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?
