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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