On Fri, Nov 12, 2021 at 03:54:40PM +0530, Pavankumar Kondeti wrote:
> Function fs endpoint file operations are synchronized via an interruptible
> mutex wait. However we see threads that do ep file operations concurrently
> are getting blocked for the mutex lock in __fdget_pos(). This is an
> uninterruptible wait and we see hung task warnings and kernel panic
> if hung_task_panic systcl is enabled if host does not send/receive
> the data for long time.
>
> The reason for threads getting blocked in __fdget_pos() is due to
> the file position protection introduced by the commit 9c225f2655e3
> ("vfs: atomic f_pos accesses as per POSIX"). Since function fs
> endpoint files does not have the notion of the file position, switch
> to the stream mode. This will bypass the file position mutex and
> threads will be blocked in interruptible state for the function fs
> mutex.
>
> It should not affects user space as we are only changing the task state
> changes the task state from UNINTERRUPTIBLE to INTERRUPTIBLE while waiting
> for the USB transfers to be finished. However there is a slight change to
> the O_NONBLOCK behavior. Earlier threads that are using O_NONBLOCK are also
> getting blocked inside fdget_pos(). Now they reach to function fs and error
> code is returned. The non blocking behavior is actually honoured now.
>
> Signed-off-by: Pavankumar Kondeti <quic_pkondeti@xxxxxxxxxxx>
It might be worth noting that this also changes pread & pwrite which
will now return ESPIPE instead of succeeding but ignoring the offset
argument.
Those operations have always been broken and I doubt anyone is using
them, but there is slightly more user visible change than just the task
state.
With that said, I think this is a good change, so:
Reviewed-by: John Keeping <john@xxxxxxxxxxxx>
> ---
> v3:
> - Added more background and user space impact in the commit description
>
> v2:
> - Removed Change-Id tag
>
> drivers/usb/gadget/function/f_fs.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index e20c19a..3c584da 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -614,7 +614,7 @@ static int ffs_ep0_open(struct inode *inode, struct file *file)
> file->private_data = ffs;
> ffs_data_opened(ffs);
>
> - return 0;
> + return stream_open(inode, file);
> }
>
> static int ffs_ep0_release(struct inode *inode, struct file *file)
> @@ -1154,7 +1154,7 @@ ffs_epfile_open(struct inode *inode, struct file *file)
> file->private_data = epfile;
> ffs_data_opened(epfile->ffs);
>
> - return 0;
> + return stream_open(inode, file);
> }
>
> static int ffs_aio_cancel(struct kiocb *kiocb)
> --
> 2.7.4
>
