linux-kernel - [PATCH 04/10] sysfs: sysfs poll keep the poll rule of regular file.

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Message-Id: <1239995074-4065-4-git-send-email-gregkh@suse.de>
Date:	Fri, 17 Apr 2009 12:04:28 -0700
From:	Greg Kroah-Hartman <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Neil Brown <neilb@...e.de>, Greg Kroah-Hartman <gregkh@...e.de>
Subject: [PATCH 04/10] sysfs: sysfs poll keep the poll rule of regular file.

From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>

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@...e.de>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.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)
-- 
1.6.2

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