Am 26.03.2015 um 09:31 schrieb Stephen Rothwell:
> Hi Christian,
>
> After merging the access_once tree, today's linux-next build (arm
> multi_v7_defconfig) produced lots of this warning:
>
> In file included from include/linux/linkage.h:4:0,
> from include/linux/preempt.h:9,
> from include/linux/spinlock.h:50,
> from include/linux/lockref.h:17,
> from lib/lockref.c:2:
> In function '__read_once_size',
> inlined from 'lockref_get' at lib/lockref.c:50:2:
> include/linux/compiler.h:216:3: warning: call to 'data_access_exceeds_word_size' declared with attribute warning: data access exceeds word size and won't be atomic
> data_access_exceeds_word_size();
> ^
>
> Introduced by commit 6becd6bd5e89 ("compiler.h: Fix word size check for
> READ/WRITE_ONCE") presumably interacting with commit 4d3199e4ca8e
> ("locking: Remove ACCESS_ONCE() usage") from the tip tree.
>
The point of my patch was to actually re-enable the formerly broken check.
And indeed on arm 32 bit
we read a 64bit variable (lock_count). There is no possible way of doing that
in an atomic fashion with READ_ONCE, so the warning is probably correct
code in lib/lockref.c
#define CMPXCHG_LOOP(CODE, SUCCESS) do { \
struct lockref old; \
BUILD_BUG_ON(sizeof(old) != 8); \
old.lock_count = READ_ONCE(lockref->lock_count); \
while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) { \
struct lockref new = old, prev = old; \
CODE \
old.lock_count = cmpxchg64_relaxed(&lockref->lock_count, \
old.lock_count, \
new.lock_count); \
if (likely(old.lock_count == prev.lock_count)) { \
SUCCESS; \
} \
cpu_relax_lowlatency(); \
} \
} while (0)
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