sysfs: fix deadlock race with module removal
Subsystems (6)
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Quoted 13 lines · David Laight
From: Luis Chamberlain
Quoted 10 lines
Sent: 17 September 2021 20:47
When sysfs attributes use a lock also used on module removal we can race to deadlock. This happens when for instance a sysfs file on a driver is used, then at the same time we have module removal call trigger. The module removal call code holds a lock, and then the sysfs file entry waits for the same lock. While holding the lock the module removal tries to remove the sysfs entries, but these cannot be removed yet as one is waiting for a lock. This won't complete as the lock is already held. Likewise module removal cannot complete, and so we deadlock.
Quoted 2 lines
Isn't the real problem the race between a sysfs file action and the removal of the sysfs node?
Nope, that is taken care of by kernfs.
Quoted 2 lines
This isn't really related to module unload - except that may well remove some sysfs nodes.
Nope, the issue is a deadlock that can happen due to a shared lock on module removal and a driver sysfs operation.
Quoted 6 lines
This is the same problem as removing any other kind of driver callback. There are three basic solutions:
1) Use a global lock - not usually useful.
2) Have the remove call sleep until any callbacks are complete.
3) Have the remove just request removal and have a final callback (from a different context).
Kernfs already does a sort of combination of 1) and 2) but 1) is using atomic reference counts.
Quoted 3 lines
If the remove can sleep (as in 2) then there is a requirement on the driver code to not hold any locks across the 'remove' that can be acquired during the callbacks.
And this is the part that kernfs has no control over since the removal and sysfs operation are implementation specific.
Quoted 5 lines
Now, for sysfs, you probably only want to sleep the remove code while a read/write is in progress - not just because the node is open.
That probably requires marking an open node 'invalid' and deferring delete to close.
This is already done by kernfs.
Quoted 1 line
None of this requires a reference count on the module.
You are missing the point to the other aspect of the try_module_get(), it lets you also check if module exit has been entered. By using try_module_get() you let the module exit trump proceeding with an operation, therefore also preventing any potential use of a shared lock on module exit and the driver specific sysfs operation.
Luis