data[] must be 64-bit aligned even on 32-bit architectures because it might be accessed by instructions that require aligned memory arguments.
One example is "atomic64_t" type accessed by special atomic instructions which may read/write entire 64-bit word.
Atomic instructions are a bit special compared to normal loads and stores. Even if normal loads and stores may deal with unaligned data, atomic instructions still require data to be aligned because it's hard to manage atomic value that spans through multiple cache lines or even MMU pages. And hardware just raises an alignment fault exception.
The problem with previously used approach is that depending on ABI "long long" type of a particular 32-bit CPU might be aligned to 8-, 16-, 32- or 64-bit boundary. Which will get in the way of mentioned above atomic instructions.
Consider the following snippet: | struct mystruct { | atomic64_t myvar; | } | | struct mystruct *p; | p = devm_kzalloc(dev, sizeof(*p), GFP_KERNEL);
Here address of "myvar" will match data[] in "struct devres", that said if "data" is not 64-bit aligned atomic instruction will fail on the first access to "myvar".
Signed-off-by: Alexey Brodkin abrodkin@synopsys.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Geert Uytterhoeven geert@linux-m68k.org Cc: David Laight David.Laight@ACULAB.COM Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Will Deacon will.deacon@arm.com Cc: Greg KH greg@kroah.com Cc: stable@vger.kernel.org # 4.8+ ---
Changes v2 -> v3:
* Align explicitly to 8 bytes [David] * Rephrased in-line comment [David] * Added more techinical details to commit message [Greg] * Mention more alignment options in commit message [Geert]
Changes v1 -> v2:
* Reworded commit message * Inserted comment right in source [Thomas]
drivers/base/devres.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/drivers/base/devres.c b/drivers/base/devres.c index f98a097e73f2..d65327cb83c9 100644 --- a/drivers/base/devres.c +++ b/drivers/base/devres.c @@ -24,8 +24,12 @@ struct devres_node {
struct devres { struct devres_node node; - /* -- 3 pointers */ - unsigned long long data[]; /* guarantee ull alignment */ + /* + * data[] must be 64 bit aligned even on 32 bit architectures + * because it might be accessed by instructions that require + * aligned memory arguments such as atomic64_t. + */ + u8 __aligned(8) data[]; };
struct devres_group {