evl/thread: hide exiting threads from sysfs attribute files

Threads are special elements in that they may exit independently from
the existence of their respective backing cdev. Make sure to hide
exiting threads from sysfs handlers before we dismantle things.

Prior to such fix, with lockdep enabled, we could receive this kernel
splat when stopping an EVL application while an evl-ps loop is
extracting data concurrently from the sysfs attributes related to an
exiting thread:

[  455.474409] DEBUG_LOCKS_WARN_ON(1)
[  455.474456] WARNING: CPU: 4 PID: 425 at kernel/locking/lockdep.c:249 __lock_acquire+0xa66/0xd10
[  455.474473] Modules linked in:
[  455.474480] CPU: 4 PID: 425 Comm: evl-ps Not tainted 5.15.64+ torvalds#369
[  455.474488] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-1.fc36 04/01/2014
[  455.474492] IRQ stage: Linux
[  455.474496] RIP: 0010:__lock_acquire+0xa66/0xd10
[  455.474504] Code: 48 b8 4a 83 e8 8b c3 25 00 8b 3d ad 3d 30 02 85 ff 0f 85 45 ff ff ff 48 c7 c6 40 e6 66 82 48 c7 c7 60 dc 66 82 e8 c5 f6 e3 00 <0f> 0b 31 ed e9 c7 f7 ff ff e8 8c 0f 62 00 85 c0 0f 84 f9 fe ff ff
[  455.474511] RSP: 0018:ffff888107cd7a78 EFLAGS: 00010082
[  455.474518] RAX: 0000000000000000 RBX: ffff888107276f98 RCX: 0000000000000000
[  455.474523] RDX: 0000000000000001 RSI: ffffffff8114d5a4 RDI: ffffed1020f9af45
[  455.474527] RBP: 0000000000000416 R08: 0000000000000001 R09: ffff8881f742d85b
[  455.474531] R10: ffffed103ee85b0b R11: 284e4f5f4e524157 R12: ffff888107276600
[  455.474536] R13: 0000000000000000 R14: 0000000000000000 R15: ffff888107276f20
[  455.474548] FS:  00007f485c93b740(0000) GS:ffff8881f7400000(0000) knlGS:0000000000000000
[  455.474554] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  455.474559] CR2: 00005653f7bb4e68 CR3: 0000000108d00000 CR4: 00000000000406e0
[  455.474564] Call Trace:
[  455.474567]  <TASK>
[  455.474570]  ? __clear_bit+0x25/0x40
[  455.474581]  lock_acquire+0x14c/0x3b0
[  455.474589]  ? sched_show+0x47/0x1b0
[  455.474598]  ? lock_downgrade+0xe0/0xe0
[  455.474605]  ? evl_get_element+0x5/0xa0
[  455.474613]  ? __test_and_set_bit.constprop.0+0xe/0x20
[  455.474622]  ? do_raw_spin_unlock+0x97/0xf0
[  455.474630]  ? device_remove_bin_file+0x20/0x20
[  455.474641]  sched_show+0x6f/0x1b0

Signed-off-by: Philippe Gerum <rpm@xenomai.org>

include/evl/factory.h

@@ -143,6 +143,14 @@ __evl_get_element_by_fundle(struct evl_index *map,
 			container_of(__e, __type, element); }) : NULL;	\
 	})
 
+/* Hide the element from sysfs operations. */
+static inline void evl_hide_element(struct evl_element *e)
+{
+	struct device *dev = e->dev;
+	if (dev)
+		dev_set_drvdata(dev, NULL);
+}
+
 static inline bool evl_element_is_public(struct evl_element *e)
 {
 	return !!(e->clone_flags & EVL_CLONE_PUBLIC);

kernel/evl/thread.c

@@ -277,6 +277,14 @@ static void uninit_thread(struct evl_thread *thread)
 	unsigned long flags;
 	struct evl_rq *rq;
 
+	/*
+	 * Threads are special elements in that they may exit
+	 * independently from the existence of their respective
+	 * backing cdev. Make sure to hide exiting threads from sysfs
+	 * handlers before we dismantle things.
+	 */
+	evl_hide_element(&thread->element);
+
 	evl_destroy_timer(&thread->rtimer);
 	evl_destroy_timer(&thread->ptimer);