[PATCH v4 2/3] coresight: tmc: refactor the tmc-etr mode setting to avoid race conditions
From: Junhao He
Date: Tue Nov 11 2025 - 07:27:18 EST
From: Junhao He <hejunhao3@xxxxxxxxxx>
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@xxxxxxxxxxxxx>
Closes: https://lore.kernel.org/linux-arm-kernel/20241202092419.11777-2-yangyicong@xxxxxxxxxx/
Signed-off-by: Junhao He <hejunhao3@xxxxxxxxxx>
Tested-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
Signed-off-by: Junhao He <hejunhao3@xxxxxxxxxxxxxx>
---
.../hwtracing/coresight/coresight-tmc-etr.c | 106 +++++++++---------
1 file changed, 55 insertions(+), 51 deletions(-)
diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
index b07fcdb3fe1a..fe1d5e8a0d2b 100644
--- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
+++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
@@ -1263,11 +1263,6 @@ 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) {
- ret = -EBUSY;
- goto out;
- }
-
/*
* If we don't have a buffer or it doesn't match the requested size,
* use the buffer allocated above. Otherwise reuse the existing buffer.
@@ -1278,7 +1273,6 @@ static struct etr_buf *tmc_etr_get_sysfs_buffer(struct coresight_device *csdev)
drvdata->sysfs_buf = new_buf;
}
-out:
raw_sp