@kernel.org> Date: Wed, 13 May 2026 21:50:42 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH tty v4 2/6] serial: Replace driver usage of UPF_CONS_FLOW To: John Ogness , Greg Kroah-Hartman , Jiri Slaby , Alim Akhtar Cc: Andy Shevchenko , linux-kernel@vger.kernel.org, "David S. Miller" , =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= , Andy Shevchenko , Thomas Fourier , Kees Cook , linux-serial@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-samsung-soc@vger.kernel.org, sparclinux@vger.kernel.org, Peter Griffin , Tudor Ambarus , =?UTF-8?Q?Andr=C3=A9_Draszik?= , Alexey Klimov References: <20260506121606.5805-1-john.ogness@linutronix.de> <20260506121606.5805-3-john.ogness@linutronix.de> <87lddvy2dl.fsf@jogness.linutronix.de> From: Krzysztof Kozlowski Content-Language: en-US Autocrypt: addr=krzk@kernel.org; keydata= xsFNBFVDQq4BEAC6KeLOfFsAvFMBsrCrJ2bCalhPv5+KQF2PS2+iwZI8BpRZoV+Bd5kWvN79 cFgcqTTuNHjAvxtUG8pQgGTHAObYs6xeYJtjUH0ZX6ndJ33FJYf5V3yXqqjcZ30FgHzJCFUu JMp7PSyMPzpUXfU12yfcRYVEMQrmplNZssmYhiTeVicuOOypWugZKVLGNm0IweVCaZ/DJDIH gNbpvVwjcKYrx85m9cBVEBUGaQP6AT7qlVCkrf50v8bofSIyVa2xmubbAwwFA1oxoOusjPIE J3iadrwpFvsZjF5uHAKS+7wHLoW9hVzOnLbX6ajk5Hf8Pb1m+VH/E8bPBNNYKkfTtypTDUCj NYcd27tjnXfG+SDs/EXNUAIRefCyvaRG7oRYF3Ec+2RgQDRnmmjCjoQNbFrJvJkFHlPeHaeS BosGY+XWKydnmsfY7SSnjAzLUGAFhLd/XDVpb1Een2XucPpKvt9ORF+48gy12FA5GduRLhQU vK4tU7ojoem/G23PcowM1CwPurC8sAVsQb9KmwTGh7rVz3ks3w/zfGBy3+WmLg++C2Wct6nM Pd8/6CBVjEWqD06/RjI2AnjIq5fSEH/BIfXXfC68nMp9BZoy3So4ZsbOlBmtAPvMYX6U8VwD TNeBxJu5Ex0Izf1NV9CzC3nNaFUYOY8KfN01X5SExAoVTr09ewARAQABzSVLcnp5c3p0b2Yg S296bG93c2tpIDxrcnprQGtlcm5lbC5vcmc+wsGVBBMBCgA/AhsDBgsJCAcDAgYVCAIJCgsE FgIDAQIeAQIXgBYhBJvQfg4MUfjVlne3VBuTQ307QWKbBQJoF1BKBQkWlnSaAAoJEBuTQ307 QWKbHukP/3t4tRp/bvDnxJfmNdNVn0gv9ep3L39IntPalBFwRKytqeQkzAju0whYWg+R/rwp +r2I1Fzwt7+PTjsnMFlh1AZxGDmP5MFkzVsMnfX1lGiXhYSOMP97XL6R1QSXxaWOpGNCDaUl ajorB0lJDcC0q3xAdwzRConxYVhlgmTrRiD8oLlSCD5baEAt5Zw17UTNDnDGmZQKR0fqLpWy 786Lm5OScb7DjEgcA2PRm17st4UQ1kF0rQHokVaotxRM74PPDB8bCsunlghJl1DRK9s1aSuN hL1Pv9VD8b4dFNvCo7b4hfAANPU67W40AaaGZ3UAfmw+1MYyo4QuAZGKzaP2ukbdCD/DYnqi tJy88XqWtyb4UQWKNoQqGKzlYXdKsldYqrLHGoMvj1UN9XcRtXHST/IaLn72o7j7/h/Ac5EL 8lSUVIG4TYn59NyxxAXa07Wi6zjVL1U11fTnFmE29ALYQEXKBI3KUO1A3p4sQWzU7uRmbuxn naUmm8RbpMcOfa9JjlXCLmQ5IP7Rr5tYZUCkZz08LIfF8UMXwH7OOEX87Y++EkAB+pzKZNNd