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

Alejandro Lucero Palau wrote:
[..]

quoted

quoted

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".