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    SubjectRe: [PATCH] Syscalls: reboot: Add options to the reboot syscall to remount filesystems ro
    Writing a single byte to /proc/sysrq-trigger is an asynchronous
    operation, with no obvious way to be informed that it has completed
    the remount. I did actually try this at first, but the reboot
    happened before the remount finished. I could of course add a sleep
    of a few seconds, but just how long to wait is not obvious, nor is it
    a guarantee that the remount will always finish in the time chosen.
    I'm heading down the path of reading /proc/mounts and remounting all
    read-wirte filesystems backed by a block device as read-only.


    On Thu, Mar 3, 2011 at 3:36 PM, Linus Torvalds
    <> wrote:
    > On Thu, Mar 3, 2011 at 10:28 AM, Mark Lord <> wrote:
    >> On 11-03-03 12:45 PM, Linus Torvalds wrote:
    >>> If you can change whatever user-land process that does the reboot
    >>> system call (and clearly you can, since you're adding new commands and
    >>> using those), then why the heck don't you just do the remount-ro from
    >>> that same user land?
    >> Is there a system call for emergency_remount_*() ?
    >> I've been writing to /proc/proc/sysrq-trigger to accomplish this.
    > So I don't know what this has to do with "emergency_remount()" - we're
    > talking about a regular controlled shutdown/reset. The fact that the
    > patch used the emergency_remount() code seems to be purely an
    > implementation issue, nothing more.
    > And while sysrq-trigger certainly works (when it's enabled, but that's
    > true of /proc too, of course), I do think it's a rather odd way of
    > solving the problem, when the simple "just remount read-only" is what
    > the code actually _wants_ to do.
    > But you're certainly right that it takes less code to open
    > /proc/sysrq-trigger and writing a single byte to it than it does to do
    > the straightforward "let's just do the normal mount thing".
    >                             Linus
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