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Reviewed-by: Yeoreum Yun > 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 mode > > Fixes: c38a9ec2b2c1 ("coresight: etm4x: moving etm_drvdata::enable to atomic field") > Signed-off-by: Leo Yan > --- > 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 42e5d37403addc6ec81f2e3184522d67d1677c04..ee405c88ea5faa130819f96b00b8307f8764d58a 100644 > --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c > +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c > @@ -590,10 +590,23 @@ static int etm4_enable_hw(struct etmv4_drvdata *drvdata) > static void etm4_enable_hw_smp_call(void *info) > { > 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); > } > > /* > @@ -809,6 +822,9 @@ 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; > + > if (WARN_ON_ONCE(drvdata->cpu != smp_processor_id())) { > ret = -EINVAL; > goto out; > @@ -828,6 +844,9 @@ 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); > return ret; > } > > @@ -880,11 +899,6 @@ 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); > @@ -896,10 +910,6 @@ 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; > } > > @@ -951,10 +961,9 @@ 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; > @@ -991,6 +1000,15 @@ 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) > +{ > + 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) > { > @@ -1020,6 +1038,8 @@ 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. > @@ -1045,7 +1065,8 @@ 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); > > @@ -1085,9 +1106,6 @@ 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) > > -- > 2.34.1 > -- Sincerely, Yeoreum Yun