task_finder holding 'mmap_sem' too long

[David Smith]

>> On Tue, 2009-05-26 at 07:40 -0500, David Smith wrote:
>> Hmm.  Looking back through the task_finder code, I believe the mmap_sem
>> is being held so that the vma list doesn't get deleted from underneath
>> the task_finder.  However, I'm not sure that can really happen in the
>> cases where it is done.  It might be possible that calling
>> 'get_task_mm()' would be enough here.
>>
>> It looks like the task_finder runs callbacks with mmap_sem held in 2 places:
>>
>> 1) When initially attaching to a "interesting" thread, it gets stopped.
>>  In the quiesce handler, the mmap callbacks are run for vma's that
>> existed before task_finder attached to it.  (This is only done for the
>> thread group leader.)  The entire vma list is processed in this matter.
>>
>> 2) At syscall exit, if the call is mmap or mmap2, the callbacks are
>> called on the new vma.  In this case it would be possible to hold
>> mmap_sem, get the information needed out of the new vma, release
>> mmap_sem, then call the callbacks.

After a bit of work, I've fixed these 2 issues (the fixes are in commits
9b59029 and bec8cf6 for the curious).  The task_finder no longer holds
the mmap_sem while making callbacks.

In case 1), the new code grabs the mmap_sem, caches information about
each vma, releases the mmap_sem, then makes the callbacks.

-- 
David Smith
dsmith@redhat.com
Red Hat
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