Re: BUG: scheduling while atomic in f_fs when gadget remove driver

[Chen Yu <chenyu56@huawei.com>]

Hi Michal,

Thanks for the patch.

在 2016/9/29 5:38, Michal Nazarewicz 写道:
> On Wed, Sep 28 2016, Michal Nazarewicz wrote:
>> With that done, the only thing which needs a mutex is
>> epfile->read_buffer.
> 
> Perhaps this would do:
> 

I tested the patch on Hikey board with adb function on android, it does fix the problem.

thanks
Chen Yu

> ---- >8 -------------------------------------------------- -------------
>>From 6416a1065203a39328311f6c58083089efe169aa Mon Sep 17 00:00:00 2001
> From: Michal Nazarewicz <mina86@mina86.com>
> Date: Wed, 28 Sep 2016 23:36:56 +0200
> Subject: [RFC] usb: gadget: f_fs: stop sleeping in ffs_func_eps_disable
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> ffs_func_eps_disable is called from atomic context so it cannot sleep
> thus cannot grab a mutex.  Change the handling of epfile->read_buffer
> to use non-sleeping synchronisation method.
> 
> Reported-by: Chen Yu <chenyu56@huawei.com>
> Signed-off-by: Michał Nazarewicz <mina86@mina86.com>
> Fixes: 9353afbbfa7b ("buffer data from ‘oversized’ OUT requests")
> ---
>  drivers/usb/gadget/function/f_fs.c | 89 +++++++++++++++++++++++++++++++-------
>  1 file changed, 73 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index 759f5d4..8db53da 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -136,8 +136,50 @@ struct ffs_epfile {
>  	/*
>  	 * Buffer for holding data from partial reads which may happen since
>  	 * we’re rounding user read requests to a multiple of a max packet size.
> +	 *
> +	 * The pointer starts with NULL value and may be initialised to other
> +	 * value by __ffs_epfile_read_data function which may need to allocate
> +	 * the temporary buffer.
> +	 *
> +	 * In normal operation, subsequent calls to __ffs_epfile_read_buffered
> +	 * will consume data from the buffer and eventually free it.
> +	 * Importantly, while the function is using the buffer, it sets the
> +	 * pointer to NULL.  This is all right since __ffs_epfile_read_data and
> +	 * __ffs_epfile_read_buffered can never run concurrently (as they are
> +	 * protected by epfile->mutex) so the latter will not assign a new value
> +	 * to the buffer.
> +	 *
> +	 * Meanwhile __ffs_func_eps_disable frees the buffer (if the pointer is
> +	 * valid) and sets the pointer to READ_BUFFER_DROP value.  This special
> +	 * value is crux of the synchronisation between __ffs_func_eps_disable
> +	 * and __ffs_epfile_read_data.
> +	 *
> +	 * Once __ffs_epfile_read_data is about to finish it will try to set the
> +	 * pointer back to its old value (as described above), but seeing as the
> +	 * pointer is not-NULL (namely READ_BUFFER_DROP) it will instead free
> +	 * the buffer.
> +	 *
> +	 * This how concurrent calls to the two functions would look like (‘<->’
> +	 * denotes xchg operation):
> +	 *
> +	 *   read_buffer = some buffer
> +	 *
> +	 *      THREAD A                 THREAD B
> +	 *   __ffs_epfile_read_data:
> +	 *      buf = NULL
> +	 *      buf <-> read_buffer
> +	 *      … do stuff on buf …
> +	 *                            __ffs_func_eps_disable:
> +	 *                                buf = READ_BUFFER_DROP
> +	 *                                buf <-> read_buffer
> +	 *                                kfree(buf);
> +	 *
