On Thu, 6 Aug 2026 22:13:04 +0100 Vincent Donnefort <[email protected]> wrote:
> trace_buffer subbuf_size is read lockless in ring_buffer_read_page() and > ring_buffer_read_start(), while it can simultaneously be resized with > ring_buffer_subbuf_order_set(). > > Instead of trace_buffer::subbuf_size, use bpage::order in > ring_buffer_read_start() and ring_buffer_read_page(). > > In ring_buffer_read_start(), even with resize_disabled, there is still a > possibility of a race with a buffer modification. Hold the trace_buffer > mutex to synchronise with any pending ring buffer order modification. > > trace_buffer::subbuf_size is now actually useless, remove it. Also, > create accessors rb_subbuf_capacity() and rb_page_capacity() which > return the actual size available for storing events, while > rb_subbuf_size() returns the actual subbuf page-size. > > Reported-by: Sashiko <[email protected]> > Fixes: f9b94daa542a ("ring-buffer: Set new size of the ring buffer sub page") > Signed-off-by: Vincent Donnefort <[email protected]> In testing a bunch of fixes, This triggered. I haven't proven that this is the culprit (currently running my tests without this commit) but it looks to be the only one that touches ring_buffer_discard_commit() (actually rb_decrement_entry() called by ring_buffer_discard_commit()). [ 441.914171] WARNING: kernel/trace/ring_buffer.c:5049 at ring_buffer_discard_commit+0x3f8/0x440, CPU#3: ftrace-test-eve/4443 [ 441.918601] Modules linked in: [ 441.920136] CPU: 3 UID: 0 PID: 4443 Comm: ftrace-test-eve Not tainted 7.2.0-rc6-test-00014-g4940cbedc775-dirty #30 PREEMPT(lazy) [ 441.924738] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-debian-1.17.0-1 04/01/2014 [ 441.928532] RIP: 0010:ring_buffer_discard_commit+0x3f8/0x440 [ 441.930958] Code: 75 fd ff ff 48 8b 42 10 f0 ff 40 08 0f 0b e9 66 fd ff ff 41 83 e1 1f 41 80 f9 1d 74 2b 44 8b 5b 04 e9 ed fd ff ff f0 ff 47 08 <0f> 0b e9 9b fc ff ff 48 8b 42 10 f0 ff 40 08 0f 0b e9 11 ff ff ff [ 441.938138] RSP: 0018:ffffcd5681657970 EFLAGS: 00010002 [ 441.940410] RAX: 000000004083b000 RBX: ffff8b734083b010 RCX: ffff8b734082b540 [ 441.943337] RDX: ffff8b73408a8000 RSI: ffff8b734082b540 RDI: ffff8b734005fc00 [ 441.946270] RBP: ffff8b734005fc00 R08: ffff8b734005fc00 R09: 0000000000000000 [ 441.949203] R10: 0000000000000003 R11: 0000000000000044 R12: 0000000000000000 [ 441.952133] R13: 0000000000000000 R14: 0000000000000000 R15: ffffffffbd75cf40 [ 441.955056] FS: 00007feb18297780(0000) GS:ffff8b73fe646000(0000) knlGS:0000000000000000 [ 441.958428] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 441.960886] CR2: 000055e57b946e20 CR3: 0000000111b30006 CR4: 0000000000172ef0 [ 441.963855] Call Trace: [ 441.965158] <TASK> [ 441.966344] trace_event_buffer_commit+0x222/0x2b0 [ 441.968132] trace_event_raw_event_sched_switch+0x129/0x190 [ 441.970070] __traceiter_sched_switch+0x45/0x60 [ 441.971729] __schedule+0x8e5/0x14c0 [ 441.973115] schedule+0x3d/0x100 [ 441.974400] schedule_timeout+0xca/0x130 [ 441.975889] wait_for_completion+0xa5/0x160 [ 441.977438] synchronize_rcu_normal+0x2c9/0x370 [ 441.979083] ? trace_buffered_event_disable+0x63/0xf0 [ 441.980884] ? trace_function+0x35/0x140 [ 441.982355] ? __pfx_synchronize_rcu_normal+0x10/0x10 [ 441.984175] ? synchronize_rcu_normal+0x5/0x370 [ 441.985824] trace_buffered_event_disable+0x63/0xf0 [ 441.987576] __ftrace_event_enable_disable+0x148/0x2c0 [ 441.989395] try_unregister_trigger.isra.0+0xa9/0x110 [ 441.991188] event_trigger_parse+0xc1/0x1b0 [ 441.992748] trigger_process_regex+0xc3/0x110 [ 441.994352] event_trigger_write+0x7b/0xf0 [ 441.995884] vfs_write+0xd0/0x5a0 [ 441.997191] ? 0xffffffffc0400095 [ 441.998507] ksys_write+0x79/0xf0 [ 441.999828] do_syscall_64+0x7e/0x7d0 [ 442.001232] ? trace_hardirqs_off+0xd/0x30 [ 442.002754] entry_SYSCALL_64_after_hwframe+0x76/0x7e [ 442.004554] RIP: 0033:0x7feb1832c7d2 static inline void rb_decrement_entry(struct ring_buffer_per_cpu *cpu_buffer, struct ring_buffer_event *event) { unsigned long addr = (unsigned long)event; struct buffer_page *bpage = cpu_buffer->commit_page; struct buffer_page *start; addr &= ~(rb_subbuf_size(cpu_buffer->buffer) - 1); /* Do the likely case first */ if (likely(bpage->page == (void *)addr)) { local_dec(&bpage->entries); return; } /* * Because the commit page may be on the reader page we * start with the next page and check the end loop there. */ rb_inc_page(&bpage); start = bpage; do { if (bpage->page == (void *)addr) { local_dec(&bpage->entries); return; } rb_inc_page(&bpage); } while (bpage != start); /* commit not part of this buffer?? */ RB_WARN_ON(cpu_buffer, 1); <<<--- WARNING from this } This patch replaced the mask algorithm with the rb_subbuf_size(). If for some reason it is incorrect, the addr will not match. Note, this is the first time I ever triggered this warning. -- Steve
