Gitweb:     http://git.kernel.org/linus/1af3557abdef34ee036a6de4cb79e24468544b8d
Commit:     1af3557abdef34ee036a6de4cb79e24468544b8d
Parent:     d094cbe998eb566d47552aa9d3c26c9123a7b858
Author:     KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
AuthorDate: Thu Apr 9 13:53:22 2009 +0900
Committer:  Greg Kroah-Hartman <gregkh@suse.de>
CommitDate: Thu Apr 16 16:17:09 2009 -0700

    sysfs: sysfs poll keep the poll rule of regular file.
    
    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 <neilb@suse.de>
    Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
 fs/sysfs/file.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 979e937..b1606e0 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -446,11 +446,11 @@ static unsigned int sysfs_poll(struct file *filp, poll_table *wait)
 	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)
--
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