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?
Thank you for your time and assistance!
Best Regards,
Zilin Guan
