From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
bp@alien8.de, Xiyu Yang <xiyuyang19@fudan.edu.cn>,
Alistair Popple <apopple@nvidia.com>,
Yang Shi <shy828301@gmail.com>,
Shakeel Butt <shakeelb@google.com>,
Hugh Dickins <hughd@google.com>,
Miaohe Lin <linmiaohe@huawei.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
yuanxzhang@fudan.edu.cn, Xin Tan <tanxin.ctf@gmail.com>,
Will Deacon <will.deacon@arm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
dhowells@redhat.com
Subject: Re: [PATCH] mm/rmap: Convert from atomic_t to refcount_t on anon_vma->refcount
Date: Thu, 19 Aug 2021 17:19:38 +0200 [thread overview]
Message-ID: <YR52igt/lJ7gQqOG@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YR5ldaQvAnCKMnkk@hirez.programming.kicks-ass.net>
On Thu, Aug 19, 2021 at 04:06:45PM +0200, Peter Zijlstra wrote:
> >
> > * We can implement (1) by checking if we hit zero (ZF=1)
> > * We can implement (2) by checking if the new value is < 0 (SF=1).
> > We then need to catch the case where the old value was < 0 but the
> > new value is 0. I think this is (SF=0 && OF=1).
> >
> > So maybe the second check is actually SF != OF? I could benefit from some
> > x86 expertise here, but hopefully you get the idea.
>
> Right, so the first condition is ZF=1, we hit zero.
> The second condition is SF=1, the result is negative.
>
> I'm not sure we need OF, if we hit that condition we've already lost.
> But it's easy enough to add I suppose.
If we can skip the OF... we can do something like this:
static inline bool refcount_dec_and_test(refcount_t *r)
{
asm_volatile_goto (LOCK_PREFIX "decl %[var]\n\t"
"jz %l[cc_zero]\n\t"
"jns 1f\n\t"
"ud1 %[var], %%ebx\n\t"
"1:"
: : [var] "m" (r->refs.counter)
: "memory" : cc_zero);
return false;
cc_zero:
smp_acquire__after_ctrl_dep();
return true;
}
where we encode the whole refcount_warn_saturate() thing into UD1. The
first argument is @r and the second argument the REFCOUNT_* thing
encoded in register space.
It would mean adding something 'clever' to the #UD handler that decodes
the trapping instruction and extracts these arguments, but this is the
smallest I could get it.
next prev parent reply other threads:[~2021-08-19 15:20 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-19 3:23 [PATCH] mm/rmap: Convert from atomic_t to refcount_t on anon_vma->refcount Xiyu Yang
2021-07-20 23:01 ` Andrew Morton
2021-08-19 13:21 ` Will Deacon
2021-08-19 14:06 ` Peter Zijlstra
2021-08-19 15:19 ` Peter Zijlstra [this message]
2021-08-19 19:09 ` Linus Torvalds
2021-08-20 6:43 ` Peter Zijlstra
2021-08-20 7:33 ` Kees Cook
2021-08-20 8:16 ` Peter Zijlstra
2021-08-20 8:24 ` Will Deacon
2021-08-20 9:03 ` Peter Zijlstra
2021-08-20 17:26 ` Kees Cook
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