This is the sixth part of the chapter which describes [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_(computer_science)) in the Linux kernel and in the previous parts we finished to consider different [readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock) synchronization primitives. We will continue to learn synchronization primitives in this part and start to consider a similar synchronization primitive which can be used to avoid the `writer starvation` problem. The name of this synchronization primitive is - `seqlock` or `sequential locks`.
This is the sixth part of the chapter which describes [synchronization primitives](https://en.wikipedia.org/wiki/Synchronization_\(computer_science\)) in the Linux kernel and in the previous parts we finished to consider different [readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock) synchronization primitives. We will continue to learn synchronization primitives in this part and start to consider a similar synchronization primitive which can be used to avoid the `writer starvation` problem. The name of this synchronization primitive is - `seqlock` or `sequential locks`.
We know from the previous [part](https://0xax.gitbooks.io/linux-insides/content/SyncPrim/sync-5.html) that [readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock) is a special lock mechanism which allows concurrent access for read-only operations, but an exclusive lock is needed for writing or modifying data. As we may guess, it may lead to a problem which is called `writer starvation`. In other words, a writer process can't acquire a lock as long as at least one reader process which acquired a lock holds it. So, in the situation when contention is high, it will lead to situation when a writer process which wants to acquire a lock will wait for it for a long time.
As we may see, these functions differ only in the initialization of spinlock. They call `spin_lock_irq` and `spin_unlock_irq` instead of `spin_lock` and `spin_unlock`.
Or for example `write_seqlock_irqsave` and `write_sequnlock_irqrestore` functions which are the same but used `spin_lock_irqsave` and `spin_unlock_irqsave` macro to use in [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_(PC_architecture)) handlers.
Or for example `write_seqlock_irqsave` and `write_sequnlock_irqrestore` functions which are the same but used `spin_lock_irqsave` and `spin_unlock_irqsave` macro to use in [IRQ](https://en.wikipedia.org/wiki/Interrupt_request_\(PC_architecture\)) handlers.
That's all.
@ -337,7 +337,7 @@ If you have questions or suggestions, feel free to ping me in twitter [0xAX](htt
@ -216,7 +216,7 @@ At the next step we check that a file is not tried to be opened via [fanotify](h
flags &= ~FMODE_NONOTIFY & ~O_CLOEXEC;
```
We do this to not leak a [file descriptor](https://en.wikipedia.org/wiki/File_descriptor). By default, the new file descriptor is set to remain open across an `execve` system call, but the `open` system call supports `O_CLOEXEC` flag that can be used to change this default behaviour. So we do this to prevent leaking of a file descriptor when one thread opens a file to set `O_CLOEXEC` flag and in the same time the second process does a [fork](https://en.wikipedia.org/wiki/Fork_(system_call)) + [execve](https://en.wikipedia.org/wiki/Exec_(system_call)) and as you may remember that child will have copies of the parent's set of open file descriptors.
We do this to not leak a [file descriptor](https://en.wikipedia.org/wiki/File_descriptor). By default, the new file descriptor is set to remain open across an `execve` system call, but the `open` system call supports `O_CLOEXEC` flag that can be used to change this default behaviour. So we do this to prevent leaking of a file descriptor when one thread opens a file to set `O_CLOEXEC` flag and in the same time the second process does a [fork](https://en.wikipedia.org/wiki/Fork_\(system_call\)) + [execve](https://en.wikipedia.org/wiki/Exec_\(system_call\)) and as you may remember that child will have copies of the parent's set of open file descriptors.
At the next step we check that if our flags contains `O_SYNC` flag, we apply `O_DSYNC` flag too:
Radek Dostál
7 years ago