If one attempts to flock() a file on a lockless NFS mount, then flock will hang even if LOCK_NB is passed.
Is it possible to detect that the filesystem does not support locking ahead of time? Perhaps with something returned from statfs?
If not, is there some suitable heuristic for when to avoid file locking? For example, by inspecting the filesystem magic numbers or mount flags?
Thanks for any help!
Related
I'm working on a Linux/C application with strict timing requirements. I want to open a directory for reading without blocking on I/O (i.e. succeed only if the information is immediately available in cache). If this request would block on I/O I would like to know so that I can abort and ignore this directory for now. I know that open() has a non-blocking option O_NONBLOCK. However, it has this caveat:
Note that this flag has no effect for regular files and
block devices; that is, I/O operations will (briefly)
block when device activity is required, regardless of
whether O_NONBLOCK is set.
I assume that a directory entry is treated like a regular file. I don't know of a good way to prove/disprove this. Is there a way to open a directory without any I/O blocking?
You could try using COPROC command in linux to run a process in background. Maybe it could work for you.
Lets say 4 simultaneous processes are running on a processor, and data needs to be copied from an HDFS (used with Spark) file system to a local directory. Now I want only one process to copy that data, while the other processes just wait for that data to be copied by the first process.
So, basically, I want some kind of a semaphore mechanism, where every process tries to obtain semaphore to try copying the data, but only one process gets the semaphore. All processes who failed to acquire the semaphore would then just wait for the semaphore to be cleared (the process who was able to acquire the semaphore would clear it after its done with copying), and when its cleared they know the data has already been copied. How can I do that in Linux?
There's a lot of different ways to implement semaphores. The classical, System V semaphore way is described in man semop and more broadly in man sem_overview.
You might still want to do something more easily scalable and modern. Many IPC frameworks (Apache has one or two of those, too!) have atomic IPC operations. These can be used to implement semaphores, but I'd be very very careful.
Generally, I regularly encourage people who write multi-process or multi-threaded applications to use C++ instead of C. It's often simpler to see where a shared state must be protected if your state is nicely encapsulated in an object which might do its own locking. Hence, I urge you to have a look at Boost's IPC synchronization mechanisms.
In addition of Marcus Müller's answer, you could use some file locking mechanism to synchronize.
File locking might not work very well on networked or remote file systems. You should use it on a locally mounted file system (e.g. Ext4, BTRFS, ...) not on a remote one (e.g. NFS)
For example, you might adopt the convention that your directory contains (or else you'll create it) some .lock file and use an advisory lock flock(2) (or a POSIX lockf(3)) on that .lock file before accessing the directory.
If using flock, you could even lock the directory directly....
The advantage of using such a file lock approach is that you could code shell scripts using flock(1)
And on Linux, you might also use inotify(7) (e.g. to be notified when some file is created in that directory)
Notice that most solutions are (advisory, so) presupposing that every process accessing that directory is following some convention (in other words, without more precautions like using flock(1), a careless user could access that directory - e.g. with a plain cp command -, or files under it, while your locking process is accessing the directory). If you don't accept that, you might look for mandatory file locking (which is a feature of some Linux kernels & filesystems, AFAIK it is sort-of deprecated).
BTW, you might read more about ACID properties and consider using some database, etc...
I would like to know if the open() system call in Linux latest kernel would block if the filesystem is mounted as remote device, for example a CEPH filesystem, or NFS , and there is a network failure of some sort?
Yes. How long depends on the speed (and state) of the uplink, but your process or thread will block until the remote operation finishes. NFS is a bit notorious for this, and some FUSE file systems handle the blocking for whatever has the file handle, but you will block on open(), read() and write(), often at the mercy of the network and the other system.
Don't use O_NONBLOCK to get around it, or you're potentially reading from or writing to a black hole (which would just block anyway).
Yes, an open() call can block when trying to open a file on a remote file system if there is a network failure of some sort.
Depending on how the remote file system is mounted, it may just take a long time (multiple seconds) to determine that the remote file system is unavailable and return unsuccessfully after what seems like an inordinate amount of time, or it may simply lock up indefinitely until the remote resource becomes available once more (or until the mapping is removed from the system).
User A asks the system to read file foo and at the same time user B wants to save his or her data onto the same file. How is this situation handled on the filesystem level?
Most filesystems (but not all) use locking to guard concurrent access to the same file. The lock can be exclusive, so the first user to get the lock gets access - subsequent users get a "access denied" error. In your example scenario, user A will be able to read the file and gets the file lock, but user B will not be able to write while user A is reading.
Some filesystems (e.g. NTFS) allow the level of locking to be specified, to allow for example concurrent readers, but no writers. Byte-range locks are also possible.
Unlike databases, filesystems typically are not transactional, not atomic and changes from different users are not isolated (if changes can even be seen - locking may prohibit this.)
Using whole-file locks is a coarse grained approach, but it will guard against inconsistent updates. Not all filesystems support whole-file locks, and so it is common practice to use a lock file - a typically empty file whose presence indicates that its associated file is in use. (Creating a file is an atomic operation on most file systems.)
Wikipedia - File Locking
For Linux, the short answer is you could get some strange information back from a file if there is a concurrent writer. The kernel does use locking internally to run each read() and write() operation serially. (Although, I forget whether the whole file is locked or if it's on a per-page granularity.) But if the application uses multiple write() calls to write information to the file, a read() could happen between any of those calls, so it could see inconsistent data. This is an atomicity violation in the operating system.
As mdma has mentioned, you could use file locking to make sure there is only one reader and one writer at a time. It sounds like NTFS uses mandatory locking, where if one program locks the file, all other programs get error messages when they try to access it.
