From: Steven Rostedt <rost...@goodmis.org> Make unwind_deferred_request() NMI-safe so tracers in NMI context can call it and safely request a user space stacktrace when the task exits.
Note, this is only allowed for architectures that implement a safe 64 bit cmpxchg. Which rules out some 32bit architectures and even some 64 bit ones. If an architecture requests a deferred stack trace from NMI context that does not support a safe NMI 64 bit cmpxchg, it will get an -EINVAL. For those architectures, they would need another method (perhaps an irqwork), to request a deferred user space stack trace. That can be dealt with later if one of theses architectures require this feature. Suggested-by: Peter Zijlstra <pet...@infradead.org> Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org> --- Changse since v10: https://lore.kernel.org/20250611010428.938845...@goodmis.org - Reworked to simply use a 64bit cmpxchg to update the timestamp. - Removed Josh Poimboeuf's authorship as the commit is completely rewritten. - Switch timestamp to local64_t type and pending to local_t type. include/linux/unwind_deferred.h | 4 +- include/linux/unwind_deferred_types.h | 7 ++- kernel/unwind/deferred.c | 74 ++++++++++++++++++++++----- 3 files changed, 69 insertions(+), 16 deletions(-) diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h index 26011b413142..718637777649 100644 --- a/include/linux/unwind_deferred.h +++ b/include/linux/unwind_deferred.h @@ -29,12 +29,12 @@ void unwind_deferred_cancel(struct unwind_work *work); static __always_inline void unwind_reset_info(void) { /* Exit out early if this was never used */ - if (likely(!current->unwind_info.timestamp)) + if (likely(!local64_read(¤t->unwind_info.timestamp))) return; if (current->unwind_info.cache) current->unwind_info.cache->nr_entries = 0; - current->unwind_info.timestamp = 0; + local64_set(¤t->unwind_info.timestamp, 0); } #else /* !CONFIG_UNWIND_USER */ diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h index 5df264cf81ad..0d722e877473 100644 --- a/include/linux/unwind_deferred_types.h +++ b/include/linux/unwind_deferred_types.h @@ -2,6 +2,9 @@ #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H +#include <asm/local64.h> +#include <asm/local.h> + struct unwind_cache { unsigned int nr_entries; unsigned long entries[]; @@ -10,8 +13,8 @@ struct unwind_cache { struct unwind_task_info { struct unwind_cache *cache; struct callback_head work; - u64 timestamp; - int pending; + local64_t timestamp; + local_t pending; }; #endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */ diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c index d5f2c004a5b0..dd36e58c8cad 100644 --- a/kernel/unwind/deferred.c +++ b/kernel/unwind/deferred.c @@ -12,6 +12,35 @@ #include <linux/slab.h> #include <linux/mm.h> +/* + * For requesting a deferred user space stack trace from NMI context + * the architecture must support a 64bit safe cmpxchg in NMI context. + * For those architectures that do not have that, then it cannot ask + * for a deferred user space stack trace from an NMI context. If it + * does, then it will get -EINVAL. + */ +#if defined(CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG) && \ + !defined(CONFIG_GENERIC_ATOMIC64) +# define CAN_USE_IN_NMI 1 +static inline u64 assign_timestamp(struct unwind_task_info *info, + u64 timestamp) +{ + u64 old = 0; + if (!local64_try_cmpxchg(&info->timestamp, &old, timestamp)) + timestamp = old; + return timestamp; +} +#else +# define CAN_USE_IN_NMI 0 +static inline u64 assign_timestamp(struct unwind_task_info *info, + u64 timestamp) +{ + /* For archs that do not allow NMI here */ + local64_set(&info->timestamp, timestamp); + return timestamp; +} +#endif + /* Make the cache fit in a 4K page */ #define UNWIND_MAX_ENTRIES \ ((SZ_4K - sizeof(struct unwind_cache)) / sizeof(long)) @@ -31,12 +60,21 @@ static LIST_HEAD(callbacks); */ static u64 get_timestamp(struct unwind_task_info *info) { + u64 timestamp; + lockdep_assert_irqs_disabled(); - if (!info->timestamp) - info->timestamp = local_clock(); + /* + * Note, the timestamp is generated on the first request. + * If it exists here, then the timestamp is earlier than + * this request and it means that this request will be + * valid for the stracktrace. + */ + timestamp = local64_read(&info->timestamp); + if (timestamp) + return timestamp; - return info->timestamp; + return assign_timestamp(info, local_clock()); } /** @@ -96,11 +134,11 @@ static void unwind_deferred_task_work(struct callback_head *head) struct unwind_work *work; u64 timestamp; - if (WARN_ON_ONCE(!info->pending)) + if (WARN_ON_ONCE(!local_read(&info->pending))) return; /* Allow work to come in again */ - WRITE_ONCE(info->pending, 0); + local_set(&info->pending, 0); /* * From here on out, the callback must always be called, even if it's @@ -111,7 +149,7 @@ static void unwind_deferred_task_work(struct callback_head *head) unwind_user_faultable(&trace); - timestamp = info->timestamp; + timestamp = local64_read(&info->timestamp); guard(mutex)(&callback_mutex); list_for_each_entry(work, &callbacks, list) { @@ -150,31 +188,43 @@ static void unwind_deferred_task_work(struct callback_head *head) int unwind_deferred_request(struct unwind_work *work, u64 *timestamp) { struct unwind_task_info *info = ¤t->unwind_info; + long pending; int ret; *timestamp = 0; - if (WARN_ON_ONCE(in_nmi())) - return -EINVAL; - if ((current->flags & (PF_KTHREAD | PF_EXITING)) || !user_mode(task_pt_regs(current))) return -EINVAL; + /* NMI requires having safe 64 bit cmpxchg operations */ + if (!CAN_USE_IN_NMI && in_nmi()) + return -EINVAL; + guard(irqsave)(); *timestamp = get_timestamp(info); /* callback already pending? */ - if (info->pending) + pending = local_read(&info->pending); + if (pending) return 1; + if (CAN_USE_IN_NMI) { + /* Claim the work unless an NMI just now swooped in to do so. */ + if (!local_try_cmpxchg(&info->pending, &pending, 1)) + return 1; + } else { + local_set(&info->pending, 1); + } + /* The work has been claimed, now schedule it. */ ret = task_work_add(current, &info->work, TWA_RESUME); - if (WARN_ON_ONCE(ret)) + if (WARN_ON_ONCE(ret)) { + local_set(&info->pending, 0); return ret; + } - info->pending = 1; return 0; } -- 2.47.2
