On 17/08/2020 19:47, Alan Stern wrote: > > Unplugging a R/W USB drive without unmounting it first is a great way to > corrupt the data. > Thank you, post development we will only mount the USB stick as R/O. >> Using perf Iidentified the hub_events workqueue was spending a lot of time in >> invalidate_partition(), I have included a cut down the captured data from perf in >> [2] which shows the additional functions where the kworker spends most of its time. > > invalidate_partition() is part of the block layer, not part of USB. It > gets called whenever a drive is removed from the system, no matter what > type of drive it is. You should ask the people involved in that > subsystem why it takes so long. > I included the linux-mm list but missed the filesystem, I will ask the question to the linux-fsdevel too. >> I realise that not unmounting the USB stick is not ideal, though I wonder what >> additional work is done when unplugging the USB stick compared to unmounting it. > > Unmounting a drive flushes all the dirty buffers from memory back to the > drive. Obviously that can't be done if the drive is unplugged first. > > As far as the USB subsystem is concerned, exactly the same amount of > work is done during disconnect regardless of whether or not the drive is > mounted. (In fact, the USB subsystem doesn't even know whether a drive > is mounted; that concept is part of the block and filesystem layers.) >>> I guess it may be waiting for a time-out during the operation without the unmount. > > That seems very unlikely. When a USB device gets unplugged the system > realizes it. Any I/O meant for that device is immediately cancelled; > there are no timeouts. > > (Okay, not strictly true; there is a fraction-of-a-second timeout during > which the system waits to see whether the disconnect was permanent or > just a temporary glitch. But you're talking about 6-second long > delays.) > Thank you, no I don't expect that to cause the issue and it is very likely the delay is in another subsystem. Regards, Jim Baxter > Alan Stern >
