linux-kernel - Re: [PATCH 2/2] ipc/sem.c: move wake_up

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Date:	Wed, 12 May 2010 19:34:31 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Zach Brown <zach.brown@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 2/2] ipc/sem.c: move wake_up_process out of the spinlock
 section

On 05/11/2010 11:21 PM, Andrew Morton wrote:
> On Wed, 28 Apr 2010 21:06:27 +0200
> Manfred Spraul<manfred@...orfullife.com>  wrote:
>
>    
>> The wake-up part of semtimedop() consists out of two steps:
>> - the right tasks must be identified.
>> - they must be woken up.
>>
>> Right now, both steps run while the array spinlock is held.
>> This patch reorders the code and moves the actual wake_up_process()
>> behind the point where the spinlock is dropped.
>>
>> The code also moves setting sem->sem_otime to one place: It does not
>> make sense to set the last modify time multiple times.
>>      
> ipc/sem.c: In function 'update_queue':
> ipc/sem.c:545: warning: 'retval' may be used uninitialized in this function
>
> which indeed was a bug.
>
>    
Duh - hiding in shame.

The effect would be that e.g. a semctl(SETALL) operation might change 
sem_otime.
semctl(SETALL) must only change sem_ctime (and sem_otime only if it 
causes a wakeup
and the woken up thread modifies the array)


> +++ a/ipc/sem.c
> @@ -542,7 +542,7 @@ static int update_queue(struct sem_array
>   	struct list_head *walk;
>   	struct list_head *pending_list;
>   	int offset;
> -	int retval;
> +	int retval = 0;
>
>   	/* if there are complex operations around, then knowing the semaphore
>   	 * that was modified doesn't help us. Assume that multiple semaphores
> _
>
> But I worry that the patch which you sent might not have been the one
> which you tested.
>    

I think I tested the patch that I sent out.

Thus I'll retest everything (including sem_ctime/sem_otime and SETALL)

The odd thing: My gcc somehow doesn't report the missing initialization :-(

 >>>
[manfred@...es linux-2.6]$ grep 'int update_queue' -A 10 ipc/sem.c
static int update_queue(struct sem_array *sma, int semnum, struct 
list_head *pt)
{
         struct sem_queue *q;
         struct list_head *walk;
         struct list_head *pending_list;
         int offset;
         int retval;

         /* if there are complex operations around, then knowing the 
semaphore
          * that was modified doesn't help us. Assume that multiple 
semaphores
          * were modified.
[manfred@...es linux-2.6]$ touch ipc/sem.o
[manfred@...es linux-2.6]$ make ipc
   CHK     include/linux/version.h
   CHK     include/generated/utsrelease.h
   CALL    scripts/checksyscalls.sh
   LD      ipc/built-in.o
[manfred@...es linux-2.6]$ gcc --version
gcc (GCC) 4.4.3 20100127 (Red Hat 4.4.3-4)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[manfred@...es linux-2.6]$ cat ipc/.sem.o.cmd
cmd_ipc/sem.o := gcc -Wp,-MD,ipc/.sem.o.d  -nostdinc -isystem 
/usr/lib/gcc/x86_64-redhat-linux/4.4.3/include 
-I/home/manfred/git/linux-2.6/arch/x86/include -Iinclude  -include 
include/generated/autoconf.h -D__KERNEL__ -Wall -Wundef 
-Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common 
-Werror-implicit-function-declaration -Wno-format-security 
-fno-delete-null-pointer-checks -Os -m64 -mtune=generic -mno-red-zone 
-mcmodel=kernel -funit-at-a-time -maccumulate-outgoing-args 
-DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -pipe -Wno-sign-compare 
-fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow 
-Wframe-larger-than=1024 -fno-stack-protector -fno-omit-frame-pointer 
-fno-optimize-sibling-calls -g -Wdeclaration-after-statement 
-Wno-pointer-sign -fno-strict-overflow -fno-dwarf2-cfi-asm 
-fconserve-stack   -D"KBUILD_STR(s)=\#s" 
-D"KBUILD_BASENAME=KBUILD_STR(sem)"  -D"KBUILD_MODNAME=KBUILD_STR(sem)"  
-c -o ipc/sem.o ipc/sem.c

<<<

--
     Manfred
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