From: Michal Hocko <mhocko@kernel.org>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
"H. Peter Anvin" <hpa@zytor.com>,
"David S. Miller" <davem@davemloft.net>,
Tony Luck <tony.luck@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Chris Zankel <chris@zankel.net>,
Max Filippov <jcmvbkbc@gmail.com>,
x86@kernel.org, linux-alpha@vger.kernel.org,
linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org,
linux-sh@vger.kernel.org, sparclinux@vger.kernel.org,
linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org,
Michal Hocko <mhocko@suse.com>
Subject: [RFC 10/12] x86, rwsem: simplify __down_write
Date: Tue, 2 Feb 2016 21:19:27 +0100 [thread overview]
Message-ID: <1454444369-2146-11-git-send-email-mhocko@kernel.org> (raw)
Message-ID: <20160202201927.GhSNB8_GMdndPxkCR8hz3YFrH-zl99VT-zh16REjwPQ@z> (raw)
In-Reply-To: <1454444369-2146-1-git-send-email-mhocko@kernel.org>
From: Michal Hocko <mhocko@suse.com>
x86 implementation of __down_write is using inline asm to optimize the
code flow. This however requires that it has go over an additional hop
for the slow path call_rwsem_down_write_failed which has to
save_common_regs/restore_common_regs to preserve the calling convention.
This, however doesn't add much because the fast path only saves one
register push/pop (rdx) when compared to the generic implementation:
Before:
0000000000000019 <down_write>:
19: e8 00 00 00 00 callq 1e <down_write+0x5>
1e: 55 push %rbp
1f: 48 ba 01 00 00 00 ff movabs $0xffffffff00000001,%rdx
26: ff ff ff
29: 48 89 f8 mov %rdi,%rax
2c: 48 89 e5 mov %rsp,%rbp
2f: f0 48 0f c1 10 lock xadd %rdx,(%rax)
34: 85 d2 test %edx,%edx
36: 74 05 je 3d <down_write+0x24>
38: e8 00 00 00 00 callq 3d <down_write+0x24>
3d: 65 48 8b 04 25 00 00 mov %gs:0x0,%rax
44: 00 00
46: 5d pop %rbp
47: 48 89 47 38 mov %rax,0x38(%rdi)
4b: c3 retq
After:
0000000000000019 <down_write>:
19: e8 00 00 00 00 callq 1e <down_write+0x5>
1e: 55 push %rbp
1f: 48 b8 01 00 00 00 ff movabs $0xffffffff00000001,%rax
26: ff ff ff
29: 48 89 e5 mov %rsp,%rbp
2c: 53 push %rbx
2d: 48 89 fb mov %rdi,%rbx
30: f0 48 0f c1 07 lock xadd %rax,(%rdi)
35: 48 85 c0 test %rax,%rax
38: 74 05 je 3f <down_write+0x26>
3a: e8 00 00 00 00 callq 3f <down_write+0x26>
3f: 65 48 8b 04 25 00 00 mov %gs:0x0,%rax
46: 00 00
48: 48 89 43 38 mov %rax,0x38(%rbx)
4c: 5b pop %rbx
4d: 5d pop %rbp
4e: c3 retq
This doesn't seem to justify the code obfuscation and complexity. Use
the generic implementation instead.
Signed-off-by: Michal Hocko <mhocko@suse.com>
---
arch/x86/include/asm/rwsem.h | 17 +++++------------
arch/x86/lib/rwsem.S | 9 ---------
2 files changed, 5 insertions(+), 21 deletions(-)
diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h
index d79a218675bc..1b5e89b3643d 100644
--- a/arch/x86/include/asm/rwsem.h
+++ b/arch/x86/include/asm/rwsem.h
@@ -102,18 +102,11 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
static inline void __down_write(struct rw_semaphore *sem)
{
long tmp;
- asm volatile("# beginning down_write\n\t"
- LOCK_PREFIX " xadd %1,(%2)\n\t"
- /* adds 0xffff0001, returns the old value */
- " test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t"
- /* was the active mask 0 before? */
- " jz 1f\n"
- " call call_rwsem_down_write_failed\n"
- "1:\n"
- "# ending down_write"
- : "+m" (sem->count), "=d" (tmp)
- : "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS)
- : "memory", "cc");
+
+ tmp = atomic_long_add_return(RWSEM_ACTIVE_WRITE_BIAS,
+ (atomic_long_t *)&sem->count);
+ if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
+ rwsem_down_write_failed(sem);
}
/*
diff --git a/arch/x86/lib/rwsem.S b/arch/x86/lib/rwsem.S
index 40027db99140..ea5c7c177483 100644
--- a/arch/x86/lib/rwsem.S
+++ b/arch/x86/lib/rwsem.S
@@ -57,7 +57,6 @@
* is also the input argument to these helpers)
*
* The following can clobber %rdx because the asm clobbers it:
- * call_rwsem_down_write_failed
* call_rwsem_wake
* but %rdi, %rsi, %rcx, %r8-r11 always need saving.
*/
@@ -93,14 +92,6 @@ ENTRY(call_rwsem_down_read_failed)
ret
ENDPROC(call_rwsem_down_read_failed)
-ENTRY(call_rwsem_down_write_failed)
- save_common_regs
- movq %rax,%rdi
- call rwsem_down_write_failed
- restore_common_regs
- ret
-ENDPROC(call_rwsem_down_write_failed)
-
ENTRY(call_rwsem_wake)
/* do nothing if still outstanding active readers */
__ASM_HALF_SIZE(dec) %__ASM_HALF_REG(dx)
--
2.7.0
next prev parent reply other threads:[~2016-02-02 20:19 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-02 20:19 [RFC 0/12] introduce down_write_killable for rw_semaphore Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` [RFC 01/12] locking, rwsem: get rid of __down_write_nested Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` [RFC 02/12] locking, rwsem: drop explicit memory barriers Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` [RFC 03/12] locking, rwsem: introduce basis for down_write_killable Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` [RFC 04/12] alpha, rwsem: provide __down_write_killable Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` [RFC 05/12] ia64, " Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` [RFC 06/12] s390, " Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` [RFC 07/12] sh, " Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-03 11:19 ` Sergei Shtylyov
2016-02-03 12:11 ` Michal Hocko
2016-02-03 12:11 ` Michal Hocko
2016-02-02 20:19 ` [RFC 08/12] sparc, " Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` [RFC 09/12] xtensa, " Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-02 20:19 ` Michal Hocko [this message]
2016-02-02 20:19 ` [RFC 10/12] x86, rwsem: simplify __down_write Michal Hocko
2016-02-03 8:10 ` Ingo Molnar
2016-02-03 8:10 ` Ingo Molnar
2016-02-03 12:10 ` Michal Hocko
2016-02-03 12:10 ` Michal Hocko
2016-06-03 16:13 ` Peter Zijlstra
2016-06-03 22:34 ` Peter Zijlstra
2016-06-03 22:34 ` Peter Zijlstra
2016-06-09 14:40 ` David Howells
2016-06-09 14:40 ` David Howells
2016-06-09 17:36 ` Peter Zijlstra
2016-06-10 16:39 ` Paul E. McKenney
2016-06-10 16:39 ` Paul E. McKenney
2016-02-02 20:19 ` [RFC 11/12] x86, rwsem: provide __down_write_killable Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-17 16:41 ` [RFC 11/12 v1] " Michal Hocko
2016-02-17 16:41 ` Michal Hocko
2016-02-02 20:19 ` [RFC 12/12] locking, rwsem: provide down_write_killable Michal Hocko
2016-02-02 20:19 ` Michal Hocko
2016-02-19 12:15 ` [RFC 0/12] introduce down_write_killable for rw_semaphore Michal Hocko
2016-02-19 12:15 ` Michal Hocko
2016-03-09 12:18 ` Ingo Molnar
2016-03-09 12:18 ` Ingo Molnar
2016-03-09 12:56 ` Michal Hocko
2016-03-09 12:56 ` Michal Hocko
2016-03-09 13:17 ` Ingo Molnar
2016-03-09 13:28 ` Michal Hocko
2016-03-09 13:43 ` Ingo Molnar
2016-03-09 14:41 ` Michal Hocko
2016-03-10 10:24 ` Ingo Molnar
2016-03-10 10:24 ` Ingo Molnar
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=1454444369-2146-11-git-send-email-mhocko@kernel.org \
--to=mhocko@kernel.org \
--cc=akpm@linux-foundation.org \
--cc=chris@zankel.net \
--cc=davem@davemloft.net \
--cc=hpa@zytor.com \
--cc=jcmvbkbc@gmail.com \
--cc=linux-alpha@vger.kernel.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-ia64@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-s390@vger.kernel.org \
--cc=linux-sh@vger.kernel.org \
--cc=linux-xtensa@linux-xtensa.org \
--cc=mhocko@suse.com \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=sparclinux@vger.kernel.org \
--cc=tglx@linutronix.de \
--cc=tony.luck@intel.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).