Stephen Warren <swar...@wwwdotorg.org> wrote @ Thu, 28 Jun 2012 21:08:12 +0200:
> On 06/28/2012 12:35 PM, Hiroshi Doyu wrote:
> > Stephen Warren <swar...@wwwdotorg.org> wrote @ Thu, 28 Jun 2012 18:57:18
> > +0200:
> >> On 06/28/2012 04:51 AM, Hiroshi DOYU wrote:
> >>> alloc_pdir() is called from smmu_iommu_domain_init() with spin_lock
> >>> held. memory allocations in alloc_pdir() had to be
> >>> atomic/unsleepable. Instead of converting into atomic allocation, this
> >>> patch once releases a lock, do the allocation, hold the lock again and
> >>> then see if it's raced or not in order to avoid introducing mutex.
>
> >>> +static int alloc_pdir(struct smmu_as *as, unsigned long *flags)
> >>
> >>> + spin_unlock_irqrestore(&as->lock, *flags);
> >>> + cnt = devm_kzalloc(smmu->dev,
> >>> + sizeof(cnt[0]) * SMMU_PDIR_COUNT, GFP_KERNEL);
> >>> + page = alloc_page(GFP_KERNEL | __GFP_DMA);
> >>> + spin_lock_irqsave(&as->lock, *flags);
> >>> +
> >>> + if (as->pdir_page) {
> >>> + /* We raced, free the redundant */
> >>> + err = -ENODEV;
> >>> + goto err_out;
> >>> }
> >>
> >> Is that really an error; it just means that something else allocated the
> >> same pdir already. Since the top of the function does:
> >>
> >> if (as->pdir_page)
> >> return 0;
> >>
> >> I'd expect to s/err = -ENODEV/err = 0/ inside that if condition that I
> >> quoted above, but still of cause "goto err_out" to free the now unneeded
> >> allocations.
> >>
> >> Aside from that, I think this looks reasonable.
> >
> > I think that, in the case of race condition, the one which comes
> > later, should retry with the another ASID, which is incremented as
> > below. So I modified that the latter one returns with -EAGAIN, and try
> > with another ASID.
> >
> > The complete patch follows this mail.
>
> incremental rather than complete, right?
Actually I was working on it.
> > Changes:
> > Modified drivers/iommu/tegra-smmu.c
> > diff --git a/drivers/iommu/tegra-smmu.c b/drivers/iommu/tegra-smmu.c
> > index ec656ec..f2c18fa 100644
> > --- a/drivers/iommu/tegra-smmu.c
> > +++ b/drivers/iommu/tegra-smmu.c
> > @@ -562,7 +562,7 @@ static int alloc_pdir(struct smmu_as *as, unsigned long
> > *flags)
> >
> > if (as->pdir_page) {
> > /* We raced, free the redundant */
> > - err = -ENODEV;
> > + err = -EAGAIN;
> > goto err_out;
> > }
> >
> > @@ -799,8 +799,15 @@ static int smmu_iommu_domain_init(struct iommu_domain
> > *domain)
> >
> > spin_lock_irqsave(&tmp->lock, flags);
> > if (!tmp->pdir_page) {
> > - as = tmp;
> > - goto found;
> > + int err;
> > +
> > + err = alloc_pdir(tmp, &flags);
> > + if (!err) {
> > + as = tmp;
> > + goto found;
> > + }
> > + if (err == -EAGAIN)
> > + continue;
>
> That loop is going to continue anyway, since that code is right at the
> end of the loop. Don't you want to replace that if block with:
>
> if (err != -EAGAIN)
> goto err_alloc_pdir;
>
> ?
Yes, I also found. I'll send the update one tomorrow.
> Also, the first thinig that alloc_pdir does is:
>
> if (as->pdir_page)
> return 0;
>
> It seems that should be removed completely, right? Since having the
> pdir_page already allocated is an error.
Yes. I thought that could be another patch. Same for the original code.
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