On Tue, Mar 13, 2018 at 4:08 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Tue, Mar 13, 2018 at 1:46 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 283 static __always_inline unsigned long
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 284 cmpxchg_size(volatile void *ptr, unsigned long old, unsigned long new, int size)
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 285 {
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 286 switch (size) {
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 287 case 1:
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 @288 return arch_cmpxchg((u8 *)ptr, (u8)old, (u8)new);
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 289 case 2:
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 290 return arch_cmpxchg((u16 *)ptr, (u16)old, (u16)new);
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 291 case 4:
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 292 return arch_cmpxchg((u32 *)ptr, (u32)old, (u32)new);
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 293 case 8:
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 294 BUILD_BUG_ON(sizeof(unsigned long) != 8);
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 295 return arch_cmpxchg((u64 *)ptr, (u64)old, (u64)new);
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 296 }
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 297 BUILD_BUG();
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 298 return 0;
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 299 }
>>> >> b06ed71a6 Dmitry Vyukov 2018-01-29 300
>>
>>> It seems that this is due to this guy:
>>>
>>> static __always_inline int trylock_clear_pending(struct qspinlock *lock)
>>> {
>>> struct __qspinlock *l = (void *)lock;
>>>
>>> return !READ_ONCE(l->locked) &&
>>> (cmpxchg_acquire(&l->locked_pending, _Q_PENDING_VAL,
>>> _Q_LOCKED_VAL) == _Q_PENDING_VAL);
>>> }
>>>
>>> _Q_PENDING_VAL is 0x100. However, locked_pending is 2 bytes. So it
>>> seems that compiler checks all switch cases, this inevitably will lead
>>> to such warnings.
So you are saying cmpxchg_size() was call with a size==2,
however sparse did not do the constant propagation or evaluation
properly. So that it did not eliminate the other branch of size==1
case statement.
Can you do a "sparse -E" on the original file, then save the
result to a new file. That will take care of the pre-processing.
Then run the sparse on the pre-processed file to get a smaller
test case?
Having a smaller test case would make it easier to reproduce
what exactly IR was issued during that warning.
>>> Off the top of my head I can think of the following solution:
>>>
>>> switch (size) {
>>> case 1:
>>> return arch_cmpxchg((u8 *)ptr, (u8)(old * (size !=
>>> 1)), (u8)(new * (size != 1)));
>>> case 2:
>>> return arch_cmpxchg((u16 *)ptr, (u16)(old * (size !=
>>> 2)), (u16)(new * (size != 2)));
>>>
>>> But it's too ugly.
>>
>> Yes agreed, that's horrendous.
Let's not do that. If it is a sparse problem, let's try to fix this sparse.
Chris
--
To unsubscribe from this list: send the line "unsubscribe linux-sparse" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
