netdev - Re: [BUG] net: smc: possible deadlock in smc_lgr_free() and smc_link_down

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Date:   Tue, 1 Feb 2022 18:06:12 +0100
From:   Karsten Graul <kgraul@...ux.ibm.com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] net: smc: possible deadlock in smc_lgr_free() and
 smc_link_down_work()

On 01/02/2022 08:51, Jia-Ju Bai wrote:
> Hello,
> 
> My static analysis tool reports a possible deadlock in the smc module in Linux 5.16:
> 
> smc_lgr_free()
>   mutex_lock(&lgr->llc_conf_mutex); --> Line 1289 (Lock A)
>   smcr_link_clear()
>     smc_wr_free_link()
>       wait_event(lnk->wr_tx_wait, ...); --> Line 648 (Wait X)
> 
> smc_link_down_work()
>   mutex_lock(&lgr->llc_conf_mutex); --> Line 1683 (Lock A)
>   smcr_link_down()
>     smcr_link_clear()
>       smc_wr_free_link()
>         smc_wr_wakeup_tx_wait()
>           wake_up_all(&lnk->wr_tx_wait); --> Line 78 (Wake X)
> 
> When smc_lgr_free() is executed, "Wait X" is performed by holding "Lock A". If smc_link_down_work() is executed at this time, "Wake X" cannot be performed to wake up "Wait X" in smc_lgr_free(), because "Lock A" has been already hold by smc_lgr_free(), causing a possible deadlock.
> 
> I am not quite sure whether this possible problem is real and how to fix it if it is real.
> Any feedback would be appreciated, thanks :)

A deeper analysis showed up that this reported possible deadlock is actually not a problem.

The wait on line 648 in smc_wr.c
	wait_event(lnk->wr_tx_wait, (!atomic_read(&lnk->wr_tx_refcnt)));
waits as long as the refcount wr_tx_refcnt is not zero.

Every time when a caller stops using a link wr_tx_refcnt is decreased, and when it reaches 
zero the wr_tx_wait is woken up in smc_wr_tx_link_put() in smc_wr.h, line 70:
		if (atomic_dec_and_test(&link->wr_tx_refcnt))
			wake_up_all(&link->wr_tx_wait);

Multiple callers of smc_wr_tx_link_put() do not run under the llc_conf_mutex lock, and those
who run under this mutex are saved against the wait_event() in smc_wr_free_link().


Thank you for reporting this finding! Which tool did you use for this analysis?