Hi Suzuki

thanks for the review


On 2025/9/18 21:29, Suzuki K Poulose wrote:

On 18/09/2025 10:15, Yicong Yang wrote:

quoted

On 2025/8/18 16:05, Junhao He wrote:

quoted

When trying to run perf and sysfs mode simultaneously, the WARN_ON()
in tmc_etr_enable_hw() is triggered sometimes:

  WARNING: CPU: 42 PID: 3911571 at 
drivers/hwtracing/coresight/coresight-tmc-etr.c:1060 
tmc_etr_enable_hw+0xc0/0xd8 [coresight_tmc]
  [..snip..]
  Call trace:
   tmc_etr_enable_hw+0xc0/0xd8 [coresight_tmc] (P)
   tmc_enable_etr_sink+0x11c/0x250 [coresight_tmc] (L)
   tmc_enable_etr_sink+0x11c/0x250 [coresight_tmc]
   coresight_enable_path+0x1c8/0x218 [coresight]
   coresight_enable_sysfs+0xa4/0x228 [coresight]
   enable_source_store+0x58/0xa8 [coresight]
   dev_attr_store+0x20/0x40
   sysfs_kf_write+0x4c/0x68
   kernfs_fop_write_iter+0x120/0x1b8
   vfs_write+0x2c8/0x388
   ksys_write+0x74/0x108
   __arm64_sys_write+0x24/0x38
   el0_svc_common.constprop.0+0x64/0x148
   do_el0_svc+0x24/0x38
   el0_svc+0x3c/0x130
   el0t_64_sync_handler+0xc8/0xd0
   el0t_64_sync+0x1ac/0x1b0
  ---[ end trace 0000000000000000 ]---

Since the sysfs buffer allocation and the hardware enablement is not
in the same critical region, it's possible to race with the perf

mode:
   [sysfs mode]                   [perf mode]
   tmc_etr_get_sysfs_buffer()
     spin_lock(&drvdata->spinlock)
     [sysfs buffer allocation]
     spin_unlock(&drvdata->spinlock)
spin_lock(&drvdata->spinlock)
                                  tmc_etr_enable_hw()
                                    drvdata->etr_buf = 
etr_perf->etr_buf
spin_unlock(&drvdata->spinlock)
  spin_lock(&drvdata->spinlock)
  tmc_etr_enable_hw()
    WARN_ON(drvdata->etr_buf) // WARN sicne etr_buf initialized at
                                 the perf side
   spin_unlock(&drvdata->spinlock)

A race condition is introduced here, perf always prioritizes 
execution, and
warnings can lead to concerns about potential hidden bugs, such as 
getting
out of sync.

To fix this, configure the tmc-etr mode before invoking 
enable_etr_perf()
or enable_etr_sysfs(), explicitly check if the tmc-etr sink is already
enabled in a different mode, and simplily the setup and checks for 
"mode".
To prevent race conditions between mode transitions.

Fixes: 296b01fd106e ("coresight: Refactor out buffer allocation 
function for ETR")
Reported-by: Yicong Yang <yangyicong@hisilicon.com>
Closes: 
https://lore.kernel.org/linux-arm-kernel/20241202092419.11777-2-yangyicong@huawei.com/ (local)
Signed-off-by: Junhao He <redacted>

Tested by running perf and sysfs mode simultaneously, no warning 
reproduced.

Tested-by: Yicong Yang <yangyicong@hisilicon.com>

quoted

---
  .../hwtracing/coresight/coresight-tmc-etr.c   | 80 
++++++++++---------
  1 file changed, 42 insertions(+), 38 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c 

b/drivers/hwtracing/coresight/coresight-tmc-etr.c
index b07fcdb3fe1a..06c74717be19 100644

--- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
+++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c

@@ -1263,7 +1263,7 @@ static struct etr_buf 

*tmc_etr_get_sysfs_buffer(struct coresight_device *csdev)
          raw_spin_lock_irqsave(&drvdata->spinlock, flags);
      }
  -    if (drvdata->reading || coresight_get_mode(csdev) == 
CS_MODE_PERF) {
+    if (drvdata->reading) {
          ret = -EBUSY;
          goto out;
      }

@@ -1300,20 +1300,18 @@ static int tmc_enable_etr_sink_sysfs(struct 

coresight_device *csdev)
      raw_spin_lock_irqsave(&drvdata->spinlock, flags);
        /*
-     * In sysFS mode we can have multiple writers per sink. Since this
-     * sink is already enabled no memory is needed and the HW need 
not be
-     * touched, even if the buffer size has changed.
+     * When two sysfs sessions race to acquire an idle sink, both 
may enter
+     * this function. We need to recheck if the sink is already in 
use to
+     * prevent duplicate hardware configuration.
       */
-    if (coresight_get_mode(csdev) == CS_MODE_SYSFS) {
+    if (csdev->refcnt) {
          csdev->refcnt++;
          goto out;
      }
        ret = tmc_etr_enable_hw(drvdata, sysfs_buf);
-    if (!ret) {
-        coresight_set_mode(csdev, CS_MODE_SYSFS);
+    if (!ret)
          csdev->refcnt++;
-    }
    out:
      raw_spin_unlock_irqrestore(&drvdata->spinlock, flags);

