Re: [PATCH v9 15/27] cxl: define a driver interface for HPA free space enumeration

Dan Williams
2025-01-21T23:44:28Z

Alejandro Lucero Palau wrote:
[..]
> >> So, I am not sure this code path has ever been tested as lockdep should
> >> complain about the double acquisition.
> >
> >
> > Oddly enough, it has been tested with two different drivers and with
> > the kernel configuring lockdep.
> >
> > It is worth to investigate ...
> >
>
> Confirmed the double lock is not an issue. Maybe the code hidden in
> those macros is checking if the current caller is the same one that the
> current owner of the lock. I will check that or investigate further.

Are you sure?

This splat:

============================================
WARNING: possible recursive locking detected
6.13.0-rc2+ #68 Tainted: G OE
--------------------------------------------
cat/1212 is trying to acquire lock:
ffffffffc0591cf0 (cxl_region_rwsem){++++}-{4:4}, at: decoders_committed_show+0x2a/0x90 [cxl_core]

but task is already holding lock:
ffffffffc0591cf0 (cxl_region_rwsem){++++}-{4:4}, at: decoders_committed_show+0x1e/0x90 [cxl_core]

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(cxl_region_rwsem);
lock(cxl_region_rwsem);

*** DEADLOCK ***

...results from this change:

diff --git a/drivers/cxl/core/port.c b/drivers/cxl/core/port.c
index 72950f631d49..9ebe9d46422b 100644
--- a/drivers/cxl/core/port.c
+++ b/drivers/cxl/core/port.c
@@ -560,9 +560,11 @@ static ssize_t decoders_committed_show(struct device *dev,
struct cxl_port *port = to_cxl_port(dev);
int rc;

+ down_read(&cxl_region_rwsem);
down_read(&cxl_region_rwsem);
rc = sysfs_emit(buf, "%d\n", cxl_num_decoders_committed(port));
up_read(&cxl_region_rwsem);
+ up_read(&cxl_region_rwsem);

return rc;
}

...and "cat /sys/bus/cxl/devices/port*/decoders_committed".