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Wed, 08 Apr 2026 10:07:32 -0700 (PDT) Date: Wed, 8 Apr 2026 10:07:29 -0700 From: Dmitry Torokhov To: Mohamad Raizudeen Cc: kees@kernel.org, linux-input@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] Input: serio - fix O(n^2) complexity in serio_unregister_driver() Message-ID: References: <20260408162849.4639-1-raizudeen.kerneldev@gmail.com> Precedence: bulk X-Mailing-List: linux-input@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260408162849.4639-1-raizudeen.kerneldev@gmail.com> Hi Mohamad, On Wed, Apr 08, 2026 at 09:58:47PM +0530, Mohamad Raizudeen wrote: > The current implementation restarts the scan from the beginning after > each disconnection(goto start_over) because serio_disconnect_port() may > delete the current list entry. This results in O(n^2) worst case > behaviour. The "big O" notation only makes sense when numbers are big. Here we are dealing with 6 serio ports max (1 KBC, 4xMUX, 1 Synaptics pass-through) unless you have a serial port expander and hooked up a few ports there. But still, the number if very limited. > > Replace with a two-phase approach: > > 1.Collect only top-level ports bound to the driver(skip those whose parent is also bound to the same driver) into a > temporary list. We do not have such setups at the moment, but what about parent's parent's parent? > > 2.Process each collected port once, moving it back to serio_list first to maintain invariants expected by serio_destroy_port(). > > This eliminates the restart loop, reduces complexity from O(n^2) to O(n) > and avoids use-after-free by never collecting child ports separately. Could you explain more about the use-after-free scenario? Thanks. -- Dmitry