From kosaki.motohiro@jp.fujitsu.com Thu Apr 9 14:32:54 2009 From: KOSAKI Motohiro Date: Thu, 9 Apr 2009 13:53:22 +0900 (JST) Subject: sysfs: sysfs poll keep the poll rule of regular file. To: Neil Brown , Greg Kroah-Hartman , Al Viro , Ram Pai , Miklos Szeredi Cc: kosaki.motohiro@jp.fujitsu.com Message-ID: <20090409134916.B3E6.A69D9226@jp.fujitsu.com> From: KOSAKI Motohiro Currently, following test programs don't finished. % ruby -e ' Thread.new { sleep } File.read("/sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies") ' strace expose the reason. ... open("/sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies", O_RDONLY|O_LARGEFILE) = 3 ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbf9fa6b8) = -1 ENOTTY (Inappropriate ioctl for device) fstat64(3, {st_mode=S_IFREG|0444, st_size=4096, ...}) = 0 _llseek(3, 0, [0], SEEK_CUR) = 0 select(4, [3], NULL, NULL, NULL) = 1 (in [3]) read(3, "1400000 1300000 1200000 1100000 1"..., 4096) = 62 select(4, [3], NULL, NULL, NULL Because Ruby (the scripting language) VM assume select system-call against regular file don't block. it because SUSv3 says "Regular files shall always poll TRUE for reading and writing". see http://www.opengroup.org/onlinepubs/009695399/functions/poll.html it seems valid assumption. But sysfs_poll() don't keep this rule although sysfs file can read and write always. This patch restore proper poll behavior to sysfs. /sys/block/md*/md/sync_action polling application and another sysfs updating sensitive application still can use POLLERR and POLLPRI. Cc: Neil Brown Signed-off-by: KOSAKI Motohiro Signed-off-by: Greg Kroah-Hartman --- fs/sysfs/file.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/fs/sysfs/file.c +++ b/fs/sysfs/file.c @@ -446,11 +446,11 @@ static unsigned int sysfs_poll(struct fi if (buffer->event != atomic_read(&od->event)) goto trigger; - return 0; + return DEFAULT_POLLMASK; trigger: buffer->needs_read_fill = 1; - return POLLERR|POLLPRI; + return DEFAULT_POLLMASK|POLLERR|POLLPRI; } void sysfs_notify_dirent(struct sysfs_dirent *sd)