From: Alexey Brodkin
Sent: 09 July 2018 05:45 Depending on ABI "long long" type of a particular 32-bit CPU might be aligned by either word (32-bits) or double word (64-bits). Make sure "data" is really 64-bit aligned for any 32-bit CPU.
At least for 32-bit ARC cores ABI requires "long long" types to be aligned by normal 32-bit word. This makes "data" field aligned to 12 bytes. Which is still OK as long as we use 32-bit data only.
But once we want to use native atomic64_t type (i.e. when we use special instructions LLOCKD/SCONDD for accessing 64-bit data) we easily hit misaligned access exception.
Shouldn't there be a typedef for the actual type. Perhaps it is even atomic64_t ? And have the __aligned(8) applied to that typedef ??
That's because even on CPUs capable of non-aligned data access LL/SC instructions require strict alignment.
...
--- 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 */
- /*
* Depending on ABI "long long" type of a particular 32-bit CPU* might be aligned by either word (32-bits) or double word (64-bits).* Make sure "data" is really 64-bit aligned for any 32-bit CPU.
Just: /* data[] must be 64 bit aligned even on 32 bit architectures * because it might be accessed by instructions that require * aligned memory arguments.
*/- unsigned long long data[] __aligned(sizeof(unsigned long long));
One day assuming that 'unsigned long long' is exactly 64 bits will bite. This probably ought to be u64 (or similar). (If not atomic64_t)
David