The patch titled Subject: stackdepot: fix stack_depot_save_flags() in NMI context has been added to the -mm mm-hotfixes-unstable branch. Its filename is stackdepot-fix-stack_depot_save_flags-in-nmi-context.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Marco Elver elver@google.com Subject: stackdepot: fix stack_depot_save_flags() in NMI context Date: Fri, 22 Nov 2024 16:39:47 +0100

Per documentation, stack_depot_save_flags() was meant to be usable from NMI context if STACK_DEPOT_FLAG_CAN_ALLOC is unset. However, it still would try to take the pool_lock in an attempt to save a stack trace in the current pool (if space is available).

This could result in deadlock if an NMI is handled while pool_lock is already held. To avoid deadlock, only try to take the lock in NMI context and give up if unsuccessful.

The documentation is fixed to clearly convey this.

Link: https://lkml.kernel.org/r/Z0CcyfbPqmxJ9uJH@elver.google.com Link: https://lkml.kernel.org/r/20241122154051.3914732-1-elver@google.com Fixes: 4434a56ec209 ("stackdepot: make fast paths lock-less again") Signed-off-by: Marco Elver elver@google.com Reported-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Reviewed-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Cc: Alexander Potapenko glider@google.com Cc: Andrey Konovalov andreyknvl@gmail.com Cc: Dmitry Vyukov dvyukov@google.com Cc: Oscar Salvador osalvador@suse.de Cc: Vlastimil Babka vbabka@suse.cz Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/stackdepot.h | 6 +++--- lib/stackdepot.c | 10 +++++++++- 2 files changed, 12 insertions(+), 4 deletions(-)

--- a/include/linux/stackdepot.h~stackdepot-fix-stack_depot_save_flags-in-nmi-context +++ a/include/linux/stackdepot.h @@ -147,7 +147,7 @@ static inline int stack_depot_early_init * If the provided stack trace comes from the interrupt context, only the part * up to the interrupt entry is saved. * - * Context: Any context, but setting STACK_DEPOT_FLAG_CAN_ALLOC is required if + * Context: Any context, but unsetting STACK_DEPOT_FLAG_CAN_ALLOC is required if * alloc_pages() cannot be used from the current context. Currently * this is the case for contexts where neither %GFP_ATOMIC nor * %GFP_NOWAIT can be used (NMI, raw_spin_lock). @@ -156,7 +156,7 @@ static inline int stack_depot_early_init */ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries, unsigned int nr_entries, - gfp_t gfp_flags, + gfp_t alloc_flags, depot_flags_t depot_flags);

/** @@ -175,7 +175,7 @@ depot_stack_handle_t stack_depot_save_fl * Return: Handle of the stack trace stored in depot, 0 on failure */ depot_stack_handle_t stack_depot_save(unsigned long *entries, - unsigned int nr_entries, gfp_t gfp_flags); + unsigned int nr_entries, gfp_t alloc_flags);

/** * __stack_depot_get_stack_record - Get a pointer to a stack_record struct --- a/lib/stackdepot.c~stackdepot-fix-stack_depot_save_flags-in-nmi-context +++ a/lib/stackdepot.c @@ -630,7 +630,15 @@ depot_stack_handle_t stack_depot_save_fl prealloc = page_address(page); }

- raw_spin_lock_irqsave(&pool_lock, flags); + if (in_nmi()) { + /* We can never allocate in NMI context. */ + WARN_ON_ONCE(can_alloc); + /* Best effort; bail if we fail to take the lock. */ + if (!raw_spin_trylock_irqsave(&pool_lock, flags)) + goto exit; + } else { + raw_spin_lock_irqsave(&pool_lock, flags); + } printk_deferred_enter();

/* Try to find again, to avoid concurrently inserting duplicates. */ _

Patches currently in -mm which might be from elver@google.com are

stackdepot-fix-stack_depot_save_flags-in-nmi-context.patch