From 47d3904fe40d62deee8cd46e79ca784e7a548acd Mon Sep 17 00:00:00 2001 From: Lawrence Rust Date: Wed, 27 Oct 2010 14:41:02 +0200 Subject: 8250: Fix tcsetattr to avoid ioctl(TIOCMIWAIT) hang Calling tcsetattr prevents any thread(s) currently suspended in ioctl TIOCMIWAIT for the same device from ever resuming. If a thread is suspended inside a call to ioctl TIOCMIWAIT, waiting for a modem status change, then the 8250 driver enables modem status interrupts (MSI). The device interrupt service routine resumes the suspended thread(s) on the next MSI. If while the thread(s) are suspended, another thread calls tcsetattr then the 8250 driver disables MSI (unless CTS/RTS handshaking is enabled) thus preventing the suspended thread(s) from ever being resumed. This patch only disables MSI in tcsetattr handling if there are no suspended threads. Program to demonstrate bug & fix: /* gcc miwait.c -o miwait -l pthread */ #include #include #include #include #include #include #include #include static void* monitor( void* pv); static int s_fd; int main( void) { const char kszDev[] = "/dev/ttyS0"; pthread_t t; struct termios tio; s_fd = open( kszDev, O_RDWR | O_NONBLOCK); if ( s_fd < 0) return fprintf( stderr, "Error(%d) opening %s: %s\n", errno, kszDev, strerror( errno)), 1; pthread_create( &t, NULL, &monitor, NULL); /* Modem status changes seen here */ puts( "Main: awaiting status changes"); sleep( 5); tcgetattr( s_fd, &tio); tio.c_cflag ^= CSTOPB; /* But not after here */ puts( "Main: tcsetattr called"); tcsetattr( s_fd, TCSANOW, &tio); for (;;) sleep( 1); } static void* monitor( void* pv) { (void)pv; for(;;) { unsigned uModem; struct serial_icounter_struct cnt; if ( ioctl( s_fd, TIOCMGET, &uModem) < 0) fprintf( stderr, "Error(%d) in TIOCMGET: %s\n", errno, strerror( errno)); printf( "Modem status:%s%s%s%s%s%s\n", (uModem & TIOCM_RTS) ? " RTS" : "", (uModem & TIOCM_DTR) ? " DTR" : "", (uModem & TIOCM_CTS) ? " CTS" : "", (uModem & TIOCM_DSR) ? " DSR" : "", (uModem & TIOCM_CD) ? " CD" : "", (uModem & TIOCM_RI) ? " RI" : "" ); if ( ioctl( s_fd, TIOCGICOUNT, &cnt) < 0) fprintf( stderr, "Error(%d) in TIOCGICOUNT: %s\n", errno, strerror( errno)); printf( "Irqs: CTS:%d DSR:%d RNG:%d DCD:%d Rx:%d Tx:%d Frame:%d Orun:%d Par:%d Brk:%d Oflow:%d\n", cnt.cts, cnt.dsr, cnt.rng, cnt.dcd, cnt.rx, cnt.tx, cnt.frame, cnt.overrun, cnt.parity, cnt.brk, cnt.buf_overrun ); fputs( "Waiting...", stdout), fflush( stdout); if ( 0 > ioctl( s_fd, TIOCMIWAIT, (unsigned long)(TIOCM_CAR | TIOCM_RNG | TIOCM_DSR | TIOCM_CTS))) fprintf( stderr, "\nError(%d) in TIOCMIWAIT: %s\n", errno, strerror( errno)); fputs( "\n", stdout); } return NULL; } Signed-off by Lawrence Rust Signed-off-by: Greg Kroah-Hartman --- drivers/serial/8250.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'drivers/serial/8250.c') diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c index 4d8e14b7aa93..dd5e1ac22251 100644 --- a/drivers/serial/8250.c +++ b/drivers/serial/8250.c @@ -2343,8 +2343,11 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios, /* * CTS flow control flag and modem status interrupts + * Only disable MSI if no threads are waiting in + * serial_core::uart_wait_modem_status */ - up->ier &= ~UART_IER_MSI; + if (!waitqueue_active(&up->port.state->port.delta_msr_wait)) + up->ier &= ~UART_IER_MSI; if (!(up->bugs & UART_BUG_NOMSR) && UART_ENABLE_MS(&up->port, termios->c_cflag)) up->ier |= UART_IER_MSI; -- cgit v1.2.3-58-ga151