Stefan Hajnoczi <stefa...@redhat.com> writes: > On Mon, Apr 16, 2018 at 05:17:32PM +0800, Peter Xu wrote: >> On Mon, Apr 16, 2018 at 04:37:48PM +0800, Stefan Hajnoczi wrote: >> > On Thu, Apr 12, 2018 at 02:11:08PM +0800, Peter Xu wrote: >> > > In the future the monitor iothread may be accessing the cur_mon as >> > > well (via monitor_qmp_dispatch_one()). Before we introduce a real >> > > Out-Of-Band command, let's convert the cur_mon variable to be a >> > > per-thread variable to make sure there won't be a race between threads. >> > > >> > > Note that thread variables are not initialized to a valid value when new >> > > thread is created. However for our case we don't need to set it up, >> > > since the cur_mon variable is only used in such a pattern: >> > > >> > > old_mon = cur_mon; >> > > cur_mon = xxx; >> > > (do something, read cur_mon if necessary in the stack) >> > > cur_mon = old_mon; >> > > >> > > It plays a role as stack variable, so no need to be initialized at all. >> > > We only need to make sure the variable won't be changed unexpectedly by >> > > other threads. >> > > >> > > Signed-off-by: Peter Xu <pet...@redhat.com> >> > > --- >> > > v3: >> > > - fix code style warning from patchew >> > > v2: >> > > - drop qemu-thread changes >> > > --- >> > > include/monitor/monitor.h | 2 +- >> > > monitor.c | 2 +- >> > > stubs/monitor.c | 2 +- >> > > tests/test-util-sockets.c | 2 +- >> > > 4 files changed, 4 insertions(+), 4 deletions(-) >> > >> > The Monitor object is not fully thread-safe, so although the correct >> > cur_mon is now accessible, code may still be unsafe. For example, >> > monitor_get_fd(cur_mon, ...) is not thread-safe and must not be used by >> > OOB commands. >> >> IMHO things like monitor_get_fd() should only be called in QMP >> context, so there should always be a monitor_qmp_dispatch_one() in the >> stack already (no matter whether it is in main thread or the monitor >> iothread), which means that cur_mon should have been setup. So IMHO >> it's a programming error if monitor_get_fd() is called without correct >> cur_mon setup after this patch. > > The pointer value of cur_mon is not the issue, you have made that work > correctly. The problem is that some monitor.h APIs do not access the > Monitor object in a thread-safe fashion. > > Two QMP commands executing simultaneously in the main loop thread and > the monitor IOThread can hit race conditions. The example I gave was > the monitor_get_fd() API, which iterates and modifies the mon->fds > QLIST without a lock. > > Please audit monitor.h and either make things thread-safe or document > the thread-safety rules (e.g. "This function cannot be called from > out-of-band QMP context"). This wasn't necessary before but now that > you are adding multi-threading it is.
Code working with the current thread's monitor via thread-local cur_mon is easier to analyze in some ways than code working with a Monitor * parameter: the latter can interfere with some other thread's monitor, and you may have to argue what values the parameter can take. You might want to replace parameters by cur_mon in certain cases. Funnily, the plan used to be the opposite. Commit 376253ece48: "On the mid or long term, those use case will be obsoleted so that [cur_mon] can be removed again."