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Commit 56fb7093 authored by Jason Wessel's avatar Jason Wessel Committed by Ingo Molnar
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kgdb: fix SMP NMI kgdb_handle_exception exit race 



Fix the problem of protecting the kgdb handle_exception exit
which had an NMI race condition, while trying to restore
normal system operation.

There was a small window after the master processor sets cpu_in_debug
to zero but before it has set kgdb_active to zero where a
non-master processor in an SMP system could receive an NMI and
re-enter the kgdb_wait() loop.

As long as the master processor sets the cpu_in_debug before sending
the cpu roundup the cpu_in_debug variable can also be used to guard
against the race condition.

The kgdb_wait() function no longer needs to check
kgdb_active because it is done in the arch specific code
and handled along with the nmi traps at the low level.
This also allows kgdb_wait() to exit correctly if it was
entered for some unknown reason due to a spurious NMI that
could not be handled by the arch specific code.

Signed-off-by: default avatarJason Wessel <jason.wessel@windriver.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 225a4424

kernel/kgdb.c

+8 −19
Original line number Diff line number Diff line
@@ -561,18 +561,6 @@ static void kgdb_wait(struct pt_regs *regs) 
	smp_wmb();

	atomic_set(&cpu_in_kgdb[cpu], 1);



	/*

	 * The primary CPU must be active to enter here, but this is

	 * guard in case the primary CPU had not been selected if

	 * this was an entry via nmi.

	 */

	while (atomic_read(&kgdb_active) == -1)

		cpu_relax();



	/* Wait till primary CPU goes completely into the debugger. */

	while (!atomic_read(&cpu_in_kgdb[atomic_read(&kgdb_active)]))

		cpu_relax();



	/* Wait till primary CPU is done with debugging */

	while (atomic_read(&passive_cpu_wait[cpu]))

		cpu_relax();
@@ -1447,18 +1435,18 @@ kgdb_handle_exception(int evector, int signo, int ecode, struct pt_regs *regs) 
			atomic_set(&passive_cpu_wait[i], 1);

	}



#ifdef CONFIG_SMP

	/* Signal the other CPUs to enter kgdb_wait() */

	if ((!kgdb_single_step || !kgdb_contthread) && kgdb_do_roundup)

		kgdb_roundup_cpus(flags);

#endif



	/*

	 * spin_lock code is good enough as a barrier so we don't

	 * need one here:

	 */

	atomic_set(&cpu_in_kgdb[ks->cpu], 1);



#ifdef CONFIG_SMP

	/* Signal the other CPUs to enter kgdb_wait() */

	if ((!kgdb_single_step || !kgdb_contthread) && kgdb_do_roundup)

		kgdb_roundup_cpus(flags);

#endif



	/*

	 * Wait for the other CPUs to be notified and be waiting for us:

	 */
@@ -1514,7 +1502,8 @@ int kgdb_nmicallback(int cpu, void *regs) 
{

#ifdef CONFIG_SMP

	if (!atomic_read(&cpu_in_kgdb[cpu]) &&

			atomic_read(&kgdb_active) != cpu) {

			atomic_read(&kgdb_active) != cpu &&

			atomic_read(&cpu_in_kgdb[atomic_read(&kgdb_active)])) {

		kgdb_wait((struct pt_regs *)regs);

		return 0;

	}