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tty: Remove tty_wait_until_sent_from_close()

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tty_wait_until_sent_from_close() drops the tty lock while waiting
for the tty driver to finish sending previously accepted data (ie.,
data remaining in its write buffer and transmit fifo).

tty_wait_until_sent_from_close() was added by commit a57a7bf3fc
("TTY: define tty_wait_until_sent_from_close") to prevent the entire
tty subsystem from being unable to open new ttys while waiting for
one tty to close while output drained.

However, since commit 0911261d4c ("tty: Don't take tty_mutex for tty
count changes"), holding a tty lock while closing does not prevent other
ttys from being opened/closed/hung up, but only prevents lifetime event
changes for the tty under lock.

Holding the tty lock while waiting for output to drain does prevent
parallel non-blocking opens (O_NONBLOCK) from advancing or returning
while the tty lock is held. However, all parallel opens _already_
block even if the tty lock is dropped while closing and the parallel
open advances. Blocking in open has been in mainline since at least 2.6.29
(see tty_port_block_til_ready(); note the test for O_NONBLOCK is _after_
the wait while ASYNC_CLOSING).

IOW, before this patch a non-blocking open will sleep anyway for the
_entire_ duration of a parallel hardware shutdown, and when it wakes, the
error return will cause a release of its tty, and it will restart with
a fresh attempt to open. Similarly with a blocking open that is already
waiting; when it's woken, the hardware shutdown has already completed
to ASYNC_INITIALIZED is not set, which forces a release and restart as
well.

So, holding the tty lock across the _entire_ close (which is what this
patch does), even while waiting for output to drain, is equivalent to
the current outcome wrt parallel opens.

Cc: Alan Cox <alan@linux.intel.com>
Cc: David Laight <David.Laight@aculab.com>
CC: Arnd Bergmann <arnd@arndb.de>
CC: Karsten Keil <isdn@linux-pingi.de>
CC: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley
2015-10-10 16:00:51 -04:00
committed by Greg Kroah-Hartman
parent 32ede4a517
commit 79c1faa451
5 changed files with 5 additions and 30 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/linux/tty.h
-18
View File
@@ -656,24 +656,6 @@ extern void __lockfunc tty_unlock(struct tty_struct *tty);
extern void __lockfunc tty_lock_slave(struct tty_struct *tty);
extern void __lockfunc tty_unlock_slave(struct tty_struct *tty);
extern void tty_set_lock_subclass(struct tty_struct *tty);
/*
* this shall be called only from where BTM is held (like close)
*
* We need this to ensure nobody waits for us to finish while we are waiting.
* Without this we were encountering system stalls.
*
* This should be indeed removed with BTM removal later.
*
* Locking: BTM required. Nobody is allowed to hold port->mutex.
*/
static inline void tty_wait_until_sent_from_close(struct tty_struct *tty,
long timeout)
{
tty_unlock(tty); /* tty->ops->close holds the BTM, drop it while waiting */
tty_wait_until_sent(tty, timeout);
tty_lock(tty);
}
/*
* wait_event_interruptible_tty -- wait for a condition with the tty lock held
*