[PATCH] udf: Fix deadlock when converting file from in-ICB one to normal one
During BKL removal, conversion of files from in-ICB format to normal format got broken. We call ->writepage with i_data_sem held but udf_get_block() also acquires i_data_sem thus creating A-A deadlock. We fix the problem by dropping i_data_sem before calling ->writepage() which is safe since i_mutex still protects us against any changes in the file. Also fix pagelock - i_data_sem lock inversion in udf_expand_file_adinicb() by dropping i_data_sem before calling find_or_create_page().
Big Kernel Lock
Linux contains a global kernel lock, kernel_flag, that was originally introduced in kernel 2.0 as the only SMP lock. During 2.2 and 2.4, much work went into removing the global lock from the kernel and replacing it with finer-grained localized locks. Today, the global lock's use is minimal. It still exists, however, and developers need to be aware of it. The global kernel lock is called the big kernel lock or BKL. It is a spinning lock that is recursive; therefore two consecutive requests for it will not deadlock the process (as they would for a spinlock). Further, a process can sleep and even enter the scheduler while holding the BKL. When a process holding the BKL enters the scheduler, the lock is dropped so other processes can obtain it. These attributes of the BKL helped ease the introduction of SMP during the 2.0 kernel series. Today, however, they should provide plenty of reason not to use the lock. source : http://www.linuxjournal.com/article/5833?page=0,2