On 10.09.19 14:41, Max Reitz wrote: > Hi, > > As reported in https://bugzilla.redhat.com/show_bug.cgi?id=1740193, our > curl block driver can spontaneously hang. This becomes visible e.g. > when reading compressed qcow2 images: > > $ qemu-img convert -p -O raw -n \ > https://download.cirros-cloud.net/0.4.0/cirros-0.4.0-x86_64-disk.img \ > null-co:// > > (Hangs at 74.21 %, usually.) > > A more direct way is: > > $ qemu-img bench -f raw http://download.qemu.org/qemu-4.1.0.tar.xz \ > -d 1 -S 524288 -c 2 > > (Which simply performs two requests, and the second one hangs. You can > use any HTTP resource (probably FTP, too) you’d like that is at least > 1 MB in size.) > > It turns out that this is because cURL 7.59.0 has added a protective > feature against some misuse we had in our code: curl_multi_add_handle() > must not be called from within a cURL callback, but in some cases we > did. As of 7.59.0, this fails, our new request is not registered and > the I/O request stalls. This is fixed by patch 6. > > Patch 7 makes us check for curl_multi_add_handle()’s return code, > because if we had done that before, debugging would have been much > simpler. > > > On the way to fixing it, I had a look over the whole cURL code and found > a suspicious QLIST_FOREACH_SAFE() loop that actually does not seem very > safe at all. I think this may lead to crashes, although I have never > seen any myself. https://bugzilla.redhat.com/show_bug.cgi?id=1744602#c5 > shows one in exactly the function in question, so I think it actually is > a problem. > > This is fixed by patch 5, patches 1, 2, and 4 prepare for it. > > (Patch 3 is kind of a misc patch that should ensure that we always end > up calling curl_multi_check_completion() whenever a request might have > been completed.)
Thanks for the review, applied to my block branch: https://git.xanclic.moe/XanClic/qemu/commits/branch/block Max
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