18.10.2012 14:39, Michael Kerrisk (man-pages) пишет:
On Thu, Oct 18, 2012 at 12:23 PM, Stanislav Kinsbursky
<[email protected]> wrote:
This patch is required for checkpoint/restore in userspace.
IOW, c/r requires some way to get all pending IPC messages without deleting
them from the queue (checkpoint can fail and in this case tasks will be resumed,
so queue have to be valid).
To achive this, new operation flag MSG_COPY for sys_msgrcv() system call was
introduced. If this flag was specified, then mtype is interpreted as number of
the message to copy.
If MSG_COPY is set, then kernel will allocate dummy message with passed size,
and then use new copy_msg() helper function to copy desired message (instead of
unlinking it from the queue).
Notes:
1) Return -ENOSYS if MSG_COPY is specified, but CONFIG_CHECKPOINT_RESTORE is
not set.
Stanislav,
A naive question, because I have not followed C/R closely. How do you
deal with the case that other processes may be reading from the queue?
(Or is that disabled during checkpointing?)
To be honest, in this case behaviour in user-space is unpredictable.
I.e. if you have, for example, 5 messages in queue and going to peek them all,
and another process is reading the queue in the same time, then, most probably,
you won't peek all the 5 and receive ENOMSG.
But this case can be easily handled by user-space application (number of
messages in queue can be discovered before peeking).
Note, that in CRIU IPC resources will be collected when all processes to migrate
are frozen.
Thanks,
Michael
--
Best regards,
Stanislav Kinsbursky
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