[PATCH 4/5] kgdb: Use atomic operators which use barriers

[Jason Wessel]

A cpu_relax() does not mandate that there is an smp memory barrier.
As a result on the arm smp architecture the kernel debugger can hang
on entry from time to time, as shown by the kgdb regression tests.

The solution is simply to use the atomic operators which include a
proper smp memory barrier, instead of using atomic_set() and
atomic_read().

Tested-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
---
 kernel/kgdb.c |   21 ++++++++++-----------
 1 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/kernel/kgdb.c b/kernel/kgdb.c
index 6882c04..7a56dc9 100644
--- a/kernel/kgdb.c
+++ b/kernel/kgdb.c
@@ -1379,8 +1379,7 @@ acquirelock:
 	 * Make sure the above info reaches the primary CPU before
 	 * our cpu_in_kgdb[] flag setting does:
 	 */
-	smp_wmb();
-	atomic_set(&cpu_in_kgdb[cpu], 1);
+	atomic_inc(&cpu_in_kgdb[cpu]);
 
 	/*
 	 * CPU will loop if it is a slave or request to become a kgdb
@@ -1391,7 +1390,7 @@ acquirelock:
 			if (atomic_cmpxchg(&kgdb_active, -1, cpu) == cpu)
 				break;
 		} else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) {
-			if (!atomic_read(&passive_cpu_wait[cpu]))
+			if (!atomic_add_return(0, &passive_cpu_wait[cpu]))
 				goto return_normal;
 		} else {
 return_normal:
@@ -1400,7 +1399,7 @@ return_normal:
 			 */
 			if (arch_kgdb_ops.correct_hw_break)
 				arch_kgdb_ops.correct_hw_break();
-			atomic_set(&cpu_in_kgdb[cpu], 0);
+			atomic_dec(&cpu_in_kgdb[cpu]);
 			touch_softlockup_watchdog_sync();
 			clocksource_touch_watchdog();
 			local_irq_restore(flags);
@@ -1449,7 +1448,7 @@ return_normal:
 	 */
 	if (!kgdb_single_step) {
 		for (i = 0; i < NR_CPUS; i++)
-			atomic_set(&passive_cpu_wait[i], 1);
+			atomic_inc(&passive_cpu_wait[i]);
 	}
 
 #ifdef CONFIG_SMP
@@ -1462,7 +1461,7 @@ return_normal:
 	 * Wait for the other CPUs to be notified and be waiting for us:
 	 */
 	for_each_online_cpu(i) {
-		while (!atomic_read(&cpu_in_kgdb[i]))
+		while (!atomic_add_return(0, &cpu_in_kgdb[i]))
 			cpu_relax();
 	}
 
@@ -1483,17 +1482,17 @@ return_normal:
 	if (kgdb_io_ops->post_exception)
 		kgdb_io_ops->post_exception();
 
-	atomic_set(&cpu_in_kgdb[ks->cpu], 0);
+	atomic_dec(&cpu_in_kgdb[ks->cpu]);
 
 	if (!kgdb_single_step) {
 		for (i = NR_CPUS-1; i >= 0; i--)
-			atomic_set(&passive_cpu_wait[i], 0);
+			atomic_dec(&passive_cpu_wait[i]);
 		/*
 		 * Wait till all the CPUs have quit
 		 * from the debugger.
 		 */
 		for_each_online_cpu(i) {
-			while (atomic_read(&cpu_in_kgdb[i]))
+			while (atomic_add_return(0, &cpu_in_kgdb[i]))
 				cpu_relax();
 		}
 	}
@@ -1736,11 +1735,11 @@ EXPORT_SYMBOL_GPL(kgdb_unregister_io_module);
  */
 void kgdb_breakpoint(void)
 {
-	atomic_set(&kgdb_setting_breakpoint, 1);
+	atomic_inc(&kgdb_setting_breakpoint);
 	wmb(); /* Sync point before breakpoint */
 	arch_kgdb_breakpoint();
 	wmb(); /* Sync point after breakpoint */
-	atomic_set(&kgdb_setting_breakpoint, 0);
+	atomic_dec(&kgdb_setting_breakpoint);
 }
 EXPORT_SYMBOL_GPL(kgdb_breakpoint);
 
-- 
1.6.3.1.9.g95405b

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