hwoXulTAgjSy+OiaLtuCys9VdXLZ3Zy314azaCU3BoWgaMV0eAW/+gprWMXQM1lrlzvwlD/k whyy9wGf0AEPpLssLVt9VVxNjo6BIkt6d1pMg6mHsUEVzsFNBFVDXDQBEADNkrQYSREUL4D3 Gws46JEoZ9HEQOKtkrwjrzlw/tCmqVzERRPvz2Xg8n7+HRCrgqnodIYoUh5WsU84N03KlLue MNsWLJBvBaubYN4JuJIdRr4dS4oyF1/fQAQPHh8Thpiz0SAZFx6iWKB7Qrz3OrGCjTPcW6ei OMheesVS5hxietSmlin+SilmIAPZHx7n242u6kdHOh+/SyLImKn/dh9RzatVpUKbv34eP1wA GldWsRxbf3WP9pFNObSzI/Bo3kA89Xx2rO2roC+Gq4LeHvo7ptzcLcrqaHUAcZ3CgFG88CnA 6z6lBZn0WyewEcPOPdcUB2Q7D/NiUY+HDiV99rAYPJztjeTrBSTnHeSBPb+qn5ZZGQwIdUW9 YegxWKvXXHTwB5eMzo/RB6vffwqcnHDoe0q7VgzRRZJwpi6aMIXLfeWZ5Wrwaw2zldFuO4Dt 91pFzBSOIpeMtfgb/Pfe/a1WJ/GgaIRIBE+NUqckM+3zJHGmVPqJP/h2Iwv6nw8U+7Yyl6gU BLHFTg2hYnLFJI4Xjg+AX1hHFVKmvl3VBHIsBv0oDcsQWXqY+NaFahT0lRPjYtrTa1v3tem/ JoFzZ4B0p27K+qQCF2R96hVvuEyjzBmdq2esyE6zIqftdo4MOJho8uctOiWbwNNq2U9pPWmu 4vXVFBYIGmpyNPYzRm0QPwARAQABwsF8BBgBCgAmAhsMFiEEm9B+DgxR+NWWd7dUG5NDfTtB YpsFAmgXUF8FCRaWWyoACgkQG5NDfTtBYptO0w//dlXJs5/42hAXKsk+PDg3wyEFb4NpyA1v qmx7SfAzk9Hf6lWwU1O6AbqNMbh6PjEwadKUk1m04S7EjdQLsj/MBSgoQtCT3MDmWUUtHZd5 RYIPnPq3WVB47GtuO6/u375tsxhtf7vt95QSYJwCB+ZUgo4T+FV4hquZ4AsRkbgavtIzQisg Dgv76tnEv3YHV8Jn9mi/Bu0FURF+5kpdMfgo1sq6RXNQ//TVf8yFgRtTUdXxW/qHjlYURrm2 H4kutobVEIxiyu6m05q3e9eZB/TaMMNVORx+1kM3j7f0rwtEYUFzY1ygQfpcMDPl7pRYoJjB dSsm0ZuzDaCwaxg2t8hqQJBzJCezTOIkjHUsWAK+tEbU4Z4SnNpCyM3fBqsgYdJxjyC/tWVT AQ18NRLtPw7tK1rdcwCl0GFQHwSwk5pDpz1NH40e6lU+NcXSeiqkDDRkHlftKPV/dV+lQXiu jWt87ecuHlpL3uuQ0ZZNWqHgZoQLXoqC2ZV5KrtKWb/jyiFX/sxSrodALf0zf+tfHv0FZWT2 zHjUqd0t4njD/UOsuIMOQn4Ig0SdivYPfZukb5cdasKJukG1NOpbW7yRNivaCnfZz6dTawXw XRIV/KDsHQiyVxKvN73bThKhONkcX2LWuD928tAR6XMM2G5ovxLe09vuOzzfTWQDsm++9UKF a/A= In-Reply-To: <87lddvy2dl.fsf@jogness.linutronix.de> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 07/05/2026 11:50, John Ogness wrote: > Hi, > > (This email is particularly directed at the Samsung folks.) > > Responding to Sashiko: > > https://sashiko.dev/#/patchset/20260506121606.5805-1-john.ogness%40linutronix.de > > On 2026-05-06, Sashiko wrote: >>> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c >>> index e27806bf2cf3e..2f94fc798cffb 100644 >>> --- a/drivers/tty/serial/samsung_tty.c >>> +++ b/drivers/tty/serial/samsung_tty.c >>> @@ -319,7 +319,7 @@ static void s3c24xx_serial_stop_tx(struct uart_port *port) >>> ourport->tx_enabled = 0; >>> ourport->tx_in_progress = 0; >>> >>> - if (port->flags & UPF_CONS_FLOW) >>> + if (uart_cons_flow_enabled(port)) >>> s3c24xx_serial_rx_enable(port); >> >> This isn't a new bug introduced by this patch, but does this code >> result in a recursive spinlock acquisition and deadlock when console >> flow control is enabled? >> >> The serial core framework calls port->ops->stop_tx() with the >> port->lock spinlock already held. When uart_cons_flow_enabled(port) >> evaluates to true, s3c24xx_serial_stop_tx() invokes >> s3c24xx_serial_rx_enable(). >> >> Looking at s3c24xx_serial_rx_enable(), it unconditionally attempts to >> acquire the exact same lock again: >> >> static void s3c24xx_serial_rx_enable(struct uart_port *port) >> { >> struct s3c24xx_uart_port *ourport = to_ourport(port); >> unsigned long flags; >> ... >> uart_port_lock_irqsave(port, &flags); >> ... >> } >> >> Because spinlocks are not recursive in the kernel, it appears this >> sequence would cause an immediate deadlock on the same CPU. > > Although out of scope for my series, the reported issue is real. Perhaps > something for Krzysztof or Alim to look into. > >>> @@ -493,7 +493,7 @@ static void s3c24xx_serial_start_tx(struct uart_port *port) >>> struct tty_port *tport = &port->state->port; >>> >>> if (!ourport->tx_enabled) { >>> - if (port->flags & UPF_CONS_FLOW) >>> + if (uart_cons_flow_enabled(port)) >>> s3c24xx_serial_rx_disable(port); >> >> Similarly here, s3c24xx_serial_start_tx() is also called with >> port->lock held, and s3c24xx_serial_rx_disable() will attempt to >> acquire port->lock again: >> >> static void s3c24xx_serial_rx_disable(struct uart_port *port) >> { >> struct s3c24xx_uart_port *ourport = to_ourport(port); >> unsigned long flags; >> ... >> uart_port_lock_irqsave(port, &flags); >> ... >> } >> >> Could this pre-existing locking issue in the samsung_tty driver be >> addressed so that the rx enable/disable helpers do not try to take the >> port lock when it is already held by the caller? > > Also legitimate. But out of scope for my series. Thanks for letting us know. Deadlock did not happen so far, so something is missing in Sashiko's report. :) I am just back from vacation, so I won't have time to dig into this, but I will keep your email in the inbox. Cc-ing also a few Linaro folks which are using this platform and might be able to help us here. Best regards, Krzysztof