Re: [PATCH net] net/smc: fix potential sleeping issue in smc_switch_conns

[Paolo Abeni <pabeni@redhat.com>]

On Sat, 2024-04-13 at 11:51 +0800, Zhengchao Shao wrote:
> Potential sleeping issue exists in the following processes:
> smc_switch_conns
>   spin_lock_bh(&conn->send_lock)
>   smc_switch_link_and_count
>     smcr_link_put
>       __smcr_link_clear
>         smc_lgr_put
>           __smc_lgr_free
>             smc_lgr_free_bufs
>               __smc_lgr_free_bufs
>                 smc_buf_free
>                   smcr_buf_free
>                     smcr_buf_unmap_link
>                       smc_ib_put_memory_region
>                         ib_dereg_mr
>                           ib_dereg_mr_user
>                             mr->device->ops.dereg_mr
> If scheduling exists when the IB driver implements .dereg_mr hook
> function, the bug "scheduling while atomic" will occur. For example,
> cxgb4 and efa driver. Use mutex lock instead of spin lock to fix it.

I tried to inspect all the lock call sites, and it *look* like they are
all in process context, so the switch should be feasible.

Still the fact that the existing lock is a BH variant is suspect.
Either the BH part was not needed or this can introduce subtle
regressions/issues. 

I think this deserves at least a 3rd party testing.

Thanks,

Paolo