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    Subject[PATCH 04/10] sysfs: sysfs poll keep the poll rule of regular file.
    Date
    From: KOSAKI Motohiro <kosaki.motohiro@jp.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@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)
    --
    1.6.2



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