From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> While the radix tree offers no ability to store IS_ERR pointers, documenting that the XArray does not led to some concern. Here is a sanctioned way to store errnos in the XArray. I'm concerned that it will confuse people who can't tell the difference between xa_is_err() and xa_is_errno(), so I've added copious kernel-doc to help them tell the difference. Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> --- Documentation/core-api/xarray.rst | 9 ++++--- include/linux/xarray.h | 46 +++++++++++++++++++++++++++++++++- tools/testing/radix-tree/xarray-test.c | 8 +++++- 3 files changed, 57 insertions(+), 6 deletions(-) diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst index 706081bfe92f..57a494026d96 100644 --- a/Documentation/core-api/xarray.rst +++ b/Documentation/core-api/xarray.rst @@ -42,11 +42,12 @@ When you retrieve an entry from the XArray, you can check whether it is a value entry by calling :c:func:`xa_is_value`, and convert it back to an integer by calling :c:func:`xa_to_value`. -The XArray does not support storing :c:func:`IS_ERR` pointers as some +The XArray does not support storing :c:func:`IS_ERR` pointers because some conflict with value entries or internal entries. If you need to store -error numbers in the array, you can store ``(errno << 2)`` as these values -will be aligned to a multiple of 4 and are not valid kernel pointers. -The values 4, 8, ... 4092 are also not valid kernel pointers. +error numbers in the array, you can encode them into error entries +with :c:func:`xa_mk_errno`, check whether a returned entry is an error +with :c:func:`xa_is_errno` and convert it back into an errno with +:c:func:`xa_to_errno`. An unusual feature of the XArray is the ability to create entries which occupy a range of indices. Once stored to, looking up any index in diff --git a/include/linux/xarray.h b/include/linux/xarray.h index bcc321fb280f..e0f8eb06b874 100644 --- a/include/linux/xarray.h +++ b/include/linux/xarray.h @@ -231,6 +231,50 @@ static inline bool xa_is_value(const void *entry) return (unsigned long)entry & 1; } +/** + * xa_mk_errno() - Create an XArray entry from an error number. + * @error: Error number to store in XArray. + * + * Return: An entry suitable for storing in the XArray. + */ +static inline void *xa_mk_errno(long error) +{ + return (void *)(error << 2); +} + +/** + * xa_to_errno() - Get error number stored in an XArray entry. + * @entry: XArray entry. + * + * Calling this function on an entry which is not an xa_is_errno() will + * yield unpredictable results. Do not confuse this function with xa_err(); + * this function is for errnos which have been stored in the XArray, and + * that function is for errors returned from the XArray implementation. + * + * Return: The error number stored in the XArray entry. + */ +static inline long xa_to_errno(const void *entry) +{ + return (long)entry >> 2; +} + +/** + * xa_is_errno() - Determine if an entry is an errno. + * @entry: XArray entry. + * + * Do not confuse this function with xa_is_err(); that function tells you + * whether the XArray implementation returned an error; this function + * tells you whether the entry you successfully stored in the XArray + * represented an errno. If you have never stored an errno in the XArray, + * you do not have to check this. + * + * Return: True if the entry is an errno, false if it is a pointer. + */ +static inline bool xa_is_errno(const void *entry) +{ + return (((unsigned long)entry & 3) == 0) && (entry > (void *)-4096); +} + /** * xa_is_internal() - Is the entry an internal entry? * @entry: Entry retrieved from the XArray @@ -277,7 +321,7 @@ static inline int xa_err(void *entry) /** * xa_store_empty() - Store this entry in the XArray unless another entry is - * already present. + * already present. * @xa: XArray. * @index: Index into array. * @entry: New entry. diff --git a/tools/testing/radix-tree/xarray-test.c b/tools/testing/radix-tree/xarray-test.c index 43111786ebdd..b843cedf3988 100644 --- a/tools/testing/radix-tree/xarray-test.c +++ b/tools/testing/radix-tree/xarray-test.c @@ -29,7 +29,13 @@ void check_xa_err(struct xarray *xa) assert(xa_err(xa_store(xa, 1, xa_mk_value(0), GFP_KERNEL)) == 0); assert(xa_err(xa_store(xa, 1, NULL, 0)) == 0); // kills the test-suite :-( -// assert(xa_err(xa_store(xa, 0, xa_mk_internal(0), 0)) == -EINVAL); +// assert(xa_err(xa_store(xa, 0, xa_mk_internal(0), 0)) == -EINVAL); + + assert(xa_err(xa_store(xa, 0, xa_mk_errno(-ENOMEM), GFP_KERNEL)) == 0); + assert(xa_err(xa_load(xa, 0)) == 0); + assert(xa_is_errno(xa_load(xa, 0)) == true); + assert(xa_to_errno(xa_load(xa, 0)) == -ENOMEM); + xa_erase(xa, 0); } void check_xa_tag(struct xarray *xa) -- 2.15.1 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html