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I know that in Linux mutexes are implemented as futexes down below and futex uses compare-and-swap mechanism. And usually for acquiring locks, a user-space thread does not need to make a system call as the lock is resolved in user-space.
Now my question is what happens when there is high contention and many threads are trying to lock a mutex at the same time. Does a system call occurs then for the kernel to decide which thread to grant the mutex? Especially when thread priorities are different? I myself think so.
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In computing, a futex (short for "fast userspace mutex") is a kernel system call that programmers can use to implement basic locking, or as a building block for higher-level locking abstractions such as semaphores and POSIX mutexes or condition variables.
In computer programming, a mutex (mutual exclusion object) is a program object that is created so that multiple program thread can take turns sharing the same resource, such as access to a file.
If the mutex is already locked by another thread, the subroutine blocks the calling thread until the mutex is unlocked. If the mutex is already locked by the calling thread, the subroutine might block forever or return an error depending on the type of mutex.
Spinlock is a type of lock that causes a thread attempting to obtain it to check for its availability while waiting in a loop continuously. On the other hand, a mutex is a program object designed to allow different processes may take turns sharing the same resource.
As long as there is no contention, there are no system calls made. If there is contention, then a system call is made to place the thread in a sleep queue that will then be used to find the first thread to wake up when the mutex becomes free. Additionally, an adjustment is made in the syscall to the value of the futex so that the currently owning thread will not go through the user-land "fast-path" unlock routine (which simply resets the futex back to a zero or "unlocked" value), but will instead make another system call to check the sleep queue for waiting threads to pass the lock ownership to. With more threads contending for a lock, there is of course going to be a higher chance of a contention being found, but again, if there is no contention, then there is no sys-call made.
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