[PATCH] coresight: etb10: restore atomic_t for shared reading state

[Runyu Xiao]

The etb10 miscdevice uses drvdata->reading as a shared exclusivity gate
for userspace buffer access. etb_open() claims that gate with
local_cmpxchg(), and etb_release() clears it with local_set().

That gate is shared per-device state rather than CPU-local state. A
running system can reach it whenever /dev/<etb> is opened, closed, and
reopened by different tasks while the device remains registered, so the
same drvdata->reading variable may be claimed on one CPU and later
cleared on another.

This code used to use atomic_t for the same gate, but commit
27b10da8fff2 ("coresight: etb10: moving to local atomic operations")
changed it to local_t even though the access pattern remained cross-task
and cross-CPU. Restore atomic_t together with atomic_cmpxchg() and
atomic_set() so the exclusivity gate again uses a primitive intended
for shared state.

The issue was found on Linux v6.18.21 by our static analysis tool while
scanning surviving local_t-on-shared-state sites, and then manually
reviewed against the live etb10 file-op path.

It was runtime-validated with a reproducible QEMU no-device KCSAN PoC
that kept the same report-local contract:

  1. use one shared struct etb_drvdata carrier and its
     drvdata->reading gate;
  2. call etb_open() and etb_release() sequentially on that gate to
     confirm the original claim/clear path;
  3. bind the open side to CPU0 and the release side to CPU1 for the
     same gate to show cross-CPU ownership;
  4. run bound workers that repeatedly race etb_open() and
     etb_release() on the same gate until KCSAN reports a target hit.

The harness recorded:

  L1 passed open=1 release=1
  reading_after_open=1 reading_after_release=0
  L2 passed open_cpu=0 release_cpu=1
  cross_cpu_release=1 reading_after=0 open_ret=0

Representative KCSAN excerpt from the no-device validation run:

  BUG: KCSAN: data-race in etb_open.constprop.0.isra.0 [vuln_msv]

  write to 0xffffffffc0003810 of 4 bytes by task 216 on cpu 1:
   etb_open.constprop.0.isra.0+0x38/0x80 [vuln_msv]
   l3_worker_thread_fn+0x4f/0xf0 [vuln_msv]
   kthread+0x17e/0x1c0
   ret_from_fork+0x22/0x30

  read to 0xffffffffc0003810 of 4 bytes by task 215 on cpu 0:
   etb_open.constprop.0.isra.0+0x18/0x80 [vuln_msv]
   l3_worker_thread_fn+0x4f/0xf0 [vuln_msv]
   kthread+0x17e/0x1c0
   ret_from_fork+0x22/0x30

  value changed: 0x00000000 -> 0x00000001

  Reported by Kernel Concurrency Sanitizer on:
  CPU: 0 PID: 215 Comm: etb10_l3_a Tainted: G           O       6.1.66 #2

This no-device harness is not a real ETB10 hardware end-to-end run, but
it preserves the same shared drvdata->reading gate and the same
etb_open()/etb_release() claim/clear contract. No real ETB10 hardware
was available for runtime testing.

Build-tested with:
  make olddefconfig
  make -j"$(nproc)" drivers/hwtracing/coresight/coresight-etb10.o

Fixes: 27b10da8fff2 ("coresight: etb10: moving to local atomic operations")
Cc: stable at vger.kernel.org
Signed-off-by: Runyu Xiao <runyu.xiao at seu.edu.cn>
---
 drivers/hwtracing/coresight/coresight-etb10.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etb10.c b/drivers/hwtracing/coresight/coresight-etb10.c
index 35db1b6093d1..98269ea6f7ae 100644
--- a/drivers/hwtracing/coresight/coresight-etb10.c
+++ b/drivers/hwtracing/coresight/coresight-etb10.c
@@ -85,7 +85,7 @@ struct etb_drvdata {
 	struct coresight_device	*csdev;
 	struct miscdevice	miscdev;
 	raw_spinlock_t		spinlock;
-	local_t			reading;
+	atomic_t		reading;
 	pid_t			pid;
 	u8			*buf;
 	u32			buffer_depth;
@@ -603,7 +603,7 @@ static int etb_open(struct inode *inode, struct file *file)
 	struct etb_drvdata *drvdata = container_of(file->private_data,
 						   struct etb_drvdata, miscdev);
 
-	if (local_cmpxchg(&drvdata->reading, 0, 1))
+	if (atomic_cmpxchg(&drvdata->reading, 0, 1))
 		return -EBUSY;
 
 	dev_dbg(&drvdata->csdev->dev, "%s: successfully opened\n", __func__);
@@ -641,7 +641,7 @@ static int etb_release(struct inode *inode, struct file *file)
 {
 	struct etb_drvdata *drvdata = container_of(file->private_data,
 						   struct etb_drvdata, miscdev);
-	local_set(&drvdata->reading, 0);
+	atomic_set(&drvdata->reading, 0);
 
 	dev_dbg(&drvdata->csdev->dev, "%s: released\n", __func__);
 	return 0;
-- 
2.34.1