Messages in this thread | | | From | Pasha Tatashin <> | Date | Wed, 27 Oct 2021 14:22:08 -0400 | Subject | Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount |
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> I found some atomic_add/dec are replaced with atomic_add/dec_return,
I am going to replace -return variants with -fetch variants, potentially -fetch
> those helpers with return value imply a full memory barrier around it, but > others without return value do not. Do you have any numbers to show > the impact? Maybe atomic_add/dec_return_relaxed can help this.
The generic variant uses arch_cmpxchg() for all atomic variants without any extra barriers. Therefore, on platforms that use generic implementations there won't be performance differences except for an extra branch that checks results when VM_BUG_ON is enabled.
On x86 the difference between the two is the following
atomic_add: lock add %eax,(%rsi)
atomic_fetch_add: lock xadd %eax,(%rsi)
atomic_fetch_add_relaxed: lock xadd %eax,(%rsi)
No differences between relaxed and non relaxed variants. However, we used lock xadd instead of lock add. I am not sure if the performance difference is going to be different.
Pasha
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