1. 20 Apr, 2020 1 commit
    • Douglas Anderson's avatar
      bdev: Reduce time holding bd_mutex in sync in blkdev_close() · b849dd84
      Douglas Anderson authored
      While trying to "dd" to the block device for a USB stick, I
      encountered a hung task warning (blocked for > 120 seconds).  I
      managed to come up with an easy way to reproduce this on my system
      (where /dev/sdb is the block device for my USB stick) with:
      
        while true; do dd if=/dev/zero of=/dev/sdb bs=4M; done
      
      With my reproduction here are the relevant bits from the hung task
      detector:
      
       INFO: task udevd:294 blocked for more than 122 seconds.
       ...
       udevd           D    0   294      1 0x00400008
       Call trace:
        ...
        mutex_lock_nested+0x40/0x50
        __blkdev_get+0x7c/0x3d4
        blkdev_get+0x118/0x138
        blkdev_open+0x94/0xa8
        do_dentry_open+0x268/0x3a0
        vfs_open+0x34/0x40
        path_openat+0x39c/0xdf4
        do_filp_open+0x90/0x10c
        do_sys_open+0x150/0x3c8
        ...
      
       ...
       Showing all locks held in the system:
       ...
       1 lock held by dd/2798:
        #0: ffffff814ac1a3b8 (&bdev->bd_mutex){+.+.}, at: __blkdev_put+0x50/0x204
       ...
       dd              D    0  2798   2764 0x00400208
       Call trace:
        ...
        schedule+0x8c/0xbc
        io_schedule+0x1c/0x40
        wait_on_page_bit_common+0x238/0x338
        __lock_page+0x5c/0x68
        write_cache_pages+0x194/0x500
        generic_writepages+0x64/0xa4
        blkdev_writepages+0x24/0x30
        do_writepages+0x48/0xa8
        __filemap_fdatawrite_range+0xac/0xd8
        filemap_write_and_wait+0x30/0x84
        __blkdev_put+0x88/0x204
        blkdev_put+0xc4/0xe4
        blkdev_close+0x28/0x38
        __fput+0xe0/0x238
        ____fput+0x1c/0x28
        task_work_run+0xb0/0xe4
        do_notify_resume+0xfc0/0x14bc
        work_pending+0x8/0x14
      
      The problem appears related to the fact that my USB disk is terribly
      slow and that I have a lot of RAM in my system to cache things.
      Specifically my writes seem to be happening at ~15 MB/s and I've got
      ~4 GB of RAM in my system that can be used for buffering.  To write 4
      GB of buffer to disk thus takes ~4000 MB / ~15 MB/s = ~267 seconds.
      
      The 267 second number is a problem because in __blkdev_put() we call
      sync_blockdev() while holding the bd_mutex.  Any other callers who
      want the bd_mutex will be blocked for the whole time.
      
      The problem is made worse because I believe blkdev_put() specifically
      tells other tasks (namely udev) to go try to access the device at right
      around the same time we're going to hold the mutex for a long time.
      
      Putting some traces around this (after disabling the hung task detector),
      I could confirm:
       dd:    437.608600: __blkdev_put() right before sync_blockdev() for sdb
       udevd: 437.623901: blkdev_open() right before blkdev_get() for sdb
       dd:    661.468451: __blkdev_put() right after sync_blockdev() for sdb
       udevd: 663.820426: blkdev_open() right after blkdev_get() for sdb
      
      A simple fix for this is to realize that sync_blockdev() works fine if
      you're not holding the mutex.  Also, it's not the end of the world if
      you sync a little early (though it can have performance impacts).
      Thus we can make a guess that we're going to need to do the sync and
      then do it without holding the mutex.  We still do one last sync with
      the mutex but it should be much, much faster.
      
      With this, my hung task warnings for my test case are gone.
      Signed-off-by: default avatarDouglas Anderson <dianders@chromium.org>
      Reviewed-by: default avatarGuenter Roeck <groeck@chromium.org>
      Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
      Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
      b849dd84
  2. 18 Apr, 2020 1 commit
  3. 17 Apr, 2020 28 commits
  4. 16 Apr, 2020 10 commits