Messages in this thread | | | From | Linus Torvalds <> | Date | Thu, 3 Mar 2011 18:33:36 -0800 | Subject | Re: [PATCH] Syscalls: reboot: Add options to the reboot syscall to remount filesystems ro |
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On Thu, Mar 3, 2011 at 6:00 PM, Ken Sumrall <ksumrall@android.com> wrote: > > 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.
Right you are. That's something of a misfeature, but it comes from the way sysrq works: obviously the "real" sysrq thing is about keyboard input, so all the sysrq stuff has to be async.
The fact that that async nature then ends up also affecting the /proc/sysrq-trigger case (which _could_ be synchronous) is a bit sad in this case.
That said, I obviously think that just doing the read-only mount yourself is the right thing to do regardless, and the sysrq thing would have been just a cute/ugly hack if it had worked.
> I'm heading down the path of reading /proc/mounts and remounting all > read-wirte filesystems backed by a block device as read-only.
So just as a practical matter: while it's quite possible that the nice seq_printf() model of /proc/mounts means that it should work correctly even if you read each line individually and then re-mount while holding the file open, I would suggest that you read the whole file into a buffer before you then start changing the mounts.
Otherwise, _if_ we ever were to actually move the filesystem on our internal list of mounts when we re-mount it, you might otherwise end up seeign the same filesystem multiple times (or the reverse - missing some filesystem).
Basically, I'm saying that you should try to avoid changing mount information in between read() calls to /proc/mounts. It might cause confusion.
Linus
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