Re: [BUG] net: smc: possible deadlock in smc_lgr_free() and smc_link_down_work()

[Jia-Ju Bai]

On 2022/2/2 1:06, Karsten Graul wrote:
> 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 :)

Hi Karsten,

Thanks for the reply and explanation :)

> 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);

Okay, you mean that wake_up_all(&link->wr_tx_wait) in 
smc_wr_tx_link_put() is used to wake up wait_event() in smc_wr_free_link().
But I wonder whether wake_up_all(&lnk->wr_tx_wait) in 
smc_wr_wakeup_tx_wait() can wake up this wait_event()?
If so, my report is in this case.

> 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().

In fact, my tool also reports some other possible deadlocks invovling 
smc_wr_tx_link_put(), which can be called by holding llc_conf_mutex.
There are three examples:

#BUG 1
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)

smcr_buf_unuse()
  mutex_lock(&lgr->llc_conf_mutex); --> Line 1087 (Lock A)
  smc_llc_do_delete_rkey()
    smc_llc_send_delete_rkey()
      smc_wr_tx_link_put()
        wake_up_all(&link->wr_tx_wait); --> Line 73 (Wake X)

#BUG 2
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()
    smc_llc_send_delete_link()
      smc_wr_tx_link_put()
        wake_up_all(&link->wr_tx_wait); --> Line 73 (Wake X)

#BUG 3
smc_llc_process_cli_delete_link()
  mutex_lock(&lgr->llc_conf_mutex); --> Line 1578 (Lock A)
  smc_llc_send_message()
    smc_llc_add_pending_send()
      smc_wr_tx_get_free_slot()
        wait_event_interruptible_timeout(link->wr_tx_wait, ...); --> 
Line 219 (Wake X)

smc_llc_process_cli_add_link()
  mutex_lock(&lgr->llc_conf_mutex); --> Line 1198 (Lock A)
  smc_llc_cli_add_link_invite()
    smc_llc_send_add_link()
      smc_wr_tx_link_put()
        wake_up_all(&link->wr_tx_wait); --> Line 73 (Wake X)

I am not quite sure whether these possible problems are real.
Any feedback would be appreciated, thanks :)

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

Thanks for your interest :)
I have implemented a static analysis tool based on LLVM, to detect 
deadlocks caused by locking cycles and improper waiting/waking operations.
However, this tool still reports some false positives, and thus I am 
still improving the accuracy of this tool.
Suggestions on deadlock detection (especially new/infrequent patterns 
causing deadlocks) or the tool are welcome ;)


Best wishes,
Jia-Ju Bai