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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: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Jason Wessel 2008-04-01 16:55:27 -05:00 committed by Ingo Molnar
parent 225a4424ad
commit 56fb709329
1 changed files with 8 additions and 19 deletions
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27
kernel/kgdb.c
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@ -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 @@ acquirelock:
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;
}