The first argument is the inode structure, which is used to retrieve information such as the user ID, group ID, access time, size, the major and minor number of a device, etc. Two arguments are passed in the my_open function. Opening the device is the first operation performed on the device file. This delay provides enough time to capture the behaviour on the screen. The delay mentioned in the above function at Line 25 is used to demonstrate how the code works, with screenshots. Copy_from_user is used for this data transfer. Here, data is transferred from the user space to the kernel space. The function my_write is similar to read. copy_to_user accepts three arguments-a pointer to the user buffer, a pointer to the kernel buffer and the size of the data to be transferred. When an application program issues the read system call, this function is invoked, and the data is to be copied from the kernel space to the user space using the copy_to_user function. The third is the size of the data to be read, while the fourth is the file pointer position. Four arguments should be passed in the read function-the first and second are pointers to the file structure and user buffer, respectively. A non-negative return value represents the number of bytes successfully read (the return value is a ‘signed size’ type, usually the native integer type for the target platform). The my_read function is used to retrieve data from the device. WithoutLock.c: In the source code, Lines 1 to 9 represent header files. We have used a pre-emptible (low-latency desktop) model as shown in Figure 2. The vanilla kernel offers the following three options for the pre-emptible kernel. This experimental work is done on Ubuntu 16.04 with kernel version 4.15, and a core processor with 8GB RAM. Figure 6: High executes after low completes execution in CS Figure 7: Low process priority gets inherited What if M is a CPU bound process which needs a lot of CPU time? In this case, M keeps on executing and apparently, L is waiting for M to get executed, and indirectly H too is waiting on M. M finishes, then L executes for the remaining portion and releases CS only then does H get resources.As M has higher priority than L and it doesn’t need to be executed in CS, L gets pre-empted by M, which then runs. L runs, and H tries to access CS while L is executing it.The real problem arises when another task of middle priority needs to be executed. The above three scenarios are normal and don’t lead to any problems. H waits for L to come out of CS and then executes. L resumes execution once H finishes its execution. Even then, H pre-empts L, and then executes. L is running in CS, H needs to run but not necessarily in CS.L then runs once H completes its execution. So, in priority based pre-emptive scheduling, H pre-empts L. L is running but not in the critical section (CS) and H needs to run.Let’s consider the following different scenarios: The shared buffer memory between H and L is protected by a lock (say semaphore). Low priority (L) Figure 4: The high priority process pre-empts the low priority process Figure 5: Low executes, high waits for low to release CS Let’s consider three processes with high, middle and low priorities. Figure 1: Priority inversion Figure 2: Kernel pre-emption model Figure 3: A low priority process is running Priority boosting is immediately removed once rt-mutex has been unlocked. If the temporarily boosted owner blocks rt-mutex, it propagates the priority boosting to the owner of the other rt-mutex on which it is blocked. A low priority owner of rt-mutex inherits the priority of a higher priority waiter until rt-mutex is released. Rt-mutex: This extends the semantics of simple mutexes by the priority inheritance protocol.
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