On Wed, Dec 04, 2024 at 06:48:18AM +0000, Zilin Guan wrote:
> Hello,
>
> I have a question regarding the use of smp_rmb() to enforce
> memory ordering in two related functions.
>
> In the function netfs_unbuffered_write_iter_locked() from the file
> fs/netfs/direct_write.c, smp_rmb() is explicitly used after the
> wait_on_bit() call to ensure that the error and transferred fields are
> read in the correct order following the NETFS_RREQ_IN_PROGRESS flag:
>
> 105 wait_on_bit(&wreq->flags, NETFS_RREQ_IN_PROGRESS,
> 106 TASK_UNINTERRUPTIBLE);
> 107 smp_rmb(); /* Read error/transferred after RIP flag */
> 108 ret = wreq->error;
> 109 if (ret == 0) {
> 110 ret = wreq->transferred;
> 111 iocb->ki_pos += ret;
> 112 }
>
> However, in the function netfs_end_writethrough() from the file
> fs/netfs/write_issue.c, there is no such use of smp_rmb() after
> the corresponding wait_on_bit() call, despite accessing the same filed
> of wreq->error and relying on the same NETFS_RREQ_IN_PROGRESS flag:
>
> 681 wait_on_bit(&wreq->flags, NETFS_RREQ_IN_PROGRESS,
> TASK_UNINTERRUPTIBLE);
> 682 ret = wreq->error;
>
> My question is why does the first function require a CPU memory barrier
> smp_rmb() to enforce ordering, whereas the second function does not?
The fence is redundant.
Per the comment in wait_on_bit:
* Returned value will be zero if the bit was cleared in which case the
* call has ACQUIRE semantics, or %-EINTR if the process received a
* signal and the mode permitted wake up on that signal.
Since both sites pass TASK_UNINTERRUPTIBLE this will only ever return
after the bit is sorted out, already providing the needed fence.