Unix programs generally don't use locking at all, and when they do, the lock is usually advisory. An advisory lock only prevents other processes from getting an advisory lock on the same file; it doesn't actually prevent the read or write. (That is, it only locks the file for those who check the lock.)
Looking for information regarding the advantages and disadvantages of both fcntl and lockf for file locking. For example which is better to use for portability? I am currently coding a linux daemon and wondering which is better suited to use for enforcing mutual exclusion.
What is the difference between lockf and fcntl:
On many systems, the lockf() library routine is just a wrapper around fcntl(). That is to say lockf offers a subset of the functionality that fcntl does.
Source
But on some systems, fcntl and lockf locks are completely independent.
Source
Since it is implementation dependent, make sure to always use the same convention. So either always use lockf from both your processes or always use fcntl. There is a good chance that they will be interchangeable, but it's safer to use the same one.
Which one you chose doesn't matter.
Some notes on mandatory vs advisory locks:
Locking in unix/linux is by default advisory, meaning other processes don't need to follow the locking rules that are set. So it doesn't matter which way you lock, as long as your co-operating processes also use the same convention.
Linux does support mandatory locking, but only if your file system is mounted with the option on and the file special attributes set. You can use mount -o mand to mount the file system and set the file attributes g-x,g+s to enable mandatory locks, then use fcntl or lockf. For more information on how mandatory locks work see here.
Note that locks are applied not to the individual file, but to the inode. This means that 2 filenames that point to the same file data will share the same lock status.
In Windows on the other hand, you can actively exclusively open a file, and that will block other processes from opening it completely. Even if they want to. I.e., the locks are mandatory. The same goes for Windows and file locks. Any process with an open file handle with appropriate access can lock a portion of the file and no other process will be able to access that portion.
How mandatory locks work in Linux:
Concerning mandatory locks, if a process locks a region of a file with a read lock, then other processes are permitted to read but not write to that region. If a process locks a region of a file with a write lock, then other processes are not permitted to read nor write to the file. What happens when a process is not permitted to access the part of the file depends on if you specified O_NONBLOCK or not. If blocking is set it will wait to perform the operation. If no blocking is set you will get an error code of EAGAIN.
NFS warning:
Be careful if you are using locking commands on an NFS mount. The behavior is undefined and the implementation widely varies whether to use a local lock only or to support remote locking.
Both interfaces are part of the POSIX standard, and nowadays both interfaces are available on most systems (I just checked Linux, FreeBSD, Mac OS X, and Solaris). Therefore, choose the one that fits better your requirements and use it.
One word of caution: it is unspecified what happens when one process locks a file using fcntl and another using lockf. In most systems these are equivalent operations (in fact under Linux lockf is implemented on top of fcntl), but POSIX says their interaction is unspecified. So, if you are interoperating with another process that uses one of the two interfaces, choose the same one.
Others have written that the locks are only advisory: you are responsible for checking whether a region is locked. Also, don't use stdio functions, if you want the to use the locking functionality.
Your main concerns, in this case (i.e. when "coding a Linux daemon and wondering which is better suited to use for enforcing mutual exclusion"), should be:
will the locked file be local or can it be on NFS?
e.g. can the user trick you into creating and locking your daemon's pid file on NFS?
how will the lock behave when forking, or when the daemon process is terminated with extreme prejudice e.g. kill -9?
The flock and fcntl commands behave differently in both cases.
My recommendation would be to use fcntl. You may refer to the File locking article on Wikipedia for an in-depth discussion of the problems involved with both solutions:
Both flock and fcntl have quirks which
occasionally puzzle programmers from
other operating systems. Whether flock
locks work on network filesystems,
such as NFS, is implementation
dependent. On BSD systems flock calls
are successful no-ops. On Linux prior
to 2.6.12 flock calls on NFS files
would only act locally. Kernel 2.6.12
and above implement flock calls on NFS
files using POSIX byte range locks.
These locks will be visible to other
NFS clients that implement
fcntl()/POSIX locks.1 Lock upgrades
and downgrades release the old lock
before applying the new lock. If an
application downgrades an exclusive
lock to a shared lock while another
application is blocked waiting for an
exclusive lock, the latter application
will get the exclusive lock and the
first application will be locked out.
All fcntl locks associated with a file
for a given process are removed when
any file descriptor for that file is
closed by that process, even if a lock
was never requested for that file
descriptor. Also, fcntl locks are not
inherited by a child process. The
fcntl close semantics are particularly
troublesome for applications which
call subroutine libraries that may
access files.
I came across an issue while using fcntl and flock recently that I felt I should report here as searching for either term shows this page near the top on both.
Be advised BSD locks, as mentioned above, are advisory. For those who do not know OSX (darwin) is BSD. This must be remembered when opening a file to write into.
To use fcntl/flock you must first open the file and get its ID. However if you have opened the file with "w" the file will instantly be zeroed out. If your process then fails to get the lock as the file is in use elsewhere, it will most likely return, leaving the file as 0kb. The process which had the lock will now find the file has vanished from underneath it, catastrophic results normally follow.
To remedy this situation, when using file locking, never open the file "w", but instead open it "a", to append. Then if the lock is successfully acquired, you can then safely clear the file as "w" would have, ie. :
fseek(fileHandle, 0, SEEK_SET);//move to the start
ftruncate(fileno((FILE *) fileHandle), 0);//clear it out
This was an unpleasant lesson for me.
As you're only coding a daemon which uses it for mutual exclusion, they are equivalent, after all, your application only needs to be compatible with itself.
The trick with the file locking mechanisms is to be consistent - use one and stick to it. Varying them is a bad idea.
I am assuming here that the filesystem will be a local one - if it isn't, then all bets are off, NFS / other network filesystems handle locking with varying degrees of effectiveness (in some cases none)