Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork

[Ahmed S. Darwish]

On Sat, Oct 31, 2020 at 11:26:05AM -0400, Peter Xu wrote:

...

> Shall we document this explicitly (if this patch still needs a repost)?

Yes, this patch series needs a v3 :)

> Seems not straightforward since that seems not the usual way to use seqcount,
> not sure whether I'm the only one that feels this way, though.

Yes, this usage is correct but not common. I've proposed a more explicit
comment above the write section code, in my reply to patch #2.

...

> The other thing is, considering this use of seqcount seems to be quite special
> as explained below, I'm just not sure whether this would confuse lockdep or
> kcsan, etc., if we decide to use write_seqcount_t_begin().
>

Lockdep won't be confused as it's not used in the raw_*() variant of the
seqcount APIs.

AFAIK KCSAN also has some margin to protect itself from this: see
seqlock.h KCSAN_SEQLOCK_REGION_MAX.

Thanks,

> Peter Xu

Ahmed Darwish
Linutronix GmbH