Re: [PATCH] sched, s390: Fix the fallout of increasing the offset of 'thread_struct' within 'task_struct'

From: Ingo Molnar <mingo@kernel.org>
To: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>, Dave Hansen <dave@sr71.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH] sched, s390: Fix the fallout of increasing the offset of 'thread_struct' within 'task_struct'
Date: Mon, 20 Jul 2015 10:38:47 +0200	[thread overview]
Message-ID: <20150720083847.GC12468@gmail.com> (raw)
In-Reply-To: <20150720101219.79fb06bd@mschwide>


* Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:

> diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
> index 3238893..84062e7 100644
> --- a/arch/s390/kernel/entry.S
> +++ b/arch/s390/kernel/entry.S
> @@ -178,17 +178,21 @@ _PIF_WORK	= (_PIF_PER_TRAP)
>   */
>  ENTRY(__switch_to)
>  	stmg	%r6,%r15,__SF_GPRS(%r15)	# store gprs of prev task
> -	stg	%r15,__THREAD_ksp(%r2)		# store kernel stack of prev
> -	lg	%r4,__THREAD_info(%r2)		# get thread_info of prev
> -	lg	%r5,__THREAD_info(%r3)		# get thread_info of next
> +	lgr	%r1,%r2
> +	aghi	%r1,__TASK_thread		# thread_struct of prev task
> +	lg	%r4,__TASK_thread_info(%r2)	# get thread_info of prev
> +	lg	%r5,__TASK_thread_info(%r3)	# get thread_info of next
> +	stg	%r15,__THREAD_ksp(%r1)		# store kernel stack of prev
> +	lgr	%r1,%r3
> +	aghi	%r1,__TASK_thread		# thread_struct of next task
>  	lgr	%r15,%r5
>  	aghi	%r15,STACK_INIT			# end of kernel stack of next
>  	stg	%r3,__LC_CURRENT		# store task struct of next
>  	stg	%r5,__LC_THREAD_INFO		# store thread info of next
>  	stg	%r15,__LC_KERNEL_STACK		# store end of kernel stack
> +	lg	%r15,__THREAD_ksp(%r1)		# load kernel stack of next
>  	lctl	%c4,%c4,__TASK_pid(%r3)		# load pid to control reg. 4
>  	mvc	__LC_CURRENT_PID+4(4,%r0),__TASK_pid(%r3) # store pid of next
> -	lg	%r15,__THREAD_ksp(%r3)		# load kernel stack of next
>  	lmg	%r6,%r15,__SF_GPRS(%r15)	# load gprs of next task
>  	br	%r14

Btw., I think we'd be slightly better off with the variant I sent: that way the 
offset arithmetics are done in C code and are ready in the relevant registers by 
the time they are used in __switch_to().

>  
> @@ -417,6 +421,7 @@ ENTRY(pgm_check_handler)
>  	LAST_BREAK %r14
>  	lg	%r15,__LC_KERNEL_STACK
>  	lg	%r14,__TI_task(%r12)
> +	aghi	%r14,__TASK_thread	# pointer to thread_struct
>  	lghi	%r13,__LC_PGM_TDB
>  	tm	__LC_PGM_ILC+2,0x02	# check for transaction abort
>  	jz	2f

Don't we also need the chunk I have:

@@ -448,6 +449,7 @@ ENTRY(pgm_check_handler)
 	nill	%r10,0x007f
 	sll	%r10,2
 	je	.Lsysc_return
+	ahi     %r14,-__TASK_thread_struct # r14 now points to 'current'
 	lgf	%r1,0(%r10,%r1)		# load address of handler routine
 	lgr	%r2,%r11		# pass pointer to pt_regs
 	basr	%r14,%r1		# branch to interrupt-handler

Because the 'basr' line relies on 'r14' having task_struct, I think?

But I don't really know what I'm talking about here ...

Thanks,

	Ingo
