On Tue, Aug 16, 2022 at 11:30:45PM +0900, Hector Martin wrote:
On 16/08/2022 23.04, Will Deacon wrote:
diff --git a/include/asm-generic/bitops/atomic.h b/include/asm-generic/bitops/atomic.h index 3096f086b5a3..71ab4ba9c25d 100644 --- a/include/asm-generic/bitops/atomic.h +++ b/include/asm-generic/bitops/atomic.h @@ -39,9 +39,6 @@ arch_test_and_set_bit(unsigned int nr, volatile unsigned long *p) unsigned long mask = BIT_MASK(nr); p += BIT_WORD(nr);
- if (READ_ONCE(*p) & mask)
return 1;- old = arch_atomic_long_fetch_or(mask, (atomic_long_t *)p); return !!(old & mask);
} @@ -53,9 +50,6 @@ arch_test_and_clear_bit(unsigned int nr, volatile unsigned long *p) unsigned long mask = BIT_MASK(nr); p += BIT_WORD(nr);
- if (!(READ_ONCE(*p) & mask))
return 0;- old = arch_atomic_long_fetch_andnot(mask, (atomic_long_t *)p); return !!(old & mask);
I suppose one sad thing about this is that, on arm64, we could reasonably keep the READ_ONCE() path with a DMB LD (R->RW) barrier before the return but I don't think we can express that in the Linux memory model so we end up in RmW territory every time.
You'd need a barrier *before* the READ_ONCE(), since what we're trying to prevent is a consumer from writing to the value without being able to observe the writes that happened prior, while this side read the old value. A barrier after the READ_ONCE() doesn't do anything, as that read is the last memory operation in this thread (of the problematic sequence).
Right, having gone back to your litmus test, I now realise it's the "SB" shape from the memory ordering terminology. It's funny because the arm64 acquire/release instructions are RCsc and so upgrading the READ_ONCE() to an *arm64* acquire instruction would work for your specific case, but only because the preceeding store is a release.
At that point, I'm not sure DMB LD / early read / LSE atomic would be any faster than just always doing the LSE atomic?
It depends a lot on the configuration of the system and the state of the relevant cacheline, but generally avoiding an RmW by introducing a barrier is likely to be a win. It just gets ugly here as we'd want to avoid the DMB in the case where we end up doing the RmW. Possibly we could do something funky like a test-and-test-and-test-and-set (!) where we do the DMB+READ_ONCE() only if the first READ_ONCE() has the bit set, but even just typing that is horrible and I'd _absolutely_ want to see perf numbers to show that it's a benefit once you start taking into account things like branch prediction.
Anywho, since Linus has applied the patch and it should work, this is just an interesting aside.
Will