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drm_encoder_cleanup() modifies the encoder chain by removing bridges via
drm_bridge_detach(). Protect this whole operation by taking the mutex, so
that:
* any users iterating over the chain will not access it during the change
* other code willing to modify the list (drm_bridge_attach()) will wait
until drm_encoder_cleanup() is done
Note that the _safe macro in use here is providing a different and
orthogonal kind of protection than the mutex:
1. list_for_each_entry_safe() allows removing the current entry from the
list it is iterating on, synchronously; the non-safe version would be
unable to find the next entry after the current entry has been removed
2. the mutex being added allows to ensure that the list is not used
asynchronously by other code while it is being modified; this prevents
such other concurrent code to derail because it is iterating over an
element while it is removed
The _safe macro, which works by taking the "next" pointer in addition to
the "current" one, does not even try to provide the protection at item 2
above. This is visible e.g. when the "next" element is removed by other
concurrent code. This is what would happen without the added mutex:
1. start loop: list_for_each_entry_safe(pos, n, ...) sets:
pos = list_first_entry() = (bridge 1)
n = list_next_entry(pos) = (bridge 2)
2. enter the loop 1st time, do something with *pos (bridge 1)
3. in the meanwhile bridge 2 is hot-unplugged
-> another thread removes bridge 2
-> drm_bridge_detach()
-> list_del() sets (bridge 2)->next = LIST_POISON1
4. loop iteration 1 finishes, list_for_each_entry_safe() sets:
pos = n (previously set to bridge 2)
n = (bridge 2)->next = LIST_POISON1
5. enter the loop 2nd time, do something with *pos (bridge 2)
6. loop iteration 2 finishes, list_for_each_entry_safe() sets:
pos = n = LIST_POISON1 ==> bug!
However, simply adding mutex_[un]lock(&encoder->bridge_chain_mutex)
before/after the list_for_each_entry_safe() seems a simple and good
solution, but it is introducing a possible ABBA deadlock (found by
PROVE_LOCKING). The two code paths involved are:
* drm_encoder_cleanup():
- takes the bridge_chain_mutex (A)
- calls drm_bridge_detach -> drm_atomic_private_obj_fini ->
DRM_MODESET_LOCK_ALL_BEGIN() which takes all locks in the
acquisition context (B)
* drm_mode_getconnector() (and other code paths):
- calls drm_helper_probe_single_connector_modes() which:
- takes a drm_modeset_lock in the acquisition context (B)
- calls __drm_helper_update_and_validate ->
drm_bridge_chain_mode_valid -> drm_for_each_bridge_in_chain_from()
which takes the bridge_chain_mutex (A)
To avoid this potential ABBA deadlock, move all list items to a temporary
list while holding the bridge_chain_mutex, then detach all elements from
the temporary list without the mutex.
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://patch.msgid.link/20260324-drm-bridge-alloc-encoder-chain-mutex-v5-2-8bf786c5c7e6@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
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