[PATCH] libcxgb: use raw_cpu_ptr() in ppm_get_cpu_entries()

[PATCH] libcxgb: use raw_cpu_ptr() in ppm_get_cpu_entries()

[Haoran Wang]

ppm_get_cpu_entries() disables preemption with get_cpu(), reads the
current CPU's pool with per_cpu_ptr(), then acquires spin_lock_bh()
while preemption is still disabled, and finally put_cpu().

    cpu = get_cpu();
    pool = per_cpu_ptr(ppm->pool, cpu);
    spin_lock_bh(&pool->lock);
    put_cpu();

On PREEMPT_RT kernels spin_lock_bh() becomes a sleeping lock
(rt_spin_lock).  get_cpu() raises preempt_count, so acquiring the
lock right after sleeps with preempt_count still positive.  Sleeping
in that state is the scheduling-while-atomic case, which under
CONFIG_DEBUG_ATOMIC_SLEEP reports a BUG and, if the lock is contended,
deadlocks on the spot.  The trigger path is any iSCSI offload I/O
through a Chelsio NIC that reaches ppm_get_cpu_entries().

The preemption disable exists only to stabilize the per-CPU accessor,
the pool is serialized by pool->lock, not by preemption.  Replace
get_cpu() + per_cpu_ptr() + put_cpu() with raw_smp_processor_id() +
raw_cpu_ptr(), which read the current CPU without disabling preemption.

On non-RT kernels spin_lock_bh() does not sleep, so the BUG does not
fire there; the defect is PREEMPT_RT-specific.

With the fix, preempt_count is 0 at spin_lock_bh(), so rt_spin_lock()'s
might_sleep() no longer triggers.

Signed-off-by: Haoran Wang <haoranwangsec@gmail.com>
---
 drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c b/drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c
index 2e3973a32..5f68c5d57 100644
--- a/drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c
+++ b/drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c
@@ -125,10 +125,9 @@ static int ppm_get_cpu_entries(struct cxgbi_ppm *ppm, unsigned int count,
 	if (!ppm->pool)
 		return -EINVAL;
 
-	cpu = get_cpu();
-	pool = per_cpu_ptr(ppm->pool, cpu);
+	cpu = raw_smp_processor_id();
+	pool = raw_cpu_ptr(ppm->pool);
 	spin_lock_bh(&pool->lock);
-	put_cpu();
 
 	i = ppm_find_unused_entries(pool->bmap, ppm->pool_index_max,
 				    pool->next, count, 0);

Re: [PATCH] libcxgb: use raw_cpu_ptr() in ppm_get_cpu_entries()

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

 - What hardware the change was tested on. For driver fixes please
   mention the device (and if relevant firmware version) used for
   testing, or say that the change was not tested on real hardware.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
other reasons, please include the information in the commit messages
then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.

Re: [PATCH] libcxgb: use raw_cpu_ptr() in ppm_get_cpu_entries()

[Sebastian Andrzej Siewior]

On 2026-10-19 21:22:27 [+0800], Haoran Wang wrote:
> diff --git a/drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c b/drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c
> index 2e3973a32..5f68c5d57 100644
> --- a/drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c
> +++ b/drivers/net/ethernet/chelsio/libcxgb/libcxgb_ppm.c
> @@ -125,10 +125,9 @@ static int ppm_get_cpu_entries(struct cxgbi_ppm *ppm, unsigned int count,
>  	if (!ppm->pool)
>  		return -EINVAL;
>  
> -	cpu = get_cpu();
> -	pool = per_cpu_ptr(ppm->pool, cpu);
> +	cpu = raw_smp_processor_id();
> +	pool = raw_cpu_ptr(ppm->pool);

cpu needs too be obtained after the lock has been acquired.
Please also explain why it is okay to obtain pool belonging to CPU1 on
CPU0.

>  	spin_lock_bh(&pool->lock);
> -	put_cpu();
>  
>  	i = ppm_find_unused_entries(pool->bmap, ppm->pool_index_max,
>  				    pool->next, count, 0);


Sebastian