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Patch 1 fixes a deadlock between port removal and a tcdrain() in flight, present since 8ca59f7baee7 ("serial: fix ioctl hangup race"). Patch 2 fixes a read of the port after its last reference was dropped, found by review of the same path and not reproduced. Reproduced on a MAX14830 (max310x) board, on v7.4-rc1 plus the board support, by unbinding the driver while four processes drained a few kilobytes each at 300 baud: the first unbind never returned, and the hung-task detector reported the unbind in wait_event() and a drain in mutex_lock(), with the drain "blocked on a mutex likely owned by" the unbind; a power cycle was needed. With the two patches the same test completed 46 of 46 unbinds with blocked tcdrain() waiters, each unbind taking 2 to 3.5 s, the time for the drains to be hung up. A two-hour soak with lockdep and kmemleak enabled, 1472 unbind and rebind cycles every 3 s while other ports of the same driver carried full traffic, completed with nothing reported. The change is independent of the driver: it only moves the wait for the reference holders out from under the mutex those holders need. Tool use: Claude Code (Anthropic) was used throughout. It analysed the hung-task reports and the /proc stacks from the stress bench, located the lock order between serial_core_remove_one_port() and uart_wait_until_sent(), drafted this change and the commit message, and wrote and drove the test harness that reproduced the deadlock and verified the fix. I reviewed the locking argument against the code, decided the shape of the change, ran the hardware tests and read every result. The code and the message are mine to defend. 8ca59f7baee7 is in v7.3 and is tagged for stable back to 2.6.26, so patch 1 has to follow it into every tree that takes it. I will send backports for the stable trees where the patches do not apply. --- Changes in v2: - Patch 1: dropped the paragraph on stable backports and the comment on the Cc: stable line (Johan). - Added Johan's Reviewed-by on both patches and Tested-by on patch 1. - No code changes. - Link to v1: https://lore.kernel.org/r/20261104-serial-core-remove-deadlock-v1-0-6a30597adcbf@vaisala.com --- Tapio Reijonen (2): serial: core: do not hold the port mutex while waiting for references serial: core: do not touch the port after dropping the last reference drivers/tty/serial/serial_core.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-) --- base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4 change-id: 20261103-serial-core-remove-deadlock-0310bbca5554 Best regards, -- Tapio Reijonen