[bug report] Potential refcounting

[Ginger]

Dear Linux kernel maintainers,

My research-based static analyzer found a potential
refcounting/atomicity bug within the
'drivers/net/ethernet/mellanox/mlx4' subsystem, more specifically, in
'drivers/net/ethernet/mellanox/mlx4/cq.c'.

Kernel version: long-term kernel v6.18.9

Potential concurrent triggering executions:
T0:
mlx4_cq_tasklet_cb
     --> if (refcount_dec_and_test(&mcq->refcount))
     --> complete(&mcq->free)

T1:
mlx4_cq_completion
    --> cq->comp(cq);
        --> mlx4_add_cq_to_tasklet(struct mlx4_cq *cq)
            --> spin_lock_irqsave(&tasklet_ctx->lock, flags);
            --> refcount_inc(&cq->refcount);
            --> spin_unlock_irqrestore(&tasklet_ctx->lock, flags);

In T1, the refcounting increment on 'cq->refcount)', although within
the protection range of the 'tasklet_ctx->locl', is not synchronized
against T0 because 'refcount_inc()' does not check whether the
refcount has reached zero in T0. This case is potentially problematic
because T0 decrements he 'mcq->refcount' and can enable the
'mlx4_cq_free()' to proceed.

Thank you for your time and consideration.

Best regards,
Ginger