Re: [Qemu-devel] [PATCH 1/2] vl.c: set NULL upon deleting handlers in qemu_set_fd_handler2()

[Stefan Hajnoczi]

On Tue, Jan 25, 2011 at 10:13 AM, Corentin Chary
<address@hidden> wrote:
> On Tue, Jan 25, 2011 at 10:03 AM, Stefan Hajnoczi <address@hidden> wrote:
>> On Tue, Jan 25, 2011 at 8:33 AM, Corentin Chary
>> <address@hidden> wrote:
>>> From: Yoshiaki Tamura <address@hidden>
>>>
>>> Currently qemu_set_fd_handler2() is only setting ioh->deleted upon
>>> deleting.  This may cause a crash when a read handler calls
>>> qemu_set_fd_handler2() to delete handlers, but a write handler is
>>> still invoked from main_loop_wait().  Because main_loop_wait() checks
>>> handlers before calling, setting NULL upon deleting will protect
>>> handlers being called if already deleted.
>>>
>>> One example is the new threaded vnc server.  When an error occurs in
>>> the context of a read handler, it'll releases resources and deletes
>>> handlers.  However, because the write handler still exists, it'll be
>>> called, and then crashes because of lack of resources.  This patch
>>> fixes it.
>>
>> Does this case still happen with qemu.git/master?  In November I sent
>> a patch to check for deleted handlers:
>>
>> commit 0290b57bdfec83ca78b6d119ea9847bb17943328
>> Author: Stefan Hajnoczi <address@hidden>
>> Date:   Wed Nov 3 14:29:44 2010 +0000
>>
>>    Delete IOHandlers after potentially running them
>>
>>    Since commit 4bed9837309e58d208183f81d8344996744292cf an .fd_read()
>>    handler that deletes its IOHandler is exposed to .fd_write() being
>>    called on the deleted IOHandler.
>>
>>    This patch fixes deletion so that .fd_read() and .fd_write() are never
>>    called on an IOHandler that is marked for deletion.
>>
>>    Signed-off-by: Stefan Hajnoczi <address@hidden>
>>    Signed-off-by: Anthony Liguori <address@hidden>
>>
>> So I don't think Yoshi's patch is necessary anymore?
>
> Ho I didn't see that one.
> It's probably not necessary, but it stills make sense to apply this
> patch since there is
> absolutly no reasons to keep the old value in fd_read and fd_write when
> the user explicitly asked to set them to NULL.

That's true, I don't see a good reason why we shouldn't clear them.
The only minor advantage to keeping them is that it helps when
debugging QEMU - you can identify the fd handler by its
fd_read/fd_write function pointers easily.

Stefan