> +	 *      old = cmpxchg(read_buffer, NULL, buf)
> +	 *      if (old)
> +	 *          kfree(buf)
>  	 */
> -	struct ffs_buffer		*read_buffer;	/* P: epfile->mutex */
> +	struct ffs_buffer		*read_buffer;
> +#define READ_BUFFER_DROP ((struct ffs_buffer *)ERR_PTR(-ESHUTDOWN))
>  
>  	char				name[5];
>  
> @@ -740,21 +782,31 @@ static void ffs_epfile_async_io_complete(struct usb_ep *_ep,
>  static ssize_t __ffs_epfile_read_buffered(struct ffs_epfile *epfile,
>  					  struct iov_iter *iter)
>  {
> -	struct ffs_buffer *buf = epfile->read_buffer;
> +	/*
> +	 * Null out epfile->read_buffer so ffs_func_eps_disable does not free
> +	 * the buffer while we are using it.
> +	 */
> +	struct ffs_buffer *buf = xchg(&epfile->read_buffer, NULL);
>  	ssize_t ret;
> -	if (!buf)
> +	if (!buf || buf == READ_BUFFER_DROP)
>  		return 0;
>  
>  	ret = copy_to_iter(buf->data, buf->length, iter);
>  	if (buf->length == ret) {
>  		kfree(buf);
> -		epfile->read_buffer = NULL;
> -	} else if (unlikely(iov_iter_count(iter))) {
> +		return ret;
> +	}
> +
> +	if (unlikely(iov_iter_count(iter))) {
>  		ret = -EFAULT;
>  	} else {
>  		buf->length -= ret;
>  		buf->data += ret;
>  	}
> +
> +	if (cmpxchg(&epfile->read_buffer, NULL, buf))
> +		kfree(buf);
> +
>  	return ret;
>  }
>  
> @@ -783,7 +835,10 @@ static ssize_t __ffs_epfile_read_data(struct ffs_epfile *epfile,
>  	buf->length = data_len;
>  	buf->data = buf->storage;
>  	memcpy(buf->storage, data + ret, data_len);
> -	epfile->read_buffer = buf;
> +
> +	buf = xchg(&epfile->read_buffer, buf);
> +	if (buf && buf != READ_BUFFER_DROP)
> +		kfree(buf);
>  
>  	return ret;
>  }
> @@ -1094,11 +1149,14 @@ static int
>  ffs_epfile_release(struct inode *inode, struct file *file)
>  {
>  	struct ffs_epfile *epfile = inode->i_private;
> +	struct ffs_buffer *buf;
>  
>  	ENTER();
>  
> -	kfree(epfile->read_buffer);
> -	epfile->read_buffer = NULL;
> +	buf = xchg(&epfile->read_buffer, NULL);
> +	if (buf && buf != READ_BUFFER_DROP)
> +		kfree(buf);
> +
>  	ffs_data_closed(epfile->ffs);
>  
>  	return 0;
> @@ -1721,27 +1779,26 @@ static void ffs_func_eps_disable(struct ffs_function *func)
>  {
>  	struct ffs_ep *ep         = func->eps;
>  	struct ffs_epfile *epfile = func->ffs->epfiles;
> +	struct ffs_buffer *buf;
>  	unsigned count            = func->ffs->eps_count;
>  	unsigned long flags;
>  
> +	spin_lock_irqsave(&func->ffs->eps_lock, flags);
>  	do {
> -		spin_lock_irqsave(&func->ffs->eps_lock, flags);
>  		/* pending requests get nuked */
>  		if (likely(ep->ep))
>  			usb_ep_disable(ep->ep);
>  		++ep;
> -		if (epfile)
> -			epfile->ep = NULL;
> -		spin_unlock_irqrestore(&func->ffs->eps_lock, flags);
>  
>  		if (epfile) {
> -			mutex_lock(&epfile->mutex);
> -			kfree(epfile->read_buffer);
> -			epfile->read_buffer = NULL;
> -			mutex_unlock(&epfile->mutex);
> +			epfile->ep = NULL;
> +			buf = xchg(&epfile->read_buffer, READ_BUFFER_DROP);
> +			if (buf && buf != READ_BUFFER_DROP)
> +				kfree(buf);
>  			++epfile;
>  		}
>  	} while (--count);
> +	spin_unlock_irqrestore(&func->ffs->eps_lock, flags);
>  }
>  
>  static int ffs_func_eps_enable(struct ffs_function *func)
> ---- >8 -------------------------------------------------- -------------
> 
> Note: This has not been tested in *any* way.  It’s more to demonstrate
> the concept even though it is likely that it does actually work.
>