On 4/20/20 8:51 AM, Doug Anderson wrote: > Hi Alexander, > > On Tue, Mar 24, 2020 at 2:48 PM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote: >> >> 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: Douglas Anderson <dianders@xxxxxxxxxxxx> >> --- >> I didn't put a "Fixes" annotation here because, as far as I can tell, >> this issue has been here "forever" unless someone knows of something >> else that changed that made this possible to hit. This could probably >> get picked back to any stable tree that anyone is still maintaining. >> >> Changes in v2: >> - Don't bother holding the mutex when checking "bd_openers". >> >> fs/block_dev.c | 10 ++++++++++ >> 1 file changed, 10 insertions(+) >> >> diff --git a/fs/block_dev.c b/fs/block_dev.c >> index 9501880dff5e..40c57a9cc91a 100644 >> --- a/fs/block_dev.c >> +++ b/fs/block_dev.c >> @@ -1892,6 +1892,16 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part) >> struct gendisk *disk = bdev->bd_disk; >> struct block_device *victim = NULL; >> >> + /* >> + * Sync early if it looks like we're the last one. If someone else >> + * opens the block device between now and the decrement of bd_openers >> + * then we did a sync that we didn't need to, but that's not the end >> + * of the world and we want to avoid long (could be several minute) >> + * syncs while holding the mutex. >> + */ >> + if (bdev->bd_openers == 1) >> + sync_blockdev(bdev); >> + >> mutex_lock_nested(&bdev->bd_mutex, for_part); >> if (for_part) >> bdev->bd_part_count--; >> -- >> 2.25.1.696.g5e7596f4ac-goog > > Are you the right person to land this patch? If so, is there anything > else that needs to be done? Jens: if you should be the person to land > (as suggested by "git log" but not by "get_maintainer") I'm happy to > repost with collected tags. Originally I trusted "get_maintainer" to > help point me to the right person. I can queue it up, looks good to me. -- Jens Axboe