Re: [PATCH RFC 0/7] rcu: v2 Inlinable preemptible rcu_read_lock() and rcu_read_unlock()
From: Linus Torvalds
Date: Sat Apr 14 2012 - 12:28:49 EST
On Sat, Apr 14, 2012 at 9:19 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>
> It also compiles this:
>
> void rcu_read_unlock_code(void)
> {
> rcu_read_unlock();
> }
>
> to this:
>
> 000000e0 <rcu_read_unlock_code>:
> e0: 64 a1 00 00 00 00 mov %fs:0x0,%eax
> e6: 83 f8 01 cmp $0x1,%eax
> e9: 74 0d je f8 <rcu_read_unlock_code+0x18>
> eb: 64 ff 0d 00 00 00 00 decl %fs:0x0
> f2: c3 ret
> f3: 90 nop
> f4: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
> f8: 64 c7 05 00 00 00 00 movl $0x80000000,%fs:0x0
> ff: 00 00 00 80
> 103: 64 a1 00 00 00 00 mov %fs:0x0,%eax
> 109: 85 c0 test %eax,%eax
> 10b: 75 0c jne 119 <rcu_read_unlock_code+0x39>
> 10d: 64 c7 05 00 00 00 00 movl $0x0,%fs:0x0
> 114: 00 00 00 00
> 118: c3 ret
> 119: 8d b4 26 00 00 00 00 lea 0x0(%esi,%eiz,1),%esi
> 120: e8 fc ff ff ff call 121 <rcu_read_unlock_code+0x41>
> 125: eb e6 jmp 10d <rcu_read_unlock_code+0x2d>
>
> It is therefore not at all clear to me that the final patch in this
> series is worthwhile.
Yeah, the read-unlock does not seem to be worth inlining as-is. As
mentioned earlier, it tends to be the rcu_read_lock() that we really
want to inline anyway (not just because it's small, but also because
the call clobbers registers, and that has other bad interaction).
Linus
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