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Commit eda0c6d6 authored by Martin Schwidefsky's avatar Martin Schwidefsky
Browse files

s390: fix race on TIF_MCCK_PENDING 



There is a small race window in the __switch_to code in regard to
the transfer of the TIF_MCCK_PENDING bit from the previous to the
next task. The bit is transferred before the task struct pointer
and the thread-info pointer for the next task has been stored to
lowcore. If a machine check sets the TIF_MCCK_PENDING bit between
the transfer code and the store of current/thread_info the bit
is still set for the previous task. And if the previous task has
terminated it can get lost. The effect is that a pending CRW is
not retrieved until the next machine checks sets TIF_MCCK_PENDING.
To fix this reorder __switch_to to first store the task struct
and thread-info pointer and then do the transfer of the bit.

Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
parent e5b8d755

arch/s390/kernel/entry.S

+11 −10
Original line number Diff line number Diff line
@@ -145,22 +145,23 @@ STACK_SIZE = 1 << STACK_SHIFT 
 *  gpr2 = prev

 */

ENTRY(__switch_to)

	stm	%r6,%r15,__SF_GPRS(%r15)	# store gprs of prev task

	st	%r15,__THREAD_ksp(%r2)		# store kernel stack of prev

	l	%r4,__THREAD_info(%r2)		# get thread_info of prev

	l	%r5,__THREAD_info(%r3)		# get thread_info of next

	lr	%r15,%r5

	ahi	%r15,STACK_SIZE			# end of kernel stack of next

	st	%r3,__LC_CURRENT		# store task struct of next

	st	%r5,__LC_THREAD_INFO		# store thread info of next

	st	%r15,__LC_KERNEL_STACK		# store end of kernel stack

	lctl	%c4,%c4,__TASK_pid(%r3)		# load pid to control reg. 4

	mvc	__LC_CURRENT_PID(4,%r0),__TASK_pid(%r3)	# store pid of next

	l	%r15,__THREAD_ksp(%r3)		# load kernel stack of next

	tm	__TI_flags+3(%r4),_TIF_MCCK_PENDING # machine check pending?

	jz	0f

	ni	__TI_flags+3(%r4),255-_TIF_MCCK_PENDING	# clear flag in prev

	oi	__TI_flags+3(%r5),_TIF_MCCK_PENDING	# set it in next

0:	stm	%r6,%r15,__SF_GPRS(%r15)	# store gprs of prev task

	st	%r15,__THREAD_ksp(%r2)		# store kernel stack of prev

	l	%r15,__THREAD_ksp(%r3)		# load kernel stack of next

	lctl	%c4,%c4,__TASK_pid(%r3)		# load pid to control reg. 4

	lm	%r6,%r15,__SF_GPRS(%r15)	# load gprs of next task

	st	%r3,__LC_CURRENT		# store task struct of next

	mvc	__LC_CURRENT_PID(4,%r0),__TASK_pid(%r3)	# store pid of next

	st	%r5,__LC_THREAD_INFO		# store thread info of next

	ahi	%r5,STACK_SIZE			# end of kernel stack of next

	st	%r5,__LC_KERNEL_STACK		# store end of kernel stack

0:	lm	%r6,%r15,__SF_GPRS(%r15)	# load gprs of next task

	br	%r14



__critical_start:

arch/s390/kernel/entry64.S

+11 −10
Original line number Diff line number Diff line
@@ -164,22 +164,23 @@ _TIF_EXIT_SIE = (_TIF_SIGPENDING | _TIF_NEED_RESCHED | _TIF_MCCK_PENDING) 
 *  gpr2 = prev

 */

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	%r15,%r5

	aghi	%r15,STACK_SIZE			# 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

	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

	tm	__TI_flags+7(%r4),_TIF_MCCK_PENDING # machine check pending?

	jz	0f

	ni	__TI_flags+7(%r4),255-_TIF_MCCK_PENDING	# clear flag in prev

	oi	__TI_flags+7(%r5),_TIF_MCCK_PENDING	# set it in next

0:	stmg	%r6,%r15,__SF_GPRS(%r15)	# store gprs of prev task

	stg	%r15,__THREAD_ksp(%r2)		# store kernel stack of prev

	lg	%r15,__THREAD_ksp(%r3)		# load kernel stack of next

	lctl	%c4,%c4,__TASK_pid(%r3)		# load pid to control reg. 4

	lmg	%r6,%r15,__SF_GPRS(%r15)	# load gprs of next task

	stg	%r3,__LC_CURRENT		# store task struct of next

	mvc	__LC_CURRENT_PID+4(4,%r0),__TASK_pid(%r3) # store pid of next

	stg	%r5,__LC_THREAD_INFO		# store thread info of next

	aghi	%r5,STACK_SIZE			# end of kernel stack of next

	stg	%r5,__LC_KERNEL_STACK		# store end of kernel stack

0:	lmg	%r6,%r15,__SF_GPRS(%r15)	# load gprs of next task

	br	%r14



__critical_start: