What to do if PHP file lock is deadlocked

What is a deadlock

Everyone who has studied operating systems knows the concept of multi-threading. Accessing public resources in multiple threads requires locking the resources. After the access is completed, release the lock. If the lock is not released, then the next thread will never be able to acquire the resource lock when it comes to acquire the resource, so the thread is deadlocked. So is CGI a deadlock caused by multi-threaded public resource access? The answer is NO.

1. CGI is a single-threaded process and can be seen through ps. (The process status Sl is a multi-threaded process).

2. Even if it is multi-threaded, the deadlock occurs when the time function in glibc is called during PHP's shutdown process, and is not caused by the PHP module. The time-related functions in glibc are thread-safe and will not cause deadlock.

What causes deadlock?

By analyzing the mechanism of deadlock in Linux, we found that in addition to multi-threading, signal processing functions can also cause deadlock. So is cgi deadlock due to signal handling? Before that, let me introduce a thought.

Reentrancy and signal safety of functions

The reentrancy of a function means that no matter how many times the function is entered, the function can execute normally and return the result. . So are thread-safe functions reentrant? The answer is NO. Thread-safe functions acquire global locks when accessing public resources for the first time. If the function has not been executed and the lock has not been released, the process is interrupted. Then in the interrupt handling function, accessing the function again will cause a deadlock.

So what kind of functions can be accessed in the interrupt handling function?

In addition to functions that do not use global locks, there are also some signal safe system calls that can be used. Calling any other non-signal safe function will have unpredictable consequences (such as deadlock). See man signal for details. Before analyzing the causes of deadlock, let's first look at the cgi execution process and analyze whether there is a possibility of deadlock.

PHP-CGI execution process

The time function in Glibc uses a global lock to ensure the thread safety of the function, but does not guarantee signal safety. After previous analysis, we initially suspected that the deadlock was due to the PHP-CGI process receiving a signal and then executing a non-signal safe function in the signal handle. Before the main process is interrupted, it is executing the time function in glibc. Before the lock acquired by the function is released, the interrupt process is entered. During the interruption process, the time function in glibc is accessed. This resulted in a deadlock.

The execution process of PHP-CGI is as shown below:

Solution:

Remove or Simplify the hook function registered by qalarm into shutdown. Avoid unsafe function calls.

The above content is for reference only!

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