Are calls to open()/close() serialized by tty layer?

[Grant Edwards <grant.b.edwards@gmail.com>]

Are calls to a tty driver's open()/close() methods serialized by the
tty layer?  If so, has this always been the case?

I'm working on converting an old tty driver over to using the "new"
tty_port_* functions and the standard "struct tty_port" port data
structure.

[I thought about converting it to a uart driver for serial_core, but
AFAICT, that API doesn't provide any way to return errors for
userspace open() and write() calls and would therefore break my
user-land API.]

>From looking at example tty drivers in the 3.7.2 source tree, it
appears that they assume calls to open()/close() are serialized.

The old driver I'm working on had its own internal locking to protect
race conditiions when incrementing and decrementing of a port's open
count.  That counter is being prelaced by tty->port->count, and in
existing in-kernel tty drivers it doesn't look like there is any
locking when that count is incremented during an open().

For example, from rocket.c:

   891	static int rp_open(struct tty_struct *tty, struct file *filp)
   892	{
[...]   
   924	        tty->driver_data = info;
   925	        tty_port_tty_set(port, tty);
   926	
   927	        if (port->count++ == 0) {
   928	                atomic_inc(&rp_num_ports_open);
   929	
   930	#ifdef ROCKET_DEBUG_OPEN
   931	                printk(KERN_INFO "rocket mod++ = %d...\n",
   932	                                atomic_read(&rp_num_ports_open));
   933	#endif
   934	        }

If calls to open()/close() aren't serialized, then line 927 would
cause a race condition...

-- 
Grant Edwards               grant.b.edwards        Yow! Everybody gets free
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