On Fri, 14 Jun 2019 at 17:42, Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > > Hi, > > On Fri, Jun 14, 2019 at 4:56 AM Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote: > > > > > I was more worried about the safety of mmc_card_set_suspended() > > > itself. That is: > > > > > > #define mmc_card_set_suspended(c) ((c)->state |= MMC_STATE_SUSPENDED) > > > > > > ...so it's doing a read-modify-write of "state". Is that safe to do > > > without any type of locking? > > > > In this case, yes I think so. > > > > The point is, it really doesn't matter if the reader (work or thread), > > reads a non-updated value, because the synchronization is managed by > > the later mmc_claim_host() and the cancel_delayed_work_sync(). > > If this were just an "int" then perhaps, but this is a bitfield. So > if someone else updates the bitfield at the same time then we can > fully clobber their modification or they can clobber ours, right? > > task 1: load "state" from memory into CPU register on cpu0 > task 2: load "state" from memory into CPU register on cpu1 > task 1: OR in MMC_CARD_REMOVED > task 1: write "state" from CPU register on cpu0 > task 2: OR in MMC_STATE_SUSPENDED > task 2: write "state" from CPU register on cpu1 > > ...so now we've clobbered MMC_CARD_REMOVED. ...or am I just being > paranoid here and everything else in "state" is somehow guaranteed to > not be touched at the same time this function is running? I understand your concern. It's not obvious by looking at the code, but yes, there should be no other writing to the "state" at the same time mmc_sdio_supend() is running. MMC_CARD_REMOVED for example, is set from _mmc_detect_card_removed(), but because the detect work (mmc_recan()) has been disabled and the block device driver has been suspended, it can't be called. Anyway, perhaps we get reasons to add a lock to the card struct when going forward, but at this point I think we are fine. Kind regards Uffe
