[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH v6 03/13] vpci: move lock outside of struct vpci
On 04.02.2022 13:37, Oleksandr Andrushchenko wrote: > > > On 04.02.22 13:37, Jan Beulich wrote: >> On 04.02.2022 12:13, Roger Pau Monné wrote: >>> On Fri, Feb 04, 2022 at 11:49:18AM +0100, Jan Beulich wrote: >>>> On 04.02.2022 11:12, Oleksandr Andrushchenko wrote: >>>>> On 04.02.22 11:15, Jan Beulich wrote: >>>>>> On 04.02.2022 09:58, Oleksandr Andrushchenko wrote: >>>>>>> On 04.02.22 09:52, Jan Beulich wrote: >>>>>>>> On 04.02.2022 07:34, Oleksandr Andrushchenko wrote: >>>>>>>>> @@ -285,6 +286,12 @@ static int modify_bars(const struct pci_dev >>>>>>>>> *pdev, uint16_t cmd, bool rom_only) >>>>>>>>> continue; >>>>>>>>> } >>>>>>>>> >>>>>>>>> + spin_lock(&tmp->vpci_lock); >>>>>>>>> + if ( !tmp->vpci ) >>>>>>>>> + { >>>>>>>>> + spin_unlock(&tmp->vpci_lock); >>>>>>>>> + continue; >>>>>>>>> + } >>>>>>>>> for ( i = 0; i < ARRAY_SIZE(tmp->vpci->header.bars); i++ ) >>>>>>>>> { >>>>>>>>> const struct vpci_bar *bar = >>>>>>>>> &tmp->vpci->header.bars[i]; >>>>>>>>> @@ -303,12 +310,14 @@ static int modify_bars(const struct pci_dev >>>>>>>>> *pdev, uint16_t cmd, bool rom_only) >>>>>>>>> rc = rangeset_remove_range(mem, start, end); >>>>>>>>> if ( rc ) >>>>>>>>> { >>>>>>>>> + spin_unlock(&tmp->vpci_lock); >>>>>>>>> printk(XENLOG_G_WARNING "Failed to remove [%lx, >>>>>>>>> %lx]: %d\n", >>>>>>>>> start, end, rc); >>>>>>>>> rangeset_destroy(mem); >>>>>>>>> return rc; >>>>>>>>> } >>>>>>>>> } >>>>>>>>> + spin_unlock(&tmp->vpci_lock); >>>>>>>>> } >>>>>>>> At the first glance this simply looks like another unjustified (in the >>>>>>>> description) change, as you're not converting anything here but you >>>>>>>> actually add locking (and I realize this was there before, so I'm sorry >>>>>>>> for not pointing this out earlier). >>>>>>> Well, I thought that the description already has "...the lock can be >>>>>>> used (and in a few cases is used right away) to check whether vpci >>>>>>> is present" and this is enough for such uses as here. >>>>>>>> But then I wonder whether you >>>>>>>> actually tested this, since I can't help getting the impression that >>>>>>>> you're introducing a live-lock: The function is called from cmd_write() >>>>>>>> and rom_write(), which in turn are called out of vpci_write(). Yet that >>>>>>>> function already holds the lock, and the lock is not (currently) >>>>>>>> recursive. (For the 3rd caller of the function - init_bars() - otoh >>>>>>>> the locking looks to be entirely unnecessary.) >>>>>>> Well, you are correct: if tmp != pdev then it is correct to acquire >>>>>>> the lock. But if tmp == pdev and rom_only == true >>>>>>> then we'll deadlock. >>>>>>> >>>>>>> It seems we need to have the locking conditional, e.g. only lock >>>>>>> if tmp != pdev >>>>>> Which will address the live-lock, but introduce ABBA deadlock potential >>>>>> between the two locks. >>>>> I am not sure I can suggest a better solution here >>>>> @Roger, @Jan, could you please help here? >>>> Well, first of all I'd like to mention that while it may have been okay to >>>> not hold pcidevs_lock here for Dom0, it surely needs acquiring when dealing >>>> with DomU-s' lists of PCI devices. The requirement really applies to the >>>> other use of for_each_pdev() as well (in vpci_dump_msi()), except that >>>> there it probably wants to be a try-lock. >>>> >>>> Next I'd like to point out that here we have the still pending issue of >>>> how to deal with hidden devices, which Dom0 can access. See my RFC patch >>>> "vPCI: account for hidden devices in modify_bars()". Whatever the solution >>>> here, I think it wants to at least account for the extra need there. >>> Yes, sorry, I should take care of that. >>> >>>> Now it is quite clear that pcidevs_lock isn't going to help with avoiding >>>> the deadlock, as it's imo not an option at all to acquire that lock >>>> everywhere else you access ->vpci (or else the vpci lock itself would be >>>> pointless). But a per-domain auxiliary r/w lock may help: Other paths >>>> would acquire it in read mode, and here you'd acquire it in write mode (in >>>> the former case around the vpci lock, while in the latter case there may >>>> then not be any need to acquire the individual vpci locks at all). FTAOD: >>>> I haven't fully thought through all implications (and hence whether this is >>>> viable in the first place); I expect you will, documenting what you've >>>> found in the resulting patch description. Of course the double lock >>>> acquire/release would then likely want hiding in helper functions. >>> I've been also thinking about this, and whether it's really worth to >>> have a per-device lock rather than a per-domain one that protects all >>> vpci regions of the devices assigned to the domain. >>> >>> The OS is likely to serialize accesses to the PCI config space anyway, >>> and the only place I could see a benefit of having per-device locks is >>> in the handling of MSI-X tables, as the handling of the mask bit is >>> likely very performance sensitive, so adding a per-domain lock there >>> could be a bottleneck. >> Hmm, with method 1 accesses serializing globally is basically >> unavoidable, but with MMCFG I see no reason why OSes may not (move >> to) permit(ting) parallel accesses, with serialization perhaps done >> only at device level. See our own pci_config_lock, which applies to >> only method 1 accesses; we don't look to be serializing MMCFG >> accesses at all. >> >>> We could alternatively do a per-domain rwlock for vpci and special case >>> the MSI-X area to also have a per-device specific lock. At which point >>> it becomes fairly similar to what you propose. > @Jan, @Roger > > 1. d->vpci_lock - rwlock <- this protects vpci > 2. pdev->vpci->msix_tbl_lock - rwlock <- this protects MSI-X tables > or should it better be pdev->msix_tbl_lock as MSI-X tables don't > really depend on vPCI? If so, perhaps indeed better the latter. But as said in reply to Roger, I'm not convinced (yet) that doing away with the per-device lock is a good move. As said there - we're ourselves doing fully parallel MMCFG accesses, so OSes ought to be fine to do so, too. Jan
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