@@ -1729,39 +1727,24 @@ static int tmc_enable_etr_sink_perf(struct 

coresight_device *csdev, void *data)
  {
      int rc = 0;
      pid_t pid;
-    unsigned long flags;
      struct tmc_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
      struct perf_output_handle *handle = data;
      struct etr_perf_buffer *etr_perf = etm_perf_sink_config(handle);
  -    raw_spin_lock_irqsave(&drvdata->spinlock, flags);
-     /* Don't use this sink if it is already claimed by sysFS */
-    if (coresight_get_mode(csdev) == CS_MODE_SYSFS) {
-        rc = -EBUSY;
-        goto unlock_out;
-    }
-
-    if (WARN_ON(!etr_perf || !etr_perf->etr_buf)) {
-        rc = -EINVAL;
-        goto unlock_out;
-    }
+    if (WARN_ON(!etr_perf || !etr_perf->etr_buf))
+        return -EINVAL;
        /* Get a handle on the pid of the session owner */
      pid = etr_perf->pid;
        /* Do not proceed if this device is associated with another 
session */
-    if (drvdata->pid != -1 && drvdata->pid != pid) {
-        rc = -EBUSY;
-        goto unlock_out;
-    }
+    if (drvdata->pid != -1 && drvdata->pid != pid)
+        return -EBUSY;
  -    /*
-     * No HW configuration is needed if the sink is already in
-     * use for this session.
-     */
+    /* The sink is already in use for this session */
      if (drvdata->pid == pid) {
          csdev->refcnt++;
-        goto unlock_out;
+        return rc;
      }
        rc = tmc_etr_enable_hw(drvdata, etr_perf->etr_buf);

@@ -1773,22 +1756,43 @@ static int tmc_enable_etr_sink_perf(struct 

coresight_device *csdev, void *data)
          csdev->refcnt++;
      }
  -unlock_out:
-    raw_spin_unlock_irqrestore(&drvdata->spinlock, flags);
      return rc;
  }
    static int tmc_enable_etr_sink(struct coresight_device *csdev,
                     enum cs_mode mode, void *data)
  {
-    switch (mode) {
-    case CS_MODE_SYSFS:
-        return tmc_enable_etr_sink_sysfs(csdev);
-    case CS_MODE_PERF:
-        return tmc_enable_etr_sink_perf(csdev, data);
-    default:
-        return -EINVAL;
+    struct tmc_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
+    int rc;
+
+    scoped_guard(raw_spinlock_irqsave, &drvdata->spinlock) {
+        if (coresight_get_mode(csdev) != CS_MODE_DISABLED &&
+            coresight_get_mode(csdev) != mode)
+            return -EBUSY;
+
+        switch (mode) {
+        case CS_MODE_SYSFS:
+            if (csdev->refcnt) {
+                /* The sink is already enabled */
+                csdev->refcnt++;
+                return 0;
+            }
+            coresight_set_mode(csdev, mode);

Why are we spilling bits here in the common code for sysfs ? More on 
this, see below.

This is to ensure that all operations on the mode are placed within a 
spinlock critical section.
If the above mode setting operation is removed, when the sysfs session 
and the perf session
enable the sink simultaneously, the sysfs session will release the 
spinlock multiple times during
the process of enabling the sink. The perf session can acquire the lock 
at multiple points in time
when the spinlock is released by sysfs and complete the entire sink 
enabling process.
Therefore, the sysfs session needs to check whether the sink is occupied 
by perf each time
before reacquiring the lock.

quoted

quoted

+            break;
+        case CS_MODE_PERF:
+            return tmc_enable_etr_sink_perf(csdev, data);
+        default:
+            return -EINVAL;
+        }
+    }
+
+    rc = tmc_enable_etr_sink_sysfs(csdev);

We now call the above function without the spinlock and the refcnt is
managed with and without the spinlock by the users. This is problematic,
with refcnt being a non-atomic type.

Please fix. I don't see why we can't set the mode in
tmc_enable_etr_sink_sysfs() with the locks held and
reset the mode if we failed enable it properly.

Suzuki

The internal of the `tmc_enable_etr_sink_sysfs()` function will acquire 
a spinlock.
It manages the `refcnt` within the spinlock context, ensuring that the 
operation is safe.

And tmc_enable_etr_sink_sysfs() will call tmc_etr_get_sysfs_buffer(), 
the function
`tmc_etr_get_sysfs_buffer()` will release the spinlock when allocating 
the buffer, so
it is not possible to move `tmc_enable_etr_sink_sysfs()` into the 
context of the spinlock.

Or is there another way to do it?

Best regards,
Junhao.

quoted

quoted

+    if (rc) {
+        scoped_guard(raw_spinlock_irqsave, &drvdata->spinlock)
+        coresight_set_mode(csdev, CS_MODE_DISABLED);
      }
+
+    return rc;
  }
    static int tmc_disable_etr_sink(struct coresight_device *csdev)

.