From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
Thomas Gleixner <tglx@linutronix.de>,
Andrew Lutomirski <luto@kernel.org>,
Nadav Amit <namit@vmware.com>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
Nadav Amit <nadav.amit@gmail.com>
Subject: Re: [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and __this_cpu_{}()
Date: Wed, 27 Feb 2019 08:14:09 -0800 [thread overview]
Message-ID: <CAHk-=wiv6dgJDj81wukpJCJBVhAVn-CYJdhj+o8JGN1LfRwb0g@mail.gmail.com> (raw)
In-Reply-To: <20190227101544.210748832@infradead.org>
[-- Attachment #1: Type: text/plain, Size: 1078 bytes --]
On Wed, Feb 27, 2019 at 2:16 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Nadav Amit reported that commit:
>
> b59167ac7baf ("x86/percpu: Fix this_cpu_read()")
>
> added a bunch of constraints to all sorts of code; and while some of
> that was correct and desired, some of that seems superfluous.
Hmm.
I have the strong feeling that we should instead relax this_cpu_read()
again a bit.
In particular, making it "asm volatile" really is a big hammer
approach. It's worth noting that the *other* this_cpu_xyz ops don't
even do that.
I would suggest that instead of making "this_cpu_read()" be asm
volatile, we mark it as potentially changing the memory location it is
touching - the same way the modify/write ops do.
That still means that the read will be forced (like READ_ONCE()), but
allows gcc a bit more flexibility in instruction scheduling, I think.
Trivial (but entirely untested) patch attached.
That said, I didn't actually check how it affects code generation.
Nadav, would you check the code sequences you originally noticed?
Linus
[-- Attachment #2: patch.diff --]
[-- Type: text/x-patch, Size: 1348 bytes --]
arch/x86/include/asm/percpu.h | 24 ++++++++++++------------
1 file changed, 12 insertions(+), 12 deletions(-)
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 1a19d11cfbbd..63b0f361533f 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -185,24 +185,24 @@ do { \
typeof(var) pfo_ret__; \
switch (sizeof(var)) { \
case 1: \
- asm volatile(op "b "__percpu_arg(1)",%0"\
- : "=q" (pfo_ret__) \
- : "m" (var)); \
+ asm(op "b "__percpu_arg(1)",%0"\
+ : "=q" (pfo_ret__), \
+ "+m" (var)); \
break; \
case 2: \
- asm volatile(op "w "__percpu_arg(1)",%0"\
- : "=r" (pfo_ret__) \
- : "m" (var)); \
+ asm(op "w "__percpu_arg(1)",%0"\
+ : "=r" (pfo_ret__), \
+ "+m" (var)); \
break; \
case 4: \
- asm volatile(op "l "__percpu_arg(1)",%0"\
- : "=r" (pfo_ret__) \
- : "m" (var)); \
+ asm(op "l "__percpu_arg(1)",%0"\
+ : "=r" (pfo_ret__), \
+ "+m" (var)); \
break; \
case 8: \
- asm volatile(op "q "__percpu_arg(1)",%0"\
- : "=r" (pfo_ret__) \
- : "m" (var)); \
+ asm(op "q "__percpu_arg(1)",%0"\
+ : "=r" (pfo_ret__), \
+ "+m" (var)); \
break; \
default: __bad_percpu_size(); \
} \
next prev parent reply other threads:[~2019-02-27 16:14 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-27 10:12 [PATCH 0/5] x86/percpu semantics and fixes Peter Zijlstra
2019-02-27 10:12 ` [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and __this_cpu_{}() Peter Zijlstra
2019-02-27 16:14 ` Linus Torvalds [this message]
2019-02-27 16:48 ` Peter Zijlstra
2019-02-27 17:17 ` Linus Torvalds
2019-02-27 17:34 ` Peter Zijlstra
2019-02-27 17:38 ` Linus Torvalds
2019-02-27 17:57 ` Nadav Amit
2019-02-27 18:55 ` Nadav Amit
2019-02-27 19:41 ` Linus Torvalds
2019-03-08 13:35 ` Peter Zijlstra
2019-02-27 10:12 ` [PATCH 2/5] x86/percpu: Relax smp_processor_id() Peter Zijlstra
2019-02-27 10:12 ` [PATCH 3/5] x86/percpu, x86/irq: Relax {set,get}_irq_regs() Peter Zijlstra
2019-02-27 10:12 ` [PATCH 4/5] x86/percpu, x86/tlb: Relax cpu_tlbstate accesses Peter Zijlstra
2019-02-27 10:12 ` [PATCH 5/5] x86/percpu, sched/fair: Avoid local_clock() Peter Zijlstra
2019-02-27 10:24 ` [PATCH 6/5] x86/percpu: Optimize raw_cpu_xchg() Peter Zijlstra
2019-02-27 14:43 ` Peter Zijlstra
2019-02-27 23:16 ` [PATCH 0/5] x86/percpu semantics and fixes Nadav Amit
2019-03-08 14:50 ` Peter Zijlstra
2019-03-08 19:35 ` Nadav Amit
2019-03-08 20:56 ` Peter Zijlstra
2019-03-10 12:46 ` Peter Zijlstra
2019-03-08 22:48 ` Peter Zijlstra
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