On 15/09/2025 11:33, Leo Yan wrote:
When enabling a tracer via SysFS interface, the device mode may be set by any CPU - not necessarily the target CPU. This can lead to race condition in SMP, and may result in incorrect mode values being read.
Consider the following example, where CPU0 attempts to enable the tracer on CPU1 (the target CPU):
CPU0 CPU1 etm4_enable() ` coresight_take_mode(SYSFS) ` etm4_enable_sysfs() ` smp_call_function_single() ----> etm4_enable_hw_smp_call() / / CPU idle: / etm4_cpu_save() / ` coresight_get_mode() Failed to enable h/w / ^^^ ` coresight_set_mode(DISABLED) <-' Read the intermediate SYSFS mode
In this case, CPU0 initiates the operation by taking the SYSFS mode to avoid conflicts with the Perf mode. It then sends an IPI to CPU1 to configure the tracer registers. If any error occurs during this process, CPU0 rolls back by setting the mode to DISABLED.
However, if CPU1 enters an idle state during this time, it might read the intermediate SYSFS mode. As a result, the CPU PM flow could wrongly save and restore tracer context that is actually disabled.
To resolve the issue, this commit moves the device mode setting logic on the target CPU. This ensures that the device mode is only modified by the target CPU, eliminating race condition between mode writes and reads across CPUs.
An additional change introduces the etm4_disable_hw_smp_call() function for SMP calls, which disables the tracer and explicitly set the mode to DISABLED during SysFS operations.
The flow is updated with this change:
CPU0 CPU1 etm4_enable() ` etm4_enable_sysfs() ` smp_call_function_single() ----> etm4_enable_hw_smp_call() ` coresight_take_mode(SYSFS) Failed, set back to DISABLED ` coresight_set_mode(DISABLED)
CPU idle: etm4_cpu_save() ` coresight_get_mode() ^^^ Read out the DISABLED modeFixes: c38a9ec2b2c1 ("coresight: etm4x: moving etm_drvdata::enable to atomic field") Reviewed-by: Yeoreum Yun yeoreum.yun@arm.com Signed-off-by: Leo Yan leo.yan@arm.com
drivers/hwtracing/coresight/coresight-etm4x-core.c | 48 +++++++++++++++------- 1 file changed, 33 insertions(+), 15 deletions(-)
diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c index 020f070bf17dc0557dfb3ae4f282b0a0c1778bd8..02ad41da7356547a67c53ff0a9146aec844f89da 100644 --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c @@ -592,10 +592,23 @@ static int etm4_enable_hw(struct etmv4_drvdata *drvdata) static void etm4_enable_hw_smp_call(void *info)
While at this, please could you rename this function to make it explicit that this is only for sysfs mode ?
e.g., etm4_enable_hw_sysfs_smp_call()
{ struct etm4_enable_arg *arg = info;
- struct coresight_device *csdev;
if (WARN_ON(!arg)) return;
- csdev = arg->drvdata->csdev;
- if (!coresight_take_mode(csdev, CS_MODE_SYSFS)) {
/* Someone is already using the tracer */arg->rc = -EBUSY;return;- }
- arg->rc = etm4_enable_hw(arg->drvdata);
- /* The tracer didn't start */
- if (arg->rc)
coresight_set_mode(csdev, CS_MODE_DISABLED);/* @@ -811,6 +824,9 @@ static int etm4_enable_perf(struct coresight_device *csdev, int ret = 0; struct etmv4_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
- if (!coresight_take_mode(csdev, CS_MODE_PERF))
return -EBUSY;
Should be done after the CPU check below ? Otherwise you are undoing the fix on sysfs side.
if (WARN_ON_ONCE(drvdata->cpu != smp_processor_id())) { ret = -EINVAL; goto out; @@ -830,6 +846,9 @@ static int etm4_enable_perf(struct coresight_device *csdev, ret = etm4_enable_hw(drvdata); out:
- /* The tracer didn't start */
- if (ret)
coresight_set_mode(csdev, CS_MODE_DISABLED);@@ -882,11 +901,6 @@ static int etm4_enable(struct coresight_device *csdev, struct perf_event *event, { int ret;
- if (!coresight_take_mode(csdev, mode)) {
/* Someone is already using the tracer */return -EBUSY;- }
- switch (mode) { case CS_MODE_SYSFS: ret = etm4_enable_sysfs(csdev, path);
@@ -898,10 +912,6 @@ static int etm4_enable(struct coresight_device *csdev, struct perf_event *event, ret = -EINVAL; }
- /* The tracer didn't start */
- if (ret)
coresight_set_mode(csdev, CS_MODE_DISABLED);- return ret; }
@@ -953,10 +963,9 @@ static void etm4_disable_trace_unit(struct etmv4_drvdata *drvdata) isb(); } -static void etm4_disable_hw(void *info) +static void etm4_disable_hw(struct etmv4_drvdata *drvdata) { u32 control;
- struct etmv4_drvdata *drvdata = info; struct etmv4_config *config = &drvdata->config; struct coresight_device *csdev = drvdata->csdev; struct csdev_access *csa = &csdev->access;
@@ -993,6 +1002,15 @@ static void etm4_disable_hw(void *info) "cpu: %d disable smp call done\n", drvdata->cpu); } +static void etm4_disable_hw_smp_call(void *info)
sysfs mode again.
Rest looks fine
Suzuki
+{
- struct etmv4_drvdata *drvdata = info;
- etm4_disable_hw(drvdata);
- coresight_set_mode(drvdata->csdev, CS_MODE_DISABLED);
+}
- static int etm4_disable_perf(struct coresight_device *csdev, struct perf_event *event) {
@@ -1022,6 +1040,8 @@ static int etm4_disable_perf(struct coresight_device *csdev, /* TRCVICTLR::SSSTATUS, bit[9] */ filters->ssstatus = (control & BIT(9));
- coresight_set_mode(drvdata->csdev, CS_MODE_DISABLED);
- /*
- perf will release trace ids when _free_aux() is
- called at the end of the session.
@@ -1047,7 +1067,8 @@ static void etm4_disable_sysfs(struct coresight_device *csdev) * Executing etm4_disable_hw on the cpu whose ETM is being disabled * ensures that register writes occur when cpu is powered. */
- smp_call_function_single(drvdata->cpu, etm4_disable_hw, drvdata, 1);
- smp_call_function_single(drvdata->cpu, etm4_disable_hw_smp_call,
drvdata, 1);raw_spin_unlock(&drvdata->spinlock); @@ -1087,9 +1108,6 @@ static void etm4_disable(struct coresight_device *csdev, etm4_disable_perf(csdev, event); break; }
- if (mode)
coresight_set_mode(csdev, CS_MODE_DISABLED);static int etm4_resume_perf(struct coresight_device *csdev)