Re: [PATCH v3 05/13] generic/1226: Add atomic write test using fio crc check verifier

[John Garry]

On 28/07/2025 07:43, Ojaswin Mujoo wrote:
> > > What do you mean by not safe?
> > Multiple threads issuing atomic writes may trample over one another.
> >
> > It is due to the steps used to issue an atomic write in xfs by software
> > method. Here we do 3x steps:
> > a. allocate blocks for out-of-place write
> > b. do write in those blocks
> > c. atomically update extent mapping.
> >
> > In this, threads wanting to atomic write to the same address will use the
> > new blocks and can trample over one another before we atomically update the
> > mapping.
> So iiuc, w/ software fallback, a thread atomically writing to a range
> will use a new block A. Another parallel thread trying to atomically
> write to the same range will also use A, and there is no serialization
> b/w the 2 so A could end up with a mix of data from both threads.

right

> If this is true, aren't we violating the atomic guarantees. Nothing
> prevents the userspace from doing overlapping parallel atomic writes and
> it is kernels duty to error out if the write could get torn.

Correct, but simply userspace should not do this. Direct I/O 
applications are responsible for ordering.

We guarantee that the write is committed all-or-nothing, but do rely on 
userspace not issuing racing atomic writes or racing regular writes.

I can easily change this, as I mentioned, but I am not convinced that it 
is a must.

Thanks,
John