Red Hat Bugzilla – Bug 163528
/dev/tty won't open during blocking /dev/ttyS1 open
Last modified: 2013-03-06 00:58:58 EST
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Description of problem:
One of the Red Hat patches to 2.6.9 moves the tty_sem to encompass uart_open in tty_io.c. After issuing a "blocking" uart_open (wait for carrier detect on the modem) ALL subsequent opens to any /dev/tty device are blocking on the tty_sem semaphore which is being held during the sleep cycle in uart_open that's waiting for carrier-detect from the modem.
This behaviour was introduced in the Red Hat "bits" patch. The tty_sem semaphore cannot be held during the call to uart_open.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. Issue a blocking open to /dev/ttyS1 with CLOCAL cleared and w/o O_NDELAY.
2. Start a shell prompt (this will open /dev/tty)
3. The open of /dev/tty will hang on tty_sem.
Actual Results: Unable to start new sessions or open other major-device 5 inodes.
Expected Results: The subsequent opens should succeed immediately, rather than block.
Whoever "cleaned up" tty_io.c in the kernel in "2.6.10-ac-selected-bits-patch" made an assumption that the underlying device driver open would not sleep with the semaphore held open.
This issue has been addressed in our laest builds of U2. Please see:
http://people.redhat.com/~jbaron/rhel4/ for a test kernel.
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.