[PATCH -rcu/dev 3/4] hazptr: Do not warn when a backup slot is released on another CPU

[Andrea Parri]

With CONFIG_HAZPTR_DEBUG, hazptr_release_debug() warns when a hazard
pointer is released on a CPU other than the one that acquired it,
unless ->detach_cpu is set.  hazptr_detach() sets it, and so does
hazptr_note_context_switch() when it promotes a per-CPU slot to the
backup slot.

__hazptr_acquire() can also put the hazard pointer in the backup slot
directly, when all per-CPU slots are in use or the pointer changes
under __hazptr_try_acquire().  Nothing sets ->detach_cpu then, so if
the task is migrated before the release, the check warns although the
backup slot can be released on any CPU.

>From hazptrtorture, with CPU hotplug and readers that nest a second
hazard pointer, as "hazptrtorture: Nest a second hazard pointer in
readers" makes them do (without the nesting, 30-minute PREEMPT and
NOPREEMPT runs do not hit it):

    Hazard Pointer (addr=00000000e2979b1e) released on remote cpu without context switch. Acquire: caller=hazptr_torture_read_lock+0x43/0xb0, pid=80, cpu=0. Release: pid=80, cpu=1.
    WARNING: ./include/linux/hazptr.h:293 at hazptr_torture_read_unlock+0x80/0x160, CPU#1: hazptr_torture_/80
    Call Trace:
     <TASK>
     hazptr_torture_reader_tail+0x88/0x250
     hazptr_torture_nested_reader+0x6c/0x90
     hazptr_torture_reader+0x2f5/0x550
     kthread+0xdf/0x110
     ret_from_fork+0x179/0x240
     ret_from_fork_asm+0x1a/0x30
     </TASK>

Set ->detach_cpu when __hazptr_acquire() falls back to the backup slot.
The remote-task check still applies to such a hazard pointer.

The false positive was found with the help of an LLM.

Fixes: 3d48872eb74f ("hazptr: Introduce CONFIG_HAZPTR_DEBUG misuse detection")
Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
---
 kernel/hazptr.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/hazptr.c b/kernel/hazptr.c
index b15b84ebd4ceb..b06164aac3a70 100644
--- a/kernel/hazptr.c
+++ b/kernel/hazptr.c
@@ -112,6 +112,10 @@ void *__hazptr_acquire(struct hazptr_ctx *ctx, void * const *addr_p)
 	 *   synchronize forward progress.
 	 */
 	slot = hazptr_chain_backup_slot(ctx);
+#ifdef CONFIG_HAZPTR_DEBUG
+	/* Unlike a per-CPU slot, the backup slot can be released on any CPU. */
+	ctx->detach_cpu = true;
+#endif
 	WRITE_ONCE(slot->addr, HAZPTR_WILDCARD);	/* Store B */
 
 	/* Memory ordering: Store B before Load A. */
-- 
2.53.0