Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 26 Oct 2020 17:14:35 -0700 | Subject | Re: [PATCH 2/2] mm: prevent gup_fast from racing with COW during fork |
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On Mon, Oct 26, 2020 at 4:50 PM Jason Gunthorpe <jgg@nvidia.com> wrote: > > Right, it isn't a "seqcount" because the read side doesn't spin.
Not all readers spin for seqcount either.
Sure, it's the *common* case, but look at name resolution - it will basically fail on a seqcount failure in RCU mode, and go to the "slow and safe" version:
if (unlikely(read_seqcount_retry(&d->d_seq, nd->seq))) return -ECHILD;
Exactly like gup_fast vs fork, in fact.
So no, you don't need to retry for seqcount failures, doing
seq = seqcount_begin(..); ... optimistic fast case ... if (!read_seqcount_retry(.., seq)) return 0; // success .. do slow case ...
is valid and happens.
Linus
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