Re: [Qemu-devel] [Qemu-block] [PATCH] file-posix: unlock qemu_global_mutex before pread when attach disk

[Paolo Bonzini <pbonzini@redhat.com>]

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