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

[Andrea Parri <parri.andrea@gmail.com>]

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@gmail.com>
---
 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