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sampling mode PMC is allocated, hwpmc calls linker_hwpmc_list_objects() while already holding an exclusive lock on pmc-sx lock. list_objects() tries to acquire an exclusive lock on the kld_sx lock. When a KLD module is loaded or unloaded successfully, kern_kld(un)load calls into the pmc hook while already holding an exclusive lock on the kld_sx lock. Calling the pmc hook requires acquiring a shared lock on the pmc-sx lock. Fix this by only acquiring a shared lock on the kld_sx lock in linker_hwpmc_list_objects(), and also downgrading to a shared lock on the kld_sx lock in kern_kld(un)load before calling into the pmc hook. In kern_kldload this required moving some modifications of the linker_file_t to happen before calling into the pmc hook. This fixes the deadlock by ensuring that the hwpmc -> list_objects() case is always able to proceed. Without this patch, I was able to deadlock a multicore system within minutes by constantly loading and unloading an KLD module while I simultaneously started a sampling mode PMC in a loop. MFC after: 1 month
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