Re: [PATCH block/for-4.2-writeback] v9fs: fix error handling in v9fs_session_init()

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On 06/07/2015 11:57 PM, Tejun Heo wrote:
On failure, v9fs_session_init() returns with the v9fs_session_info
struct partially initialized and expects the caller to invoke
v9fs_session_close() to clean it up; however, it doesn't track whether
the bdi is initialized or not and curiously invokes bdi_destroy() in
both vfs_session_init() failure path too.

A. If v9fs_session_init() fails before the bdi is initialized, the
    follow-up v9fs_session_close() will invoke bdi_destroy() on an
    uninitialized bdi.

B. If v9fs_session_init() fails after the bdi is initialized,
    bdi_destroy() will be called twice on the same bdi - once in the
    failure path of v9fs_session_init() and then by
    v9fs_session_close().

A is broken no matter what.  B used to be okay because bdi_destroy()
allowed being invoked multiple times on the same bdi, which BTW was
broken in its own way - if bdi_destroy() was invoked on an initialiezd
but !registered bdi, it'd fail to free percpu counters.  Since
f0054bb1e1f3 ("writeback: move backing_dev_info->wb_lock and
->worklist into bdi_writeback"), this no longer work - bdi_destroy()
on an initialized but not registered bdi works correctly but multiple
invocations of bdi_destroy() is no longer allowed.

The obvious culprit here is v9fs_session_init()'s odd and broken error
behavior.  It should simply clean up after itself on failures.  This
patch makes the following updates to v9fs_session_init().

* @rc -> @retval error return propagation removed.  It didn't serve
   any purpose.  Just use @rc.

* Move addition to v9fs_sessionlist to the end of the function so that
   incomplete sessions are not put on the list or iterated and error
   path doesn't have to worry about it.

* Update error handling so that it cleans up after itself.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>

Added to for-4.2/writeback, thanks.

--
Jens Axboe

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