The function rb_check_pages() validates the integrity of a specified per-CPU tracing ring buffer. It does so by traversing the underlying linked list and checking its next and prev links.
To guarantee that the list isn't modified during the check, a caller typically needs to take cpu_buffer->reader_lock. This prevents the check from running concurrently, for example, with a potential reader which can make the list temporarily inconsistent when swapping its old reader page into the buffer. A problem with this approach is that the time when interrupts are disabled is non-deterministic, dependent on the ring buffer size. This particularly affects PREEMPT_RT because the reader_lock is a raw spinlock which doesn't become sleepable on PREEMPT_RT kernels. Modify the check so it still attempts to traverse the entire list, but gives up the reader_lock between checking individual pages. Introduce for this purpose a new variable ring_buffer_per_cpu.pages_era which is bumped any time the list is modified. The value is used by rb_check_pages() to detect such a change and restart the check. Signed-off-by: Petr Pavlu <petr.pa...@suse.com> --- This is a follow-up to the discussion about improving this check [1]. Changes since v1 [2]: * Correct the case when rb_check_pages() is invoked concurrently with the resize code which modifies the list. Introduce ring_buffer_per_cpu.pages_era for this purpose. [1] https://lore.kernel.org/linux-trace-kernel/20240517134008.24529-1-petr.pa...@suse.com/ [2] https://lore.kernel.org/linux-trace-kernel/20240621150956.24814-1-petr.pa...@suse.com/ kernel/trace/ring_buffer.c | 97 ++++++++++++++++++++++++++++---------- 1 file changed, 71 insertions(+), 26 deletions(-) diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c index 28853966aa9a..ff9a98c06043 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -462,6 +462,7 @@ struct ring_buffer_per_cpu { unsigned long nr_pages; unsigned int current_context; struct list_head *pages; + unsigned long pages_era; /* list state era */ struct buffer_page *head_page; /* read from head */ struct buffer_page *tail_page; /* write to tail */ struct buffer_page *commit_page; /* committed pages */ @@ -1454,40 +1455,87 @@ static void rb_check_bpage(struct ring_buffer_per_cpu *cpu_buffer, RB_WARN_ON(cpu_buffer, val & RB_FLAG_MASK); } +static bool rb_check_links(struct ring_buffer_per_cpu *cpu_buffer, + struct list_head *list) +{ + if (RB_WARN_ON(cpu_buffer, + rb_list_head(rb_list_head(list->next)->prev) != list)) + return false; + + if (RB_WARN_ON(cpu_buffer, + rb_list_head(rb_list_head(list->prev)->next) != list)) + return false; + + return true; +} + /** * rb_check_pages - integrity check of buffer pages * @cpu_buffer: CPU buffer with pages to test * * As a safety measure we check to make sure the data pages have not * been corrupted. - * - * Callers of this function need to guarantee that the list of pages doesn't get - * modified during the check. In particular, if it's possible that the function - * is invoked with concurrent readers which can swap in a new reader page then - * the caller should take cpu_buffer->reader_lock. */ static void rb_check_pages(struct ring_buffer_per_cpu *cpu_buffer) { - struct list_head *head = rb_list_head(cpu_buffer->pages); - struct list_head *tmp; + struct list_head *head, *tmp; + unsigned long pages_era; + unsigned long flags; + int nr_loops = 0; - if (RB_WARN_ON(cpu_buffer, - rb_list_head(rb_list_head(head->next)->prev) != head)) + /* + * Walk the linked list underpinning the ring buffer and validate all + * its next and prev links. + * + * The check acquires the reader_lock to avoid concurrent processing + * with code that could be modifying the list. However, the lock cannot + * be held for the entire duration of the walk, as this would make the + * time when interrupts are disabled non-deterministic, dependent on the + * ring buffer size. Therefore, the code releases and re-acquires the + * lock after checking each page. The pages_era variable is then used to + * detect if the list was modified while the lock was not held, in which + * case the check needs to be restarted. + * + * The code attempts to perform the check at most three times before + * giving up. This is acceptable because this is only a self-validation + * to detect problems early on. In practice, the list modification + * operations are fairly spaced, and so this check typically succeeds at + * most on the second try. + */ +again: + if (++nr_loops > 3) return; - if (RB_WARN_ON(cpu_buffer, - rb_list_head(rb_list_head(head->prev)->next) != head)) - return; + raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags); + head = rb_list_head(cpu_buffer->pages); + if (!rb_check_links(cpu_buffer, head)) + goto out_locked; + pages_era = cpu_buffer->pages_era; + tmp = head; + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); - for (tmp = rb_list_head(head->next); tmp != head; tmp = rb_list_head(tmp->next)) { - if (RB_WARN_ON(cpu_buffer, - rb_list_head(rb_list_head(tmp->next)->prev) != tmp)) - return; + while (true) { + raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags); - if (RB_WARN_ON(cpu_buffer, - rb_list_head(rb_list_head(tmp->prev)->next) != tmp)) - return; + if (pages_era != cpu_buffer->pages_era) { + /* The list was updated, try again. */ + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); + goto again; + } + + tmp = rb_list_head(tmp->next); + if (tmp == head) + /* The iteration circled back, all is done. */ + goto out_locked; + + if (!rb_check_links(cpu_buffer, tmp)) + goto out_locked; + + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); } + +out_locked: + raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); } static int __rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer, @@ -1874,6 +1922,7 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages) /* make sure pages points to a valid page in the ring buffer */ cpu_buffer->pages = next_page; + cpu_buffer->pages_era++; /* update head page */ if (head_bit) @@ -1980,6 +2029,7 @@ rb_insert_pages(struct ring_buffer_per_cpu *cpu_buffer) * pointer to point to end of list */ head_page->prev = last_page; + cpu_buffer->pages_era++; success = true; break; } @@ -2215,12 +2265,8 @@ int ring_buffer_resize(struct trace_buffer *buffer, unsigned long size, */ synchronize_rcu(); for_each_buffer_cpu(buffer, cpu) { - unsigned long flags; - cpu_buffer = buffer->buffers[cpu]; - raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags); rb_check_pages(cpu_buffer); - raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); } atomic_dec(&buffer->record_disabled); } @@ -4611,6 +4657,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer) rb_list_head(reader->list.next)->prev = &cpu_buffer->reader_page->list; rb_inc_page(&cpu_buffer->head_page); + cpu_buffer->pages_era++; local_inc(&cpu_buffer->pages_read); /* Finally update the reader page to the new head */ @@ -5150,12 +5197,9 @@ void ring_buffer_read_finish(struct ring_buffer_iter *iter) { struct ring_buffer_per_cpu *cpu_buffer = iter->cpu_buffer; - unsigned long flags; /* Use this opportunity to check the integrity of the ring buffer. */ - raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags); rb_check_pages(cpu_buffer); - raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags); atomic_dec(&cpu_buffer->resize_disabled); kfree(iter->event); @@ -6050,6 +6094,7 @@ int ring_buffer_subbuf_order_set(struct trace_buffer *buffer, int order) cpu_buffer->pages = cpu_buffer->new_pages.next; cpu_buffer->new_pages.next->prev = cpu_buffer->new_pages.prev; cpu_buffer->new_pages.prev->next = cpu_buffer->new_pages.next; + cpu_buffer->pages_era++; /* Clear the new_pages list */ INIT_LIST_HEAD(&cpu_buffer->new_pages); base-commit: 1dfe225e9af5bd3399a1dbc6a4df6a6041ff9c23 -- 2.35.3
