On 10/06/19 15:34, Zhengui li wrote: > > when do qmp sush as drive_add, qemu main thread locks the > qemu_global_mutex and do pread in raw_probe_alignmen. Pread is a > synchronous operation. If backend storage network has a large delay > or IO pressure is too large, the pread operation will not return for > a long time, which make vcpu thread can't acquire qemu_global_mutex > for a long time and make the vcpu thread unable to be scheduled for a > long time. So virtual machine cpu soft lockup happened. > > qemu main thread should not hold qemu_global_mutex for a long time > when do qmp that involving IO synchronous operation sush pread , > ioctl, etc. So this patch unlock qemu_global_mutex before IO > synchronous operation sush pread.
These preads are for 512-4096 bytes, can they really last much longer than the "open" that precedes them? If pread of 4K can trigger a soft lockup, things are really screwed up---and it's hard to be sure that all callers of raw_probe_alignment are okay with releasing the global mutex. Paolo
