Messages in this thread | | | Date | Thu, 9 Mar 2023 11:59:16 +0100 | From | Roger Pau Monné <> | Subject | Re: [PATCH v2] hvc/xen: prevent concurrent accesses to the shared ring |
| |
Hello,
It's been 3 months and no reply.
On Mon, Dec 12, 2022 at 01:36:48PM +0100, Roger Pau Monné wrote: > Hello, > > Gentle ping regarding the locking question below. > > Thanks, Roger. > > On Fri, Dec 02, 2022 at 12:40:05PM +0100, Roger Pau Monné wrote: > > On Wed, Nov 30, 2022 at 05:08:06PM -0800, Stefano Stabellini wrote: > > > On Wed, 30 Nov 2022, Roger Pau Monne wrote: > > > > The hvc machinery registers both a console and a tty device based on > > > > the hv ops provided by the specific implementation. Those two > > > > interfaces however have different locks, and there's no single locks > > > > that's shared between the tty and the console implementations, hence > > > > the driver needs to protect itself against concurrent accesses. > > > > Otherwise concurrent calls using the split interfaces are likely to > > > > corrupt the ring indexes, leaving the console unusable. > > > > > > > > Introduce a lock to xencons_info to serialize accesses to the shared > > > > ring. This is only required when using the shared memory console, > > > > concurrent accesses to the hypercall based console implementation are > > > > not an issue. > > > > > > > > Note the conditional logic in domU_read_console() is slightly modified > > > > so the notify_daemon() call can be done outside of the locked region: > > > > it's an hypercall and there's no need for it to be done with the lock > > > > held. > > > > > > For domU_read_console: I don't mean to block this patch but we need to > > > be sure about the semantics of hv_ops.get_chars. Either it is expected > > > to be already locked, then we definitely shouldn't add another lock to > > > domU_read_console. Or it is not expected to be already locked, then we > > > should add the lock. > > > > > > My impression is that it is expected to be already locked, but I think > > > we need Greg or Jiri to confirm one way or the other. > > > > Let me move both to the 'To:' field then. > > > > My main concern is the usage of hv_ops.get_chars hook in > > hvc_poll_get_char(), as it's not obvious to me that callers of > > tty->poll_get_char hook as returned by tty_find_polling_driver() will > > always do so with the tty lock held (in fact the only user right now > > doesn't seem to hold the tty lock). > > > > > Aside from that the rest looks fine.
I guess I could reluctantly remove the lock in the get_chars hook, albeit I'm not convinced at all the lock is not needed there.
Roger.
| |