xtensa: add missing exclusive access state management

The result of the s32ex opcode is recorded in the ATOMCTL special
register and must be retrieved with the getex opcode. Context switch
between s32ex and getex may trash the ATOMCTL register and result in
duplicate update or missing update of the atomic variable.
Add atomctl8 field to the struct thread_info and use getex to swap
ATOMCTL bit 8 as a part of context switch.
Clear exclusive access monitor on kernel entry.

Cc: stable@vger.kernel.org
Fixes: f7c3487 ("xtensa: add exclusive atomics support")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/thread_info.h

@@ -55,6 +55,10 @@ struct thread_info {
 	mm_segment_t		addr_limit;	/* thread address space */
 
 	unsigned long		cpenable;
+#if XCHAL_HAVE_EXCLUSIVE
+	/* result of the most recent exclusive store */
+	unsigned long		atomctl8;
+#endif
 
 	/* Allocate storage for extra user states and coprocessor states. */
 #if XTENSA_HAVE_COPROCESSORS

arch/xtensa/kernel/asm-offsets.c

@@ -93,6 +93,9 @@ int main(void)
 	DEFINE(THREAD_RA, offsetof (struct task_struct, thread.ra));
 	DEFINE(THREAD_SP, offsetof (struct task_struct, thread.sp));
 	DEFINE(THREAD_CPENABLE, offsetof (struct thread_info, cpenable));
+#if XCHAL_HAVE_EXCLUSIVE
+	DEFINE(THREAD_ATOMCTL8, offsetof (struct thread_info, atomctl8));
+#endif
 #if XTENSA_HAVE_COPROCESSORS
 	DEFINE(THREAD_XTREGS_CP0, offsetof(struct thread_info, xtregs_cp.cp0));
 	DEFINE(THREAD_XTREGS_CP1, offsetof(struct thread_info, xtregs_cp.cp1));

arch/xtensa/kernel/entry.S

@@ -374,6 +374,11 @@ common_exception:
 	s32i	a2, a1, PT_LCOUNT
 #endif
 
+#if XCHAL_HAVE_EXCLUSIVE
+	/* Clear exclusive access monitor set by interrupted code */
+	clrex
+#endif
+
 	/* It is now save to restore the EXC_TABLE_FIXUP variable. */
 
 	rsr	a2, exccause
@@ -2020,6 +2025,12 @@ ENTRY(_switch_to)
 	s32i	a3, a4, THREAD_CPENABLE
 #endif
 
+#if XCHAL_HAVE_EXCLUSIVE
+	l32i	a3, a5, THREAD_ATOMCTL8
+	getex	a3
+	s32i	a3, a4, THREAD_ATOMCTL8
+#endif
+
 	/* Flush register file. */
 
 	spill_registers_kernel