parasite: send PARASITE_CMD_FINI before resuming the target process

The control socket has enough buffer for one command and the target
process will not wait a new command, so we will avoid extra context
switches.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
dump: use breakpoints instead of tracing syscalls (v2)

Currently CRIU traces syscalls to catch a moment, when sigreturn() is
called. Now we trace recv(cmd), close(logfd), close(cmdfd), sigreturn().

We can reduce a number of steps by using hw breakpoints. A breakpoint is
set before sigreturn, so we will need to trace only it.

v2: In the first version a breakpoint is set after sigreturn. In this
case we have a problem with signals. If a process has pending signals,
it will start to precess them after exiting from sigreturn(), but before
returning to userspace. So the breakpoint will not be triggered.

And at the end Here are a few numbers how we catch sigreturn.
Before this patch criu executes 36 syscalls and gets 12 signals.
With this patch criu executes 18 syscalls and gets 5 signals.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
restore: use breakpoints instead of tracing syscalls

Currently CRIU traces syscalls to catch a moment, when sigreturn() is
called. Now we trace recv(cmd), close(logfd), close(cmdfd), sigreturn().

We can reduce a number of steps by using hw breakpoints. A breakpoint is
set before sigreturn, so we will need to trace only it.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>