On 10/14/2023 2:15 AM, Yury Norov wrote:
Restore LKML
On Thu, Oct 12, 2023 at 02:21:10PM +0200, Jan Kara wrote:
On Wed 11-10-23 11:26:29, Yury Norov wrote:
Long story short: KCSAN found some potential issues related to how
people use bitmap API. And instead of working through that issues,
the following code shuts down KCSAN by applying READ_ONCE() here
and there.
I'm sorry but this is not what the patch does. I'm not sure how to get the
message across so maybe let me start from a different angle:
Bitmaps are perfectly fine to be used without any external locking if
only atomic bit ops (set_bit, clear_bit, test_and_{set/clear}_bit) are
used. This is a significant performance gain compared to using a spinlock
or other locking and people do this for a long time. I hope we agree on
that.
Now it is also common that you need to find a set / clear bit in a bitmap.
To maintain lockless protocol and deal with races people employ schemes
like (the dumbest form):
do {
bit = find_first_bit(bitmap, n);
if (bit >= n)
abort...
} while (!test_and_clear_bit(bit, bitmap));
So the code loops until it finds a set bit that is successfully cleared by
it. This is perfectly fine and safe lockless code and such use should be
supported. Agreed?
Great example. When you're running non-atomic functions concurrently,
the result may easily become incorrect, and this is what you're
demonstrating here.
Regarding find_first_bit() it means that:
- it may erroneously return unset bit;
- it may erroneously return non-first set bit;
- it may erroneously return no bits for non-empty bitmap.
Effectively it means that find_first bit may just return a random number.
Let's take another example:
do {
bit = get_random_number();
if (bit >= n)
abort...
} while (!test_and_clear_bit(bit, bitmap));
When running concurrently, the difference between this and your code
is only in probability of getting set bit somewhere from around the
beginning of bitmap.
The key point is that find_bit() may return undef even if READ_ONCE() is
used. If bitmap gets changed anytime in the process, the result becomes
invalid. It may happen even after returning from find_first_bit().
And if my understanding correct, your code is designed in the
assumption that find_first_bit() may return garbage, so handles it
correctly.
*Except* that the above actually is not safe due to find_first_bit()
implementation and KCSAN warns about that. The problem is that:
Assume *addr == 1
CPU1 CPU2
find_first_bit(addr, 64)
val = *addr;
if (val) -> true
clear_bit(0, addr)
val = *addr -> compiler decided to refetch addr contents for whatever
reason in the generated assembly
__ffs(val) -> now executed for value 0 which has undefined results.
Yes, __ffs(0) is undef. But the whole function is undef when accessing
bitmap concurrently.
And the READ_ONCE() this patch adds prevents the compiler from adding the
refetching of addr into the assembly.
That's true. But it doesn't improve on the situation. It was an undef
before, and it's undef after, but a 2% slower undef.
Now on that KCSAN warning. If I understand things correctly, for the
example above, KCSAN warning is false-positive, because you're
intentionally running lockless.
But for some other people it may be a true error, and now they'll have
no chance to catch it if KCSAN is forced to ignore find_bit() entirely.
We've got the whole class of lockless algorithms that allow safe concurrent
access to the memory. And now that there's a tool that searches for them
(concurrent accesses), we need to have an option to somehow teach it
to suppress irrelevant warnings. Maybe something like this?
lockless_algorithm_begin(bitmap, bitmap_size(nbits));
do {
bit = find_first_bit(bitmap, nbits);
if (bit >= nbits)
break;
} while (!test_and_clear_bit(bit, bitmap));
lockless_algorithm_end(bitmap, bitmap_size(nbits));
And, of course, as I suggested a couple iterations ago, you can invent
a thread-safe version of find_bit(), that would be perfectly correct
for lockless use:
unsigned long _find_and_clear_bit(volatile unsigned long *addr, unsigned long size)
{
unsigned long bit = 0;
while (!test_and_clear_bit(bit, bitmap) {
bit = FIND_FIRST_BIT(addr[idx], /* nop */, size);
if (bit >= size)
return size;
}
return bit;
}
Hi, Yuri,
But the code above effectively does the same as the READ_ONCE() macro
as defined in rwonce.h:
#ifndef __READ_ONCE
#define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x))
#endif
#define READ_ONCE(x) \
({ \
compiletime_assert_rwonce_type(x); \
__READ_ONCE(x); \
})
Both uses only prevent the funny stuff the compiler might have done to the
read of the addr[idx], there's no black magic in READ_ONCE().
Both examples would probably result in the same assembly and produce the
same 2% slowdown ...
Only you declare volatile in one place, and READ_ONCE() in each read, but
this will only compile a bit slower and generate the same machine code.
Best regards,
Mirsad Todorovac
Didn't test that, but I hope 'volatile' specifier should be enough
for compiler to realize that it shouldn't optimize memory access, and
for KCSAN that everything's OK here.
By the way, thank you for respectful and professional communication.
Thanks,
Yury
--
Mirsad Todorovac
Sistem inženjer
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb, Republic of Croatia
tel. +385 (0)1 3711 451
mob. +385 91 57 88 355
