Messages in this thread |  | | Date | Wed, 09 Jul 2014 09:57:50 -0400 | From | Peter Hurley <> | Subject | Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close() |
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On 06/17/2014 07:32 AM, Peter Hurley wrote: > On 06/17/2014 07:03 AM, David Laight wrote: >> From: Peter Hurley >> ... >>>> I don't understand the second half of the changelog, it doesn't seem >>>> to fit here: there deadlock that we are trying to avoid here happens >>>> when the *same* tty needs the lock to complete the function that >>>> sends the pending data. I don't think we do still do that any more, >>>> but it doesn't seem related to the tty lock being system-wide or not. >>> >>> The tty lock is not used in the i/o path; it's purpose is to >>> mutually exclude state changes in open(), close() and hangup(). >>> >>> The commit that added this [1] comments that _other_ ttys may wait >>> for this tty to complete, and comments in the code note that this >>> function should be removed when the system-wide tty mutex was removed >>> (which happened with the commit noted in the changelog). >> >> What happens if another process tries to do a non-blocking open >> while you are sleeping in close waiting for output to drain? >> >> Hopefully this returns before that data has drained. > > Good point. > > tty_open() should be trylocking both mutexes anyway in O_NONBLOCK.
Further, the tty lock should not be nested within the tty_mutex lock in a reopen, regardless of O_NONBLOCK.
AFAICT, the tty_mutex in the reopen scenario is only protecting the tty count bump of the linked tty (if the tty is a pty).
I think with some refactoring and returning with a tty reference held from both tty_open_current_tty() and tty_driver_lookup_tty(), the tty lock in tty_open() can be attempted without nesting in the tty_mutex.
Regardless, I'll be splitting this series and I'll be sure to cc you all when I resubmit these changes (after testing).
Regards, Peter Hurley
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