Re: [PATCH v3] coresight: Fix possible deadlock in coresight_panic_cb

Suzuki K Poulose
2025-09-16T16:09:17Z

On 16/09/2025 17:00, Leo Yan wrote:
> On Mon, Sep 15, 2025 at 10:31:24AM -0400, Sean Anderson wrote:
>> On 9/15/25 05:58, Leo Yan wrote:
>>> On Fri, Sep 12, 2025 at 11:13:14AM -0400, Sean Anderson wrote:
>>>> coresight_panic_cb is called with interrupts disabled during panics.
>>>> However, bus_for_each_dev calls bus_to_subsys which takes
>>>> bus_kset->list_lock without disabling IRQs. This may cause a deadlock.
>>>
>>> I would rephrase it to make it clearer for anyone reading it later:
>>>
>>> coresight_panic_cb() is called during panics, which can preempt a flow
>>> that triggers exceptions (such as data or instruction aborts).
>>
>> I don't see what exceptions have to do with it. You can also panic
>> during a regular interrupt.
>
> The commit mentioned "without disabling IRQs" gives the impression that
> the deadlock is caused by IRQ-unsafe locking, which might mislead into
> thinking why the issue cannot be fixed with IRQ-safe locking.
>
> Regardless of whether IRQs are disabled, and regardless of the context
> (interrupt, bottom-half, or normal thread), the conditions for the
> deadlock are only about:
>
> (a) The bus lock has been acquired;
> (b) A panic is triggered to try to acquire the same lock.
>
> [...]
>
>>> When I review this patch, I recognize we can consolidate panic notifier
>>> in coresight-tmc-core.c, so we don't need to distribute the changes
>>> into ETF and ETR drivers (sorry if I misled you in my previous reply).
>>
>> And this kind of thing is why I went with the straightforward fix
>> initially. I do not want to bikeshed the extent that this gets removed.
>> IMO the whole "panic ops" stuff should be done directly with the panic
>> notifier, hence this patch. If you do not agree with that, then ack v2
>> and send a follow up of your own to fix it how you see fit.
>
> I would fix it in one go.
>
> I agree with you that "the whole panic ops stuff should be done directly
> with the panic". The only difference between us is that I would keep the
> `panic_ops` callback. To me, this encapsulates panic callbacks into
> different modules, to make the code more general.
>
> Could you check if the drafted patch below looks good to you? If so, I
> will send out a formal patch.
>
> ---8<---
>
> From ea78dd22cbdd97f709c5991d5bd3be97be6e137e Mon Sep 17 00:00:00 2001
> From: Sean Anderson <sean.anderson@linux.dev>
> Date: Tue, 16 Sep 2025 16:03:58 +0100
> Subject: [PATCH] coresight: Fix possible deadlock in coresight_panic_cb()
>
> coresight_panic_cb() is called during a panic. It invokes
> bus_for_each_dev(), which then calls bus_to_subsys() and takes the
> 'bus_kset->list_lock'. If a panic occurs after the lock has been
> acquired, it can lead to a deadlock.
>
> Instead of using a common panic notifier to iterate the bus, this commit
> directly registers the TMC device's panic notifier. This avoids bus
> iteration and effectively eliminates the race condition that could cause
> the deadlock.

Well, if you are going that far, why not register the notifier from
coresight-core ? We could always check the panic_ops->sync, and
then register/unregister the callback. Add the notifer to the csdev.
Otherwise, there is no point in keeping the panic_ops in
coresight_device, it is just for the TMC device.

