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

[l00284672]

The pread will hang in attaching disk just when backend storage network 
disconnection .

I think the locking range of qemu_global_mutex is too large when do qmp 
operation. what

does the qemu_global_mutex  really protect?  what is the risk of 
unlocking qemu_global_mutex

in qmp?


On 2019/6/10 21:51, Paolo Bonzini wrote:
> 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
>
> .

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