On Tue, 21 Mar 2017 09:01:50 -0500 Eric Blake <ebl...@redhat.com> wrote:
> On 03/16/2017 11:33 AM, Greg Kurz wrote: > > According to the 9P spec [*], when a client wants to cancel a pending I/O > > request identified by a given tag (uint16), it must send a Tflush message > > and wait for the server to respond with a Rflush message before reusing this > > tag for another I/O. The server may still send a completion message for the > > I/O if it wasn't actually cancelled but the Rflush message must arrive after > > that. > > > > QEMU hence waits for the flushed PDU to complete before sending the Rflush > > message back to the client. > > > > If a client sends 'Tflush tag oldtag' and tag == oldtag, QEMU will then > > allocate a PDU identified by tag, find it in the PDU list and wait for > > this same PDU to complete... i.e. wait for a completion that will never > > happen. This causes a tag and ring slot leak in the guest, and a PDU > > leak in QEMU, all of them limited by the maximal number of PDUs (128). > > But, worse, this causes QEMU to hang on device reset since v9fs_reset() > > wants to drain all pending I/O. > > > > This insane behavior is likely to denote a bug in the client, and it would > > deserve an Rerror message to be sent back. Unfortunately, the protocol > > allows it and requires all flush requests to suceed (only a Tflush response > > > > s/suceed/succeed/ > > > is expected). > > > > The only option is to detect when we have to handle a self-referencing > > flush request and report success to the client right away. > > > > [*] http://man.cat-v.org/plan_9/5/flush > > > > Reported-by: Al Viro <v...@zeniv.linux.org.uk> > > Signed-off-by: Greg Kurz <gr...@kaod.org> > > --- > > hw/9pfs/9p.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > Reviewed-by: Eric Blake <ebl...@redhat.com> > Oh, I've sent a v2 for this patch (error_report() a warning) and it is actually part of the pull request I've sent earlier today... dunno how to have your Reviewed-by: added there. Thanks. -- Greg
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