Suzuki

>
> Fixes: 46006ceb5d02 ("coresight: core: Add provision for panic callbacks")
> Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
> Signed-off-by: Leo Yan <leo.yan@arm.com>
> ---
> drivers/hwtracing/coresight/coresight-core.c | 42 -------------------
> .../hwtracing/coresight/coresight-tmc-core.c | 26 ++++++++++++
> drivers/hwtracing/coresight/coresight-tmc.h | 2 +
> 3 files changed, 28 insertions(+), 42 deletions(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-core.c b/drivers/hwtracing/coresight/coresight-core.c
> index 3267192f0c1c..cb0cc8d77056 100644
> --- a/drivers/hwtracing/coresight/coresight-core.c
> +++ b/drivers/hwtracing/coresight/coresight-core.c
> @@ -21,7 +21,6 @@
> #include <linux/property.h>
> #include <linux/delay.h>
> #include <linux/pm_runtime.h>
> -#include <linux/panic_notifier.h>
>
> #include "coresight-etm-perf.h"
> #include "coresight-priv.h"
> @@ -1566,36 +1565,6 @@ const struct bus_type coresight_bustype = {
> .name = "coresight",
> };
>
> -static int coresight_panic_sync(struct device *dev, void *data)
> -{
> - int mode;
> - struct coresight_device *csdev;
> -
> - /* Run through panic sync handlers for all enabled devices */
> - csdev = container_of(dev, struct coresight_device, dev);
> - mode = coresight_get_mode(csdev);
> -
> - if ((mode == CS_MODE_SYSFS) || (mode == CS_MODE_PERF)) {
> - if (panic_ops(csdev))
> - panic_ops(csdev)->sync(csdev);
> - }
> -
> - return 0;
> -}
> -
> -static int coresight_panic_cb(struct notifier_block *self,
> - unsigned long v, void *p)
> -{
> - bus_for_each_dev(&coresight_bustype, NULL, NULL,
> - coresight_panic_sync);
> -
> - return 0;
> -}
> -
> -static struct notifier_block coresight_notifier = {
> - .notifier_call = coresight_panic_cb,
> -};
> -
> static int __init coresight_init(void)
> {
> int ret;
> @@ -1608,20 +1577,11 @@ static int __init coresight_init(void)
> if (ret)
> goto exit_bus_unregister;
>
> - /* Register function to be called for panic */
> - ret = atomic_notifier_chain_register(&panic_notifier_list,
> - &coresight_notifier);
> - if (ret)
> - goto exit_perf;
> -
> /* initialise the coresight syscfg API */
> ret = cscfg_init();
> if (!ret)
> return 0;
>
> - atomic_notifier_chain_unregister(&panic_notifier_list,
> - &coresight_notifier);
> -exit_perf:
> etm_perf_exit();
> exit_bus_unregister:
> bus_unregister(&coresight_bustype);
> @@ -1631,8 +1591,6 @@ static int __init coresight_init(void)
> static void __exit coresight_exit(void)
> {
> cscfg_exit();
> - atomic_notifier_chain_unregister(&panic_notifier_list,
> - &coresight_notifier);
> etm_perf_exit();
> bus_unregister(&coresight_bustype);
> }
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-core.c b/drivers/hwtracing/coresight/coresight-tmc-core.c
> index 36599c431be6..108ed9daf56d 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-core.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-core.c
> @@ -21,6 +21,7 @@
> #include <linux/slab.h>
> #include <linux/dma-mapping.h>
> #include <linux/spinlock.h>
> +#include <linux/panic_notifier.h>
> #include <linux/pm_runtime.h>
> #include <linux/of.h>
> #include <linux/of_address.h>
> @@ -769,6 +770,21 @@ static void register_crash_dev_interface(struct tmc_drvdata *drvdata,
> "Valid crash tracedata found\n");
> }
>
> +static int tmc_panic_cb(struct notifier_block *nb, unsigned long v, void *p)
> +{
> + struct tmc_drvdata *drvdata = container_of(nb, struct tmc_drvdata,
> + panic_notifier);
> + struct coresight_device *csdev = drvdata->csdev;
> +
> + if (coresight_get_mode(csdev) == CS_MODE_DISABLED)
> + return 0;
> +
> + if (panic_ops(csdev))
> + panic_ops(csdev)->sync(csdev);
> +
> + return 0;
> +}
> +
> static int __tmc_probe(struct device *dev, struct resource *res)
> {
> int ret = 0;
> @@ -885,6 +901,12 @@ static int __tmc_probe(struct device *dev, struct resource *res)
> goto out;
> }
>
> + if (panic_ops(drvdata->csdev)) {
> + drvdata->panic_notifier.notifier_call = tmc_panic_cb;
> + atomic_notifier_chain_register(&panic_notifier_list,
> + &drvdata->panic_notifier);
> + }
> +
> out:
> if (is_tmc_crashdata_valid(drvdata) &&
> !tmc_prepare_crashdata(drvdata))
> @@ -929,6 +951,10 @@ static void __tmc_remove(struct device *dev)
> {
> struct tmc_drvdata *drvdata = dev_get_drvdata(dev);
>
> + if (panic_ops(drvdata->csdev))
> + atomic_notifier_chain_unregister(&panic_notifier_list,
> + &drvdata->panic_notifier);
> +
> /*
> * Since misc_open() holds a refcount on the f_ops, which is
> * etb fops in this case, device is there until last file
> diff --git a/drivers/hwtracing/coresight/coresight-tmc.h b/drivers/hwtracing/coresight/coresight-tmc.h
> index cbb4ba439158..873c5427673c 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc.h
> +++ b/drivers/hwtracing/coresight/coresight-tmc.h
> @@ -243,6 +243,7 @@ struct tmc_resrv_buf {
> * (after crash) by default.
> * @crash_mdata: Reserved memory for storing tmc crash metadata.
> * Used by ETR/ETF.
> + * @panic_notifier: Notifier used to clean up during a panic
> */
> struct tmc_drvdata {
> struct clk *atclk;
> @@ -273,6 +274,7 @@ struct tmc_drvdata {
> struct etr_buf *perf_buf;
> struct tmc_resrv_buf resrv_buf;
> struct tmc_resrv_buf crash_mdata;
> + struct notifier_block panic_notifier;
> };
>
> struct etr_buf_operations {