Closed Bug 62457 Opened 21 years ago Closed 21 years ago
fcntl file locking needs to set other fields in struct flock
One implementation of NSPR file locking functions is based on fcntl(). (Currently only Neutrino uses this implementation.) It only sets the l_type field of struct flock before passing it to fcntl(). Other fields, such as l_whence, l_start, and l_len, should also be set.
See W. Richard Stevens, Advanced Programming in the Unix Environment, Program 12.2 on p. 370 for sample code. Jerry, could you test the proposed fix? mozilla/nsprpub/pr/tests/lockfile.c is a test program for PR_LockFile. Note that PR_LockFile is not used by Mozilla.
Status: NEW → ASSIGNED
Checked in the updated patch on the main trunk and NSPRPUB_RELEASE_4_1_BRANCH.
Status: ASSIGNED → RESOLVED
Closed: 21 years ago
Resolution: --- → FIXED
Target Milestone: --- → 4.1.1
There are two problem with fcntl file locking in multi-threaded applications. I'll describe them below. Because of these problems, fcntl file and record locking using F_SETLKW is unsuitable for use in multi-threaded applications, IMO. So, I wonder if it is wise to implement PR_LockFile with fcntl. fcntl locks are associated with a process, not with individual threads. This creates two problems. 1. If one thread holds a lock on a file (or record in a file) and a second thread in the same process attempts to grab the lock, the second thread does not block. Rather, the fcntl call succeeds, returning an indication that the lock has been aquired. Thereafter the two threads both think they have exclusive access when they don't. This can be worked around by using mutexes. 2. A more serious problem is the "false deadlock" problem. fcntl will sometimes return -1 with errno EDEADLK instead of blocking, even when there is not really a deadlock. An example will illustrate this. Imagine two libraries, libA and libB, each of which uses fcntl to lock a file, fileA and fileB respectively. Each library uses fcntl to lock just its one file, and never returns from the library function while holding its lock, so it cannot deadlock. Now imagine a multi-threaded multi-process program. Each process has one thread the uses libA and a second thread that uses libB. The following sequence occurs. Process 1, thread A, libA, gets lock on file A. Process 2, thread B, libB, gets lock on file B. Process 1, thread B, libB, uses F_SETLKW to get lock on file B and blocks. Process 2, thread A, libA, uses F_SETLKW to get lock on file A, and fails, getting EDEADLK. There is no real deadlock here. There is no lock ordering problem ehre. The two threads that hold the locks on files A and B are not blocked waiting on the other locks. But fcntl doesn't see the threads. It sees only the processes. To fcntl, it appears that Process 1 holds the lock on A and wants the lock on B, and Process 2 holds the lock on B and wants the lock on A, a classic deadlock. IMO, this problematic behavior of fcntl F_SETLKW in multi-threaded apps is a serious shortcoming of fcntl record locking.
This is why PR_LockFile is not in the public NSPR API.
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