In general it is not safe to call spin_lock() whilst executing in the kgdb trap handler. The trap can be entered from all sorts of execution context (NMI, IRQ, irqs disabled, etc) and the kgdb/kdb needs to be as resillient as possible.
Currently it is difficult to spot mistakes in the kgdb/kdb logic (especially so for kdb because it uses more kernel features than pure-kgdb). Let's provide a means to bring attention to deadlock risks in the debug code. Signed-off-by: Daniel Thompson <daniel.thomp...@linaro.org> --- include/linux/kgdb.h | 16 ++++++++++++++++ kernel/locking/spinlock_debug.c | 4 ++++ lib/Kconfig.kgdb | 11 +++++++++++ 3 files changed, 31 insertions(+) diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h index b072aeb1fd78..de30ce8078cf 100644 --- a/include/linux/kgdb.h +++ b/include/linux/kgdb.h @@ -332,4 +332,20 @@ extern void kgdb_panic(const char *msg); #define dbg_late_init() static inline void kgdb_panic(const char *msg) {} #endif /* ! CONFIG_KGDB */ + +#ifdef CONFIG_KGDB_DEBUG_SPINLOCK +/** + * check_kgdb_context_before() - Check whether to issue a spinlock warning + * + * Currently this only reports when the master processor violates the + * locking rules (because we are using the in_dbg_master() macro since + * we are confident that will avoid false positives). + * + * Return: True if we are executing in the debug trap + */ +static inline int check_kgdb_context_before(void) { return in_dbg_master(); } +#else +static inline int check_kgdb_context_before(void) { return 0; } +#endif + #endif /* _KGDB_H_ */ diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c index b9d93087ee66..b49789e0fed8 100644 --- a/kernel/locking/spinlock_debug.c +++ b/kernel/locking/spinlock_debug.c @@ -12,6 +12,7 @@ #include <linux/debug_locks.h> #include <linux/delay.h> #include <linux/export.h> +#include <linux/kgdb.h> void __raw_spin_lock_init(raw_spinlock_t *lock, const char *name, struct lock_class_key *key, short inner) @@ -84,6 +85,7 @@ debug_spin_lock_before(raw_spinlock_t *lock) SPIN_BUG_ON(READ_ONCE(lock->owner) == current, lock, "recursion"); SPIN_BUG_ON(READ_ONCE(lock->owner_cpu) == raw_smp_processor_id(), lock, "cpu recursion"); + SPIN_BUG_ON(check_kgdb_context_before(), lock, "in debug trap"); } static inline void debug_spin_lock_after(raw_spinlock_t *lock) @@ -174,6 +176,7 @@ int do_raw_read_trylock(rwlock_t *lock) void do_raw_read_unlock(rwlock_t *lock) { RWLOCK_BUG_ON(lock->magic != RWLOCK_MAGIC, lock, "bad magic"); + RWLOCK_BUG_ON(check_kgdb_context_before(), lock, "in debug trap"); arch_read_unlock(&lock->raw_lock); } @@ -183,6 +186,7 @@ static inline void debug_write_lock_before(rwlock_t *lock) RWLOCK_BUG_ON(lock->owner == current, lock, "recursion"); RWLOCK_BUG_ON(lock->owner_cpu == raw_smp_processor_id(), lock, "cpu recursion"); + RWLOCK_BUG_ON(check_kgdb_context_before(), lock, "in debug trap"); } static inline void debug_write_lock_after(rwlock_t *lock) diff --git a/lib/Kconfig.kgdb b/lib/Kconfig.kgdb index 933680b59e2d..4d57900d6c53 100644 --- a/lib/Kconfig.kgdb +++ b/lib/Kconfig.kgdb @@ -29,6 +29,17 @@ config KGDB_SERIAL_CONSOLE Share a serial console with kgdb. Sysrq-g must be used to break in initially. +config KGDB_DEBUG_SPINLOCK + bool "KGDB: Check for spin lock usage when system is halted" + select DEBUG_SPINLOCK + default n + help + Say Y here to catch spin lock waiting when we are running + in the kgdb trap handler and report it. When the trap handler + is executing all other system activity is halted and spin lock + contention will lead to deadlock. This makes any spin lock wait + from this execution context risky. + config KGDB_TESTS bool "KGDB: internal test suite" default n -- 2.25.4 _______________________________________________ Kgdb-bugreport mailing list Kgdb-bugreport@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kgdb-bugreport