linux-kernel - [PATCH] locking/rwsem: Fix comment on register clobbering

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Message-ID: <20160516093428.GA26108@pd.tnic>
Date:	Mon, 16 May 2016 11:34:28 +0200
From:	Borislav Petkov <bp@...e.de>
To:	x86-ml <x86@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Guenter Roeck <linux@...ck-us.net>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PATCH] locking/rwsem: Fix comment on register clobbering

On Fri, May 13, 2016 at 07:19:15PM +0200, Borislav Petkov wrote:
> The only thing that needs to be corrected then is the misleading comment
> above the 32-bit version "... Save the C-clobbered registers (%eax, %edx
> and %ecx) .." - the 64-bit version comment is correct AFAICT.

---
From: Borislav Petkov <bp@...e.de>
Date: Mon, 16 May 2016 11:29:22 +0200
Subject: [PATCH] locking/rwsem: Fix comment on register clobbering

Document explicitly that %edx can get clobbered on the slow path, on
32-bit. Something I learned the hard way. :-\

Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 arch/x86/lib/rwsem.S | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/lib/rwsem.S b/arch/x86/lib/rwsem.S
index a37462a23546..bb49caa4dd4c 100644
--- a/arch/x86/lib/rwsem.S
+++ b/arch/x86/lib/rwsem.S
@@ -29,8 +29,10 @@
  * there is contention on the semaphore.
  *
  * %eax contains the semaphore pointer on entry. Save the C-clobbered
- * registers (%eax, %edx and %ecx) except %eax whish is either a return
- * value or just clobbered..
+ * registers (%eax, %edx and %ecx) except %eax which is either a return
+ * value or just clobbered. Same is true for %edx so make sure gcc
+ * reloads it after the slow path, by making it hold a temporary, for
+ * example; see ____down_write().
  */
 
 #define save_common_regs \
-- 
2.7.3

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--