Re: [PATCH v3] sock: Make sock->sk_stamp thread-safe

From: David Miller
Date: Tue Jan 01 2019 - 12:49:19 EST

From: Deepa Dinamani <deepa.kernel@xxxxxxxxx>
Date: Thu, 27 Dec 2018 18:55:09 -0800

> Al Viro mentioned (Message-ID
> <20170626041334.GZ10672@xxxxxxxxxxxxxxxxxx>)
> that there is probably a race condition
> lurking in accesses of sk_stamp on 32-bit machines.
> sock->sk_stamp is of type ktime_t which is always an s64.
> On a 32 bit architecture, we might run into situations of
> unsafe access as the access to the field becomes non atomic.
> Use seqlocks for synchronization.
> This allows us to avoid using spinlocks for readers as
> readers do not need mutual exclusion.
> Another approach to solve this is to require sk_lock for all
> modifications of the timestamps. The current approach allows
> for timestamps to have their own lock: sk_stamp_lock.
> This allows for the patch to not compete with already
> existing critical sections, and side effects are limited
> to the paths in the patch.
> The addition of the new field maintains the data locality
> optimizations from
> commit 9115e8cd2a0c ("net: reorganize struct sock for better data
> locality")
> Note that all the instances of the sk_stamp accesses
> are either through the ioctl or the syscall recvmsg.
> Signed-off-by: Deepa Dinamani <deepa.kernel@xxxxxxxxx>

Ok, I'm fine with this, so applied and queued up for -stable.

I will note in passing that there are several 32-bit architectures
that have 64-bit loads. Sparc is one such case. And they would not
need these changes.

But I don't think it's practical or worthwhile to add that level of
consideration into your changes. I'd rather the commit stay as simple
